Reverse-engineering an airspeed/Mach indicator from 1977

How does a vintage airspeed indicator work? CuriousMarc picked one up for a project, but it didn't have any documentation, so I reverse-engineered it. This indicator was used in the cockpit panel for business jets such as the Gulfstream G-III, Cessna Citation, and Bombardier Challenger CL600. It was probably manufactured in 1977 based on the dates on its transistors.

You might expect that the indicators on an aircraft control panel are simple dials. But behind this dial is a large, 2.8-pound box with a complex system of motors, gears, and feedback potentiometers, controlled by two boards of electronics. But for all this complexity, the indicator doesn't have any smarts: the pointers just indicate voltages fed into it from an air data computer. This is a quick blog post to summarize what I found.

Front view of the indicator.

The dial has two rotating pointers: the white pointer indicates airspeed in knots while the striped pointer indicates the maximum airspeed (which varies depending on altitude). The "digital" indicator at the top shows Mach number from 0.10 to 0.99, implemented with rotating digit wheels. When the unit is operating, the OFF indicator flag switches to black. The flag switches to a bright V_MO warning if the pilot exceeds the maximum airspeed.1 On the rim of the dial, two small markers called "bugs" can be manually moved to indicate critical speeds such as takeoff speed.

In use, the indicator is connected to a Sperry air data computer and receives voltage signals to control the dial positions.3 The air data computer measures the static and dynamic air pressure from pitot tubes and determines the airspeed, Mach number, altitude, and other parameters. (These calculations become nontrival near Mach 1 as air compresses and the fluid dynamics change.) Since we didn't have the air data computer or its specifications, I needed to figure out the connections from the computer to the display.

With the unit's cover removed, you can see the internal mechanisms and circuitry. Each of the three indicators is controlled by a small DC motor with a potentiometer providing feedback. To the right, two circuit boards provide the electronics to drive the indicators.4 At the upper right, the black blob is a 26-volt 400-Hertz transformer to power the unit. Some power supply components are in front of it. Below the transformer is an orangish flexible printed-circuit board, which seems advanced for the timeframe. This flexible ribbon connects the transformer, the external connector, and the printed-circuit board sockets, providing the backplane for the system.

A side view of the unit shows the gears to control the indicators.

The diagram below shows the principle behind the servo mechanism that controls each indicator. The goal is to rotate the indicator to a position corresponding to the input voltage. A feedback loop is used to achieve this. The potentiometer provides a voltage proportional to its rotation. The input voltage and the feedback voltage are inputs to an op amp, which generates an error signal based on the difference between the inputs. The error signal rotates the DC motor in the appropriate direction until the potentiometer voltage matches the input voltage. Because the indicator and the potentiometer are geared together, the indicator will be in the correct position. As the input voltage changes, the system will continuously track the changes and keep the indicator updated.

A diagram illustrating the servo feedback loop.

Because the DC motor spins much faster than the dial moves, reduction gears slow the rotation. The photo below shows the gear train in the unit. A potentiometer is at the upper-right with three wires attached.

A closeup of the gear train. A potentiometer is on the right.

The Mach number has additional gearing to rotate the numbered wheels. When the low-digit wheel cycles around, it advances the high-digit wheel, similar to an odometer.

The mechanism to rotate the digit wheels for the Mach number.

Fault checking

One interesting feature of the indicator unit is that it implements fault checking to alert the pilot if something goes wrong. The front panel has a three-position flag. By default it's in the OFF position. Powering the coil in one direction rotates the flag to the blank side. Powering the coil in the other direction rotates the flag to the "V_MO" position which indicates that the pilot has exceeded the maximum operating speed.

I figured that powering up the unit would move the flag out of the OFF position, but it's more complicated than that. First, the unit checks that the air data computer is providing a suitable reference voltage. Second, the unit verifies that the motor voltages for the two needles are within limits; this ensures that the servo loop is operating successfully. Third, the unit checks that signals are received on status pins K and L. The unit only moves out of the OFF state if all these conditions are satisfied.5 Thus, if the unit receives bad signals or is malfunctioning, the pilot will be alerted by the OFF indicator, rather than trusting the faulty display.

The circuitry

The unit is powered by 26 volts, 400 Hz, a standard voltage for aviation. A small transformer provides multiple outputs for the various internal voltages. The unit has four power supplies: three on the first board and one on the back wall of the unit. One power supply is for the status indicator, one is for the op amps, one powers the 41.7V motors, and the fourth provides other power.

One subtlety is how the feedback potentiometers are powered. The servo loop compares the potentiometer voltage with the input voltage. But this only works if the potentiometer and the input voltage are using the same reference. One solution would be for the indicator unit and the air data computer to contain matching precision voltage regulators. Instead, the system uses a simpler, more reliable approach: the air data computer provides a reference voltage that the indicator unit uses to power the potentiometers.6 With this approach, the air data computer's voltage reference can fluctuate and the indicator will still reach the right position. (In other words, a 5V input with a 10V reference and a 6V input with a 12V reference are both 50%.)

The diagram below shows the board with the servo circuitry. The board uses dual op-amp integrated circuits, packaged in 10-pin metal cans that protected against interference.7 The ICs and some of the other components have obscure military part numbers; I don't know if this unit was built for military use or if military-grade parts were used for reliability.

The servo board is full of transistors, resistors, capacitors, diodes, and op-amp integrated circuits.

The circuitry in the lower-left corner handles the reference voltage from the air data computer. The board buffers this voltage with an op amp to power the three feedback potentiometers. The op amp also ensures that the reference voltage is at least 10 volts. If not, the indicator unit shows the "OFF" flag to alert the pilot.

The schematic below shows one of the servo circuits; the three circuits are roughly the same. The heart of the circuit is the error op amp in the center. It compares the voltage from the potentiometer with the input voltage and generates an error output that moves the motor appropriately. A positive error output will turn on the upper transistor, driving the motor with a positive voltage. Conversely, a negative error output will turn on the lower transistor, driving the motor with a negative voltage. The motor drive circuit has clamp diodes to limit the transistor base voltages.

Schematic of one of the servo circuits.

The op amp also receives a feedback signal from the motor output. I don't entirely understand this signal, which goes through a filter circuit with resistors, diodes, and a capacitor. I think it dampens the motor signal so the motor doesn't overshoot the desired position. I think it also keeps the transistor drive signal biased relative to the emitter voltage (i.e. the motor output).

On the input side, the potentiometer voltage goes through an op amp follower buffer, which simply outputs its input voltage. This may seem pointless, but the op amp provides a high-impedance input so the potentiometer's voltage doesn't get distorted.

The external input voltage goes through a resistor/capacitor circuit to scale it and filter out noise. Curiously, the circuit board was modified by cutting a trace and adding a resistor and capacitor to change the input circuit for one of the inputs. In the photo below, you can see the added resistor and capacitor; the cut trace is just to the right of the capacitor. I don't know if this modification changed the scale factor or if it filtered out noise. A label on the box says that Honeywell performed a modification on November 8, 1991, which presumably was this circuit.

A closeup of the circuit board showing the modification.

The second board implements three power supplies as well as the circuitry for the OFF/V_MO flag. The power supplies are simple and unregulated, just diode bridges to convert AC to DC, along with filter capacitors. Most of the circuitry on the board controls the status flag. Two dual op amps check the motor voltages against upper and lower limits to ensure that the motors are tracking the inputs. These outputs, along with other logic status signals, are combined with diode-transistor logic to determine the flag status. Driver transistors provide +18 or -18 volts to the flag's coil to drive it to the desired position.

This board has power supply circuitry and the control circuitry for the indicator flag.

Conclusions

After reverse-engineering the pinout, I connected the airspeed indicator to a stack of power supplies and succeeded in getting the indicators to operate (video). This unit is much more complex than I expected for a simple display, with servoed motors controlled by two boards of electronics. Air safety regulations probably account for much of the complexity, ensuring that the display provides the pilot with accurate information. For all that complexity, the unit is essentially a voltmeter, indicating three voltages on its display. This airspeed indicator is a bit different from most of the hardware I examine, but hopefully you found this look at its internal circuitry interesting.

With the case removed, the internal circuitry is visible.

You can follow me on Twitter @kenshirriff or rss. I've also started experimenting with mastodon recently as @[email protected].

Notes and references

1. Since the unit has airspeed and maximum airspeed indicators, you might expect it to display the maximum airspeed warning flag based on the two speed inputs. Instead, the flag is controlled by input pin "L". In other words, the air data computer, not the indicator unit, determines when the maximum airspeed is exceeded. ↩

2. This unit is a "Mach Airspeed Indicator", 4018366, apparently also called the SI-225,2

   

   Product label with part number 4018366-901.

   Note that the label says Sperry. In 1986, Sperry attempted to buy Honeywell but instead Burroughs made a hostile takeover bid. The merger of Sperry and Burroughs formed Unisys. A couple of months after the merger, the Sperry Aerospace Group was sold to Honeywell for $1.025 billion. Thus, the indicator became a Honeywell product. This corporate history explains why the unit has a Honeywell product support sticker.

   

   Labels on top of the unit indicate that it worked with the Sperry 4013242 and 4013244 air data computers. These became the Honeywell AZ-242 and AZ-244.

    ↩

3. The connector is a 32-pin MIL Spec round connector. Most of the 32 pins are unused. The connector has complex keying with 5 slots. I assume the keying is specific to this indicator, so the wrong indicator doesn't get connected.

   

   A closeup of the 32-pin connector, probably a MIL Spec 18-32.

   For reference, here is the pinout of the unit. Since this is based on reverse engineering, I don't guarantee it 100%. Don't use this for flight!

   Pin	Use
   A	5V illumination
   B	Chassis ground
   C	AC ground
   E	26V 400 Hz
   F	26V 400 Hz
   K	Enable
   L	Speed ok
   M	Signal ground
   N	Ref. voltage
   P	Vmax control voltage
   R	Airspeed control voltage
   S	Mach control voltage
   V	Chassis ground

   Pins D, G, H, J, T, U, W, X, Y, Z, a, b, c, d, e, f, g, h, and j are unused. ↩

4. The chassis has an empty slot for a third circuit board. My guess is that this chassis was used for multiple types of indicators and others required a third board. ↩

5. If the L pin goes low, the indicator will move to the V_MO position. ↩

6. My hypothesis is that the correct reference voltage is 11.7 volts. This yields a scale factor of 1 volt equals 50 knots. It also matches up the display's change in scale at 250 knots with the measured scale change. ↩

7. The meter uses three different integrated circuits in 10-pin metal cans with mysterious military markings: "FHL 24988", "JM38510/10102BIC 27014", and "SL14040". These appear to all be equivalent to uA747 dual op amps. (Note that JM38510 is not a part number; it is a general military specification for integrated circuits. The number after it is the relevant part number.) ↩

The unusual bootstrap drivers inside the 8086 microprocessor chip

The 8086 microprocessor is one of the most important chips ever created; it started the x86 architecture that still dominates desktop and server computing today. I've been reverse-engineering its circuitry by studying its silicon die. One of the most unusual circuits I found is a "bootstrap driver", a way to boost internal signals to improve performance.1

The bootstrap driver circuit from the 8086 processor.

This circuit consists of just three NMOS transistors, amplifying an input signal to produce an output signal, but it doesn't resemble typical NMOS logic circuits and puzzled me for a long time. Eventually, I stumbled across an explanation:2 the "bootstrap driver" uses the transistor's capacitance to boost its voltage. It produces control pulses with higher current and higher voltage than otherwise possible, increasing performance. In this blog post, I'll attempt to explain how the tricky bootstrap driver circuit works.

A die photo of the 8086 processor. The metal layer on top of the silicon is visible. Around the edge of the chip, bond wires provide connections to the chip's external pins. Click this image (or any other) for a larger version.

NMOS transistors

The 8086 is built from MOS transistors (MOSFETs), specifically NMOS transistors. Understanding the bootstrap driver requires some understanding of these transistors. If you're familiar with MOSFETs as components, they have source and drain pins and current flows from the drain to the source, controlled by the gate pin. Most of the time I treat an NMOS transistor as a digital switch between the drain and the source: a 1 input turns the transistor on, closing the switch, while a 0 turns the transistor off. However, for the bootstrap driver, we must consider the MOSFET in a bit more detail.

A MOSFET switches current from the drain to the source, under control of the gate.

The important aspect of the gate is the difference between the gate voltage and the (typically lower) source voltage; this is denoted as V_gs. Without going into semiconductor physics, a slightly more accurate model is that the transistor turns on when the voltage between the gate and the source exceeds the fixed threshold voltage, V_th. This creates a conducting channel between the transistor's source and drain. Thus, if V_gs > V_th, the transistor turns on and current flows. Otherwise, the transistor turns off and no current flows.

The voltage between the gate and the source (V_gs) controls the transistor.

The threshold voltage has an important consequence for a chip such as the 8086. The 8086, like most chips of that era, used a 5-volt power supply. The threshold voltage depends on manufacturing characteristics, but I'll use 1 volt as a typical value.3 The result is that if you put 5 volts on the drain and on the gate, the transistor can pull the source up to about 4 volts, but then V_gs falls to the threshold voltage and the transistor stops conducting. Thus, the transistor can't pull the source all the way up to the 5-volt supply, but falls short by a volt on the output. In some circumstances this is a problem, and this is the problem that the bootstrap driver fixes.

Due to the threshold voltage, the transistor doesn't pull the source all the way to the drain's voltage, but "loses" a volt.

If you get a transistor as a physical component, the source and drain are not interchangeable. However, in an integrated circuit, there is no difference between the source and the drain, and this will be important.4 The diagram below shows how a MOSFET is constructed on the silicon die. The source and drain consist of regions of silicon doped with impurities to change their property. Between them is a channel of undoped silicon, which normally does not conduct. Above the channel is the gate, made of a special type of silicon called polysilicon. The voltage on the gate controls the conductivity of the channel. A very thin insulating layer separates the gate from the channel. As a side effect, the insulating layer creates some capacitance between the gate and the underlying silicon.

Diagram of an NMOS transistor in an integrated circuit.

Basic NMOS circuits

Before getting to the bootstrap driver, I'll explain how a basic inverter is implemented in an NMOS chip like the 8086. The inverter is built from two transistors: a normal transistor on the bottom, and a special load transistor on top that acts like a pull-up resistor, providing a small constant current.5 With a 1 input, the lower transistor turns on, pulling the output to ground to produce a 0 output. With a 0 input, the lower transistor turns off and the current from the upper transistor drives the output high to produce a 1 output. Thus, the circuit implements an inverter: producing a 1 when the input is 0 and vice versa.

A standard NMOS inverter is built from two transistors. The upper transistor is a "depletion load" transistor.

The disadvantage of this inverter circuit is that when it produces a 0 output, current continuously flows through the load transistor and the lower transistor to ground. This wastes power, leading to high power consumption for NMOS circuitry. (To solve this, CMOS circuitry took over in the 1980s and is used in modern microprocessors.) This also limits the current that the inverter can provide.

If a gate needs to provide a relatively large current, for instance to drive a long bus inside the chip, a more complex circuit is used, the "superbuffer". The superbuffer uses one transistor to pull the output high and a second transistor to pull the output low.6 Because only one transistor is on at a time, a high-current output can be produced without wasting power. There are two disadvantages of the superbuffer, though. First, the superbuffer requires an inverter to control the high-side transistor, so it uses considerably more space on the die. Second, the superbuffer can't pull the high output all the way up; it loses a volt due to the threshold voltage as described earlier.

Combining two output transistors with an inverter produces a higher-current output, known as a superbuffer.

The bootstrap driver

In some circumstances, you want both a high-current output, and the full output voltage. One example is connecting a register to an internal bus. Since the 8086 is a 16-bit chip, it uses 16 transistors for the bus connection. Driving 16 transistors in parallel requires a fairly high current. But the bus transistors are "pass" transistors, which lose a volt due to the threshold voltage, so you want to start with the full voltage, not already down one volt. To provide both high current and the full voltage, bootstrap drivers are used to control the buses, as well as similar tasks such as ALU control.

The concept behind the bootstrap driver is to drive the gate voltage significantly higher than 5 volts, so even after losing the threshold voltage, the transistor can produce the full 5-volt output.7 The higher voltage is generated by a charge pump, as illustrated below. Suppose you charge a capacitor with 5 volts. Now, disconnect the bottom of the capacitor from ground, and connect it to +5 volts. The capacitor is still charged with 5 volts, so now the high side is at +10 volts with respect to ground. Thus, a capacitor can be used to create a higher voltage by "pumping" the charge to a higher level.

On the left, the "flying capacitor' is charged to 5 volts. By switching the lower terminal to +5 volts, the capacitor now outputs +10 volts

The idea of the bootstrap driver is to attach a capacitor to the gate and charge it to 5 volts. Then, the low side of the capacitor is raised to 5 volts, boosting the gate side of the capacitor to 10 volts. With this high voltage on the gate, the threshold voltage is easily exceeded and the transistor can pass the full 5 volts from the drain to the source, producing a 5-volt output.

With a large voltage on the gate, the threshold voltage is exceeded and the transistor remains on until the source reaches 5 volts.

In the 8086 bootstrap driver,8 an explicit capacitor is not used.9 Instead, the transistor's inherent capacitance is sufficient. Due to the thin insulating oxide layer between the gate and the underlying silicon, the gate acts as the plate of a capacitor relative to the source and drain. This "parasitic" capacitance is usually a bad thing, but the bootstrap driver takes advantage of it.

The diagrams below show how the bootstrap driver works. Unlike an inverter, the bootstrap driver is controlled by the chip's clock, generating an output only when the clock is high. In the first diagram, we assume that the input is a 1 and the clock is low (0). Two things happen. First, the inverted clock turns on the bottom transistor, pulling the output to ground. Second, the 5V input passes through the first transistor; the left side of the transistor acts as the drain and the right side as the source. Due to the threshold voltage, a volt is "lost" so about 4 volts reaches the gate of the second transistor. Since the source and drain of the second transistor are at 0 volts, the gate capacitors are charged with 4 volts. (Recall that these are not explicit capacitors, but are parasitic capacitors.)

The first step in the operation of the bootstrap driver. The gate capacitance is charged by the input.

In the next step, the clock switches state and things become more interesting. The second transistor is on due to the voltage on the gate, so current flows from the clock to the output. In a "normal" circuit, the output would rise to 4 volts, losing a volt due to the threshold voltage of the second transistor. However, as the output voltage rises, it boosts the voltage on the gate capacitors and thus raises the gate voltage. The increased gate voltage allows the output voltage to rise above 4 volts, pushing the gate voltage even higher, until the output reaches 5 volts.10 Thus, the bootstrap driver produces a high-current output with the full 5 volts.

The second step in the operation of the bootstrap driver. As the output rises, it boosts the gate voltage even higher.

An important factor is that the first transistor now has a higher voltage on the right than on the left, so the source and drain switch roles. Since the transistor has 5 volts on the gate and on the (now) source, V_gs is 0 and current can't flow. Thus the first transistor blocks current flow from the gate, keeping the gate at its higher voltage. This is the critical role of the first transistor in the bootstrap driver, acting as a diode to block current flow out of the gate.

The diagram below shows what happens when the clock switches state again, assuming a low input. Now the first transistor's source voltage drops, making V_gs large and turning the transistor on. This allows the second transistor's gate voltage to flow out. Note that the first transistor is no longer acting as a diode, since current can flow in the "reverse" direction. The other important action in this clock phase is that the bottom transistor turns on, pulling the output low. These actions discharge the gate capacitance, preparing it for the next bootstrap cycle.

When the clock switches off, the driver is discharged, preparing it for the next cycle.

The 8086 die

Now that I've explained the theory, how do bootstrap drivers appear on the silicon die of the 8086? The diagram below shows six drivers that control the ALU operation.11 There's a lot happening in this diagram, but I'll try to explain what's going on. For this photo, I removed the metal layer with acid to reveal the silicon underneath; the yellow lines show where the metal wiring was. The large pinkish regions are doped silicon, while the gray speckled lines are polysilicon on top. The greenish and reddish regions are undoped silicon, which doesn't conduct and can be ignored. A transistor is formed where a polysilicon line crosses silicon, with the source and drain on opposite sides. Note that some transistors share the source or drain region with a neighboring transistor, saving space. The circles are vias, connections between the metal and a lower layer.

Six bootstrap drivers as they appear on the chip.

The drivers start with six inputs at the right. Each input goes through a "diode" transistor with the gate tied to +5V. I've labeled two of these transistors and the other four are scattered around the image. Next, each signal goes to the gate of one of the drive transistors. These six large transistors pass the clock to the output when turned on. Note that the clock signal flows through large silicon regions, rather than "wires". Finally, each output has a pull-down transistor on the left, connecting it to ground (another large silicon region) under control of the inverted clock. The drive transistors are much larger than the other transistors, so they can provide much more current. Their size also provides the gate capacitance necessary for the operation of the bootstrap driver.

Although the six drivers in this diagram are electrically identical, each one has a different layout instead of repeating the same layout six times. This demonstrates how the layout has been optimized, moving transistors around to use space most efficiently.

In total, the 8086 has 81 bootstrap drivers, mostly controlling the register file and the ALU (arithmetic-logic unit). The die photo below shows the location of the drivers, indicated with red dots. Most of them are in the center-left of the chip, between the registers and ALU on the left and the control circuitry in the center.

The 8086 die with main functional blocks labeled. The bootstrap drivers are indicated with red dots.

Conclusions

For the most part, the 8086 uses standard NMOS logic circuits. However, a few of its circuits are unusual, and the bootstrap driver is one of them. This driver is a tricky circuit, depending on some subtle characteristics of MOS transistors, so I hope my explanation made sense. This driver illustrates how Intel used complex, special-case circuitry when necessary to get as much performance from the chip as possible.

If you're interested in the 8086, I wrote about the 8086 die, its die shrink process and the 8086 registers earlier. I plan to write more about the 8086 so follow me on Twitter @kenshirriff or RSS for updates.

Notes and references

1. Intel used a "bootstrap load" circuit in the 4004 and 8008 processors. The bootstrap load has many similarities to the bootstrap driver, using capacitance to boost the output voltage. But it is a different circuit, used in a different role. The bootstrap load was designed for PMOS circuits to boost the voltage from a pull-up transistor, using explicit capacitors, built with a process invented by Federico Faggin. I wrote about the bootstrap load here. ↩

2. The only explanation of a bootstrap driver that I could find is in section 2.3.1 of DRAM Circuit Design: A Tutorial. The 8086 transistors with the gate wired to +5V puzzled me for the longest time. It seemed to me that this transistor would always be on, and thus had no function. However, the high voltage of the bootstrap driver gives it a function. I was randomly reading the DRAM book and suddenly recognized that one of the circuits in that book was similar to the mysterious 8086 circuit. ↩

3. The threshold voltage was considerably higher for older PMOS transistors. To get around this, old chips used considerably higher supply voltages, so "losing" the threshold voltage wasn't as much of a problem. For instance, the Intel 4004 used a 15-volt supply. ↩

4. The reason that MOSFETs are symmetrical in an integrated circuit and asymmetrical as physical components is that MOSFETs really have four terminals: source, gate, drain, and the substrate (the underlying silicon on which the transistor is constructed). In component MOSFETs, the substrate is internally connected to the source, so the transistor has three pins. However, the source-substrate connection creates a diode, making the component MOSFET asymmetrical. Four-terminal MOSFETs such as the 3N155 exist but are rare. The MOnSter 6502 made use of 4-terminal MOSFET modules to implement the 6502's pass transistors. ↩

5. The load transistor is a special type of transistor, a depletion transistor that is doped differently. The doping produces a negative threshold voltage, so the transistor remains on and provides a relatively constant current. See Wikipedia for more on depletion loads. ↩

6. The superbuffer has some similarity with a CMOS gate. Both use separate transistors to pull the signal high or low, with only one transistor on at a time. The difference is that CMOS uses a complementary transistor, i.e. PMOS, to pull the signal high. PMOS performs better in this role than NMOS. Moreover, a PMOS transistor is turned on by a 0 on the gate. This behavior eliminates the need for the inverter in a superbuffer. ↩

7. The 8086 processor also uses completely different charge pumps to create a negative voltage for a substrate bias. I discuss that use of charge pumps here. ↩

8. Why is it called a bootstrap driver? The term originates with footwear: boots often had boot straps on the top, physical straps to help pull the boots on. In the 1800s, the saying "No man can lift himself by his own boot straps" was used as a metaphor for the impossibility of improvement solely through one's own effort. (Pulling on the straps on your boots superficially seems like it should lift you off the ground, but is of course physically impossible.) By the mid-1940s, "bootstrap" was used in electronics to describe a circuit that started itself up through positive feedback, metaphorically pulling itself up by its bootstraps. The bootstrap driver continues this tradition, pulling itself up to a higher voltage. ↩

9. Some circuits in the 8086 use physical capacitors on the die, constructed from a metal layer over silicon. The substrate bias generators use relatively large capacitors. There are also some small capacitors that appear to be used for timing reasons. ↩

10. The exact voltage on the gate will depend on the relative capacitances of different parts of the circuit, but I'm ignoring these factors. The voltages that I show in the diagram are illustrations of the principle, not accurate values. ↩

11. Some of the 8086's bootstrap drivers pre-discharge when the clock is low and produce an output when the clock is high, while other drivers operate on the opposite clock phases. The ALU drivers in the die photo operate on the opposite phases, but I've labeled the diagram to match the previous discussion. ↩

Reverse-engineering the Apollo spacecraft's FM radio

How did NASA communicate with the Apollo astronauts, hundreds of thousands of miles from Earth? The premodulation processor1 (below) was the heart of the communication system onboard the Apollo spacecraft. Its multiple functions included an FM radio for communication to the astronauts, implemented by the Voice Detector, the module second from the top. In this blog post, I reverse-engineer the circuitry for that module and explain how it worked.

With the case of the premodulation processor removed, its internal modules are visible, as well as the wiring harnesses.

The Apollo communication system was complex and full of redundancy. Most communication took place over a high-frequency radio link that supported audio, telemetry, scientific data, and television images.2 NASA's massive 85-foot dish antennas transmitted signals to the spacecraft at 2106.4 megahertz, an S-band frequency, giving the system the name "Unified S-Band". These radio signals were encoded using phase modulation;3 onboard the spacecraft, a complex box called the transponder received the S-band signal and demodulated it.4

The voice and data signals from Earth were combined through a second layer of modulation: voice was frequency-modulated (FM) onto a 30-kilohertz subcarrier while data was on a 70-kilohertz subcarrier, so the two signals wouldn't conflict.5 One of the tasks of the premodulation processor was to extract the voice and data signals from the transponder's output. These voice signals went to yet another box, the Audio Center Equipment, so the astronauts could hear the messages from the ground. The data signals were decoded by the Up-Data Link, allowing NASA to send commands to the Apollo Guidance Computer, control onboard relays, or set the spacecraft's clock.

Many systems worked together for communication, but I'm focusing on a single module: the voice detector inside the premodulation processor that performed the FM demodulation. The block diagram below shows the operation of the voice detector; I've grayed out the data detector.6 The input contains the voice signal and the data signal at different frequencies; a band-pass filter (green) separates out the voice signal at 30 kilohertz. Next, the blue triangle7 demodulates the FM signal using a "clipper discriminator" circuit. The cyan triangle is an amplifier, producing the "up voice" output signal (red), so-called because it had been transmitted "up" from a ground station. I'll explain this circuitry in detail below.

Block diagram of the data and voice detectors, with the data detector grayed out. Each "Q" indicates a transistor in the circuit. Click this image (or any other) for a larger version. Based on Command/Service Module Systems Handbook p63.

The photo below shows the premodulation processor in its case.1 The premodulation processor (PMP) weighs 14.5 pounds and measures 4.7"×6"×10.5". It used 8.5 watts of power, supplied at 28 volts DC from the spacecraft's hydrogen/oxygen fuel cells or silver-oxide zinc batteries. The PMP was mounted in the Command Module's equipment bay, along with most of the electronic equipment.8 It was fastened to a "cold plate", cooled by water-glycol loops that removed heat through radiators and water evaporators.

The premodulation processor is a bluish box with four round connectors on top.

Construction

The modules in the premodulation processor don't use printed-circuit boards, but instead are built from components that are soldered to metal pegs, forming a messy jumble of wiring. The circular transistors are mounted upside down with color-coded wiring: yellow for the emitter, green for the base, and blue for the collector. Capacitors are silver cylinders or gray squares, while the orange striped cylinder is a diode. The resistors have colored stripes, indicating their values. Point-to-point wiring provided additional connections, a mixture of color-coded insulated wires, bare wires, and wires in clear sleeves.

A closeup of the wiring in the premodulation processor. These connections are soldered, but others are spot-welded.

Since the components and wiring are visible, it seemed like these modules should be easy to reverse-engineer, but it's trickier than it seems. The components are liberally covered in what looks like hot glue but is probably silicone. (I suspect that this was only used in equipment for ground testing, while modules for spaceflight were fully encapsulated to prevent short circuits.) Much of the wiring is obscured, so I had to beep out many of the connections with a multimeter. As a result, my reverse-engineering probably has a few errors.

The modules have circuitry on both sides, which increased the density. The photo below shows the top side of the module. This module includes a few larger components that are mounted directly to the chassis, rather than to the circuit board. The large metal box at the top is the bandpass filter, built by Bulova Electronics, a division of the watch company that produced quartz crystals, oscillators, filters, servo amplifiers, and other components. This bandpass filter was built for Collins Radio, the manufacturer of the premodulation processor, and presumably contains a quartz filter, selecting the voice sub-band at 30 kilohertz and rejecting other frequencies. At the lower right is a smaller black box, an electromechanical relay that switched signals. The two grayish boxes are audio transformers to couple the module's output signals. The connector at the left has its wiring completely covered in silicone, inconvenient for reverse engineering.

Top view of the voice detector module.

The circuitry on the bottom side of the board is arranged in orderly columns, unlike the other side. I don't know why the design styles of the two sides are so different. However, in a few places they put components under other components, so the circuitry isn't as orderly as it appears. The back of the bandpass filter is visible at the bottom. The two sides of the module are connected by a few gray wires.

Bottom view of the voice detector module.

How the FM demodulator works

FM radio has a tragic history. It was invented in 1933 by Edwin Armstrong, a prolific inventor of radio technologies (no relation to Neil Armstrong, first to walk on the Moon). FM was a superior alternative to AM (amplitude modulation), an earlier radio transmission system. Unfortunately, RCA (Radio Corporation of America) had invested heavily in AM radio and lobbied to block the introduction of FM. Armstrong spent years battling RCA in court with little success, resulting in his suicide in 1954. Almost a year later, his wife obtained a million-dollar settlement from RCA (about $8 million in current dollars), followed by successful patent litigation leading to recognition of Armstrong as the inventor of FM. Eventually, in the 1960s, FM radio achieved commercial success, as well as its use in the space program.

In an FM signal, the frequency of a carrier signal is changed (modulated) depending on an input signal. That is, the varying frequency of the transmitted signal indicates the level of the input. An FM demodulation circuit undoes this process, converting a varying frequency input into the corresponding voltage to recover the original signal. Many FM receiver designs have been used with tradeoffs of linearity, noise rejection, and circuit complexity.9 The simple but inaccurate slope detector uses a high pass filter to produce more output at higher frequencies. Several techniques use a circuit tuned to the carrier frequency so the output increases with frequency deviation: the vintage Foster-Seely discriminator dating back to 1936, the ratio detector, and the simple and popular quadrature detector. The complex phase-locked loop (PLL) approach keeps an oscillator locked onto the input frequency while producing the corresponding voltage.

The premodulation processor, however, used a pulse-averaging discriminator, a high-quality but expensive demodulator used in wideband applications such as telemetry. The diagram below illustrates the FM modulation and demodulation process. The red line at the top shows an audio input signal, which modulates the purple signal: when the input is higher, the purple signal has a higher frequency. The FM purple signal is transmitted to the spacecraft. To demodulate the signal, the premodulation processor first amplifies and clips the signal (green). Next, it produces short, fixed-width pulses (gray), triggered by each green pulse; as the input frequency increases, these pulses will be closer together. Applying a low-pass filter smooths out the pulses, resulting in a higher output level when the pulses are close together.10 The result (red, bottom) matches the input.

The FM signal at various stages of processing: input, FM-modulated signal, clipped signal, fixed-width pulses, and output.

Looking at the premodulation processor's circuitry, the first step is to amplify and clip the input signal, turning the input into square waves. This step removes any variations in the input signal level, reduces the effect of noise, and creates a clean signal for the next phase. Clipping is done with a pair of diodes. A diode will turn on at about 0.6 volts, so the result is that the signal is limited to -0.6 volts to 0.6 volts. You can think of clipping as cutting the peaks off the sine waves and amplifying, so you end up with sharp transitions rather than a smooth wave.11 The schematic below shows one of the two clipping stages. The transistor amplifies the input, using a basic NPN transistor amplifier circuit. The two diodes in the middle clip the signal. The capacitors block the DC component of the signal, ensuring that it is centered around 0 for symmetrical clipping.

Schematic of one stage of the clipper.

The clipper is followed by a two-transistor pulse generator, a "single-shot monostable multivibrator". Each input pulse discharges its capacitor, which then recharges through a resistor. This resistor-capacitor delay creates a fixed-width pulse, and then the circuit waits for the next input pulse. The next stage is a two-transistor low-pass filter that turns the pulses into a smooth output, using a handful of capacitors. It is followed by a transistor amplifier (that can be turned off as needed). This feeds two audio transformers to produce the voice outputs that go to the Audio Center, and thus the astronauts.

More features

In the basic configuration, the voice detector extracts the voice signal, while the data detector extracts the data signal, using a similar circuit. However, in case the voice communication circuitry failed, the system provided an "up voice backup" circuitry, a redundant way for ground stations to send voice to the spacecraft. The backup path transmitted voice over the data sub-band, and the data detector performed the FM demodulation. By flipping a switch, the data detector's output was routed to the voice circuitry, providing a voice path for emergencies. (Backup voice was completely analog, even though it used the data detector module.) During the Apollo 13 incident, the astronauts used backup voice to conserve electricity, which was running critically low.12

If voice communication failed entirely, the astronauts could switch to "emergency key" mode and transmit Morse code using the push-to-talk button on their umbilical cable. Most of the emergency key circuitry is elsewhere, but the voice detector has an input for the emergency key tone to get mixed into the audio that the astronauts heard.

The XMIT button on the astronaut's umbilical could be used as a key to transmit Morse code in an emergency. From Apollo operations handbook.

The communication system also included "squelch", a feature that silenced the audio if the signal strength dropped too low. ("Squelch" is a curious word, an onomatopeic 17th-century word meaning "to crush", later used figuratively as "suppress", and then in the 1930s as a radio circuit to suppress noise.) The S-band radio originally didn't include squelch but NASA soon found that a loss of the carrier signal created high noise levels that could disrupt other audio channels. To avoid this problem, a squelch feature was added to the radio before the Moon landings.

The squelch circuit detected the level of the carrier signal much like an AM radio, using a diode to rectify the sine wave and track the peaks. It used two transistors to amplify the signal and a third transistor to disable the audio, triggering squelch if the carrier level fell too low. A squelch disable switch was provided to ensure that, if necessary, voice could be used even at low signal levels. Moreover, some astronauts liked disabling squelch so they could use the channel noise to determine the channel's status.

Another important feature for redundancy was relay support. If, for instance, the S-band radio failed on the Lunar Module, the Command Module could relay communication to and from the ground, using the VHF radio, as shown below. The circuit to relay communication from the ground uses a clever implementation trick. By flipping a switch, the ground up-voice signal replaced the Command Module pilot's microphone (#2, center seat), so ground communication could be transmitted just like the astronaut's speech, sending it over the VHF radio to the Lunar Module, for instance. The diagram below illustrates two scenarios: from ground to an extra-vehicular activity or from ground to the Lunar Module, relayed through the Command Module.

Illustrations of how relay mode works from the ground (MSFN, Manned Space Flight Network) to an extra-vehicular activity (EVA), as well as to the Lunar Module (LM). Adapted from Apollo CSM Logistics Training.

The voice relay circuit was implemented with an electromechanical relay in the voice decoder module—don't be confused with the two completely different meanings of "relay" in this system. Flipping a switch caused the electromechanical relay to replace the microphone signal with the up voice signal.13

The astronauts sat in front of a complex control panel full of switches and gauges. The controls for the premodulation processor were grouped in the lower-right corner of the console with other communications switches. The diagram below shows the switches that controlled the voice detector features: squelch, backup voice, and voice relay, highlighted in yellow.

The Command Module's control panel with relevant switches highlighted. Diagram based on from Command/Service Module Systems Handbook p208.

Schematic

The detailed block diagram shows the construction of the voice detector and data detector modules. Each triangle corresponds to a transistor. I've grayed out the data detector and colored external switching circuitry in blue; these switches match the ones above. You can see how the backup up-voice comes from the data detector module, and then is merged into the voice detector's output, under the control of the "data / up voice BU" switch. At the bottom, the relay switches the voice signal in place of the microphone #2 signal when relaying voice from Earth. ("AC" is the Audio Console, the audio system connected to the astronaut's headphones and microphones.)

Detailed block diagram of the voice detector and data detector modules, data detector grayed out. Based on Apollo Telecommunication System training.

After tracing out the module's circuitry, I generated the schematic below. You can match the schematic against the block diagram to see how the functional blocks are implemented,14 using relatively simple circuits with one or two transistors per function.

My reverse-engineered schematic of the voice detector module. Expect a few errors. Click for a larger version.

The photos below show how the circuitry maps onto the physical layout of the boards in the voice detector module. Signal processing starts on the right with the FM circuitry (clippers and pulse generator), and the squelch circuit. The low-pass filter and output circuitry is on the left board.

The voice detector with the main functional blocks labeled.

Conclusion

From the outside, the premodulation processor is a mysterious blue box. Opening it up reveals relatively straightforward transistor circuits, implemented with a surprisingly haphazard construction technique.

Although I reverse-engineered this module partly from curiosity, the main motivation was to uncover a pin that was missing from our documentation, specifically the pin to control squelch, missing since squelch was added relatively late in the design. We plan to wire up the premodulation processor, using an elaborate "breakout board" that Eric is designing. We can then use the premodulation processor as it would have operated during a mission, hooking it up to the transponder and giving it radio signals. I announce my latest blog posts on Twitter, so follow me @kenshirriff for updates. I also have an RSS feed.

For an overview of the premodulation processor, see my previous blog post. Also see Curious Marc's video where the premodulation processor is disassembled (below). Thanks to Mike Stewart, Curious Marc, and Eric Schlaepfer for their roles in the premodulation processor investigation. Thanks to Marcel for providing the premodulation processor.

Notes and references

1. For detailed specifications of the premodulation processor, see Command/Service Module Systems Handbook p73. ↩↩

2. The design standard for the Apollo audio system was 90% word intelligibility for the main links and 70% for the backup links. This standard seems surprisingly poor, with one out of 10 words unintelligible, but achieving this standard was challenging due to the extreme distance to the Moon. For detailed information on the voice communication system, see Apollo Experience Report - Voice Communications Techniques and Performances. It discusses the performance requirements for the Apollo communications system and how the system was designed to achieve the intelligibility requirements. ↩

3. Phase modulation (PM) varies the phase of the carrier signal, rather than varying the frequency as in frequency modulation (FM). The techniques are very similar since increasing the phase compresses the waveform, increasing the frequency. Specifically, phase modulation of an input is the same as frequency modulation of the input's derivative. Apollo used phase modulation for the overall signal because it keeps the frequency (mostly) constant so doppler ranging could be used to measure the spacecraft's speed. ↩

4. The transponder got its name because it also sent the signals back to Earth after shifting the frequency, so the distance to the spacecraft could be accurately determined; see my discussion here. ↩

5. The data signal from Earth had a third layer of modulation: the binary data was modulated with phase-shift keying at 2 kilohertz to produce an audio signal for transmission. Another box, the Up-Data Link demodulated and decoded this signal after the premodulation processor had demodulated the FM layer. I have another blog post that describes this. ↩

6. I'm not covering the data detector in this blog post, but since it's so closely tied to the voice detector, I'll give an overview. Its circuitry is similar to the voice detector, but simpler, since it doesn't have squelch or the relay. It has a similar bandpass filter module, but at 70 kilohertz rather than 30 kilohertz, reflecting the data subcarrier frequency.

   

   The data detector module.

    ↩

7. In case anyone is studying the block diagram carefully, I'll explain the labels such as "Q1-6V". This indicates transistors 1 through 6 in the voice module. "Q8D", on the other hand, indicates transistor 8 in the data module. ↩

8. The premodulation processor was one of many boxes of electronic circuitry packed into the spacecraft and linked by thick cables. The diagram below highlights where it was mounted in the lower equipment bay of the Apollo Command Module.

   

   The premodulation processor was one of many electronic boxes in the Command Module's lower equipment bay. Diagram from Command/Service Module Systems Handbook p212.

    ↩

9. Various FM detector circuits are described here. ↩

10. The technique of using varying digital pulses to generate an analog signal is similar to the PWM (pulse-width modulation) technique used for analog outputs on the Arduino. The difference is that the Arduino uses pulses with a fixed frequency and varying width, while the FM discriminator uses pulses with a varying frequency and fixed width. ↩

11. The clipping process preserves the "zero-crossings", the points where the waveform's voltage crosses zero. This throws away amplitude fluctuations and most of the noise that may be in the signal. ↩

12. The idea of backup voice was to provide a voice channel for emergencies that used less power, at the cost of garbling up to 30% of the words. After the explosion, Apollo 13 used the backup voice system so they could turn off the Lunar Module's power amplifier and conserve electrical power. (See Apollo 13 Mission Operations Report pages N-2 and N-7, as well as the transcript.) Backup voice was also used at times during Apollo 16 due to a failure of the Lunar Module's steerable S-band antenna; see Apollo 16 Mission Report page 7-3, which calls this mode "down voice backup". (I should point out that these backup voice incidents involved the Lunar Module, so the Command Module's premodulation processor didn't take part.) ↩

13. The relay circuitry was a bit more complicated than I expected. Its main task is to switch between the microphone input and the voice signal. However, it also switches a 50Ω resistor across the transformer if the voice signal is not used, presumably so the impedance remains unchanged and Audio Console level doesn't jump. In other words, the resistor gives the unused voice signal somewhere to go. ↩

14. The major difference between the block diagram and my schematic is that the block diagram shows the transformers connected to ground, while I found that they are connected to +18V. ↩

Talking with the Moon: Inside Apollo's premodulation processor

The Apollo missions to the Moon required complex hardware to communicate between Earth and the spacecraft, sending radio signals over hundreds of thousands of miles. The premodulation processor was a key component of this system, combining voice, scientific data, TV, and telemetry for transmission to Earth.1 It was built from components that were welded together and tightly packed into a 14.5-pound box.2 In this blog post, I look inside the premodulation processor, examine its construction, and describe how each module worked.

The premodulation processor with its case removed, showing some of the circuitry. (Click any image for a larger version.)

The communications systems in the Apollo Command Module were very complex, as shown in the block diagram below.3 The premodulation processor (PMP, yellow) played a central role: most of the audio (red), data (orange), and TV (purple) went through the premodulation processor, where the signals were combined for transmission by the S-band (blue) radio systems. The premodulation processor also handled most of the voice and data signals received from Earth or from the Lunar Module via the VHF (green) or S-band radio systems.

Block diagram of the Apollo communications system. From Apollo Operations Handbook: Telecommunications System page 3.

One reason for the complexity of the premodulation processor was that the audio system had to support a variety of communications configurations. The diagram below illustrates one configuration, when astronauts were walking on the Moon (i.e. extra-vehicular activity, EVA). They communicated with the Lunar Module on the Moon's surface via VHF/AM radio, which relayed their audio to Earth via the Unified S-Band (USB) radio. Meanwhile, the Command and Service Module (CSM) orbiting the Moon also communicated with Earth via S-Band. These voices were conferenced together so the astronauts and ground could all hear each other. The need for redundancy added to the complexity; for example, signals from the Moon could be relayed through the Command Module in the event of an equipment failure.

Typical Apollo communication for lunar surface operations. From Apollo Experience Report.

Construction

Like much of the Apollo electronics, the premodulation processor was packaged in a drab bluish metal case. The case has four round military-style connectors on top that linked the various audio, RF, and control signals to other components of the spacecraft.

This photo shows the premodulation processor inside its case.

We opened the case by removing the screws and inside we found 11 rectangular modules packed together tightly, from the power supply at the top to the "SCO & diff ampl" (subcarrier oscillator and differential amplifier) at the bottom, conveniently labeled with their functions. The modules were plugged into a thin backplane,5 at the right, connected by D-Sub connectors, similar to vintage RS-232 connectors but in a variety of sizes. Bundles of wires connected the backplane to the round connectors. This construction technique made it easy for us to remove the modules and inspect them individually.

A side view of the premodulation processor, showing the labeled modules.

The modules themselves don't use printed-circuit boards, but instead are built from components that are spot-welded to metal pegs, as shown below.6 These resistors, diodes, capacitors, and transistors are tightly packed with a jumble of overlapping wiring. Most of the wiring consists of the component leads, but point-to-point wiring provided additional connection. The wiring is a combination of color-coded insulated wires, bare wires, and bare wires in clear insulating tubes. The components are liberally covered in what looks like hot glue. I suspect that the hot glue was only used in equipment for ground testing, while modules for spaceflight were fully encapsulated to prevent short circuits.

A closeup of the wiring in the aux bi-phase modulator module. Most of the connections are spot-welded, although a few seem to have solder.

The modules have circuitry on both sides, which increased the density. About half of the metal pegs provide connection to the other side, while half have plastic stubs on one side. As will be seen below, many of the modules also contain rectangular metal sub-units that implement functional blocks such as oscillators or filters. It appears that these standardized functions could be bought "off-the-shelf", not as integrated circuits, but as blocks containing discrete components.

In the following sections, I'll discuss each module in more detail, starting with the power supply.

Power supply module

The premodulation processor contains a power supply that converted the spacecraft's 28-volt DC supply to 18 volts. For efficiency, it is a switching power supply, a buck converter that chops up the input power at a high frequency to drop it to the lower voltage. Although switching power supplies are now ubiquitous, in everything from phone chargers to PC power supplies, switching power supplies were expensive and rare in the 1960s, used in aerospace applications that required a compact, high-efficiency power supply.

The block diagram below shows that the power supply was implemented redundantly, with a normal regular and an auxiliary regulator. A relay switches between the two regulators, controlled by the PMP NORM/AUX switch.

Diagram of the power supply module. From Command/Service Module Systems Handbook p63.

You may know of the Apollo 12 incident where the spacecraft was hit by lightning seconds after launch, scrambling the telemetry. The problem was resolved by the famous "set SCE to AUX" switch.7 The PMP's power switch is next to the SCE switch but never played a dramatic role.8

The power switches for the signal conditioning equipment (SCE) and the premodulation processor (PMP) are in the lower-left corner of the Command Module's control panel. Each switch has positions for NORM, OFF, and AUX.

The photo below shows the power supply module. The redundant halves of the power supply are visible with the lower circuitry a mirror image of the upper circuitry. The relay to switch between the two is the black box in the center-left. The power switching transistors are above and below the relay, fastened down with screws. To the right of the transistors are cylindrical tan inductors, storing energy across each pulse. Large silver filter capacitors are between the inductors. The right half of the module is the control circuitry: resistors, capacitors, transistors, and diodes. The connector at the far right connects the power supply to the other modules via the backplane.

The power supply module for the premodulation processor.

Flipping the power supply over reveals the high-frequency power transistors, in large metal packages to dissipate heat. These packages are square, unlike the typical two-tab (TO-3) power transistor packaging. Note the second layer of discrete component circuitry on this side of the module. This illustrates how the modules have two layers of circuitry, one on each side. You can also see the tops of the smaller transistors that are wired on the other side.

Underside of the power supply with 2N3137 power transistors.

Voice and data detector module

The data and voice detectors handle signals transmitted to the spacecraft over the S-band. The S-band transceiver receives these signals, demodulates them, and passes the signal to the premodulation processor. The data and voice detectors appear as one module on the block diagram below but are implemented as two modules physically.

Diagram of the data and voice detector modules. From Command/Service Module Systems Handbook p63.

The photo below shows the voice detector module. Voice is transmitted to the spacecraft, frequency modulated onto a 30-kilohertz subcarrier. The voice detector extracts this signal through a 30-kilohertz bandpass filter, demodulates it with an FM discriminator, amplifies it, and sends it to the Audio Center, which provides it to the astronauts. The largest component of the module is the 30-kilohertz bandpass filter at the center top. This module was built by Bulova Electronics, a division of the watch company that produced quartz crystals, oscillators, filters, servo amplifiers, and other components. Two gray transformers are also visible; these coupled the audio signals. The black relay in the lower right was controlled by the "Up Voice Relay" console switch. (Don't be confused by the two completely different definitions of "relay".)

The circuitry of the voice detector module. The connector is on the left.

More circuitry is on the other side of the voice detector. The transformers, relay, and bandpass filter are visible through openings in the module's metal frame. The discrete components are arranged in orderly columns, unlike the other modules.

The other side of the voice detector module.

The data detector module operated similarly to the voice detector, except that it extracted the data link signals from ground. From the data detector module, data was processed by the Up-Data Link box, giving the ground control over multiple spacecraft systems. For instance, commands could be entered into the Apollo Guidance Computer. The spacecraft clock (CTE, Central Timing Equipment) could be set. Various relays could be controlled, overriding some of the switches on the console.

The data detector module. It contains a 70-kilohertz bandpass filter produced by Bulova.

The implementation of the data detector module (above) is similar to the voice detector module, but simpler since it doesn't have the summing and switching circuitry. It uses a 70-kilohertz bandpass filter module, rather than the voice detector's 30-kilohertz filter. In case of a malfunction with the voice detector, backup voice communication could be transmitted to the spacecraft over the 70-kilohertz subcarrier, and extracted by the data detector module. This mode was controlled by the "Up-voice backup" switch.

Bi-phase modulator modules

The role of the bi-phase modulator modules was to modulate telemetry data using bi-phase modulation. In total, data had three layers of modulation. First, data was digitally encoded using pulse-code modulation (PCM). Next, this module applied bi-phase modulation to the bits at 1.024 MHz. Finally, the S-band transceiver used FM or PM (frequency or phase modulation) for the communication to Earth.

Bi-phase modulation encodes a bit using a sine wave for a 1 and an inverted sine wave for a 0 (i.e. a phase shift of 180°). Bi-phase modulation is a type of phase-shift keying. The PCM data was at 51.2 kilobits per second ("high bit rate") or 1.6 kilobits per second ("low bit rate"). Since the data was modulated at 1.024 MHz, a bit was encoded by at least 20 cycles of the waveform. This gave the receiver plenty of time to determine the phase and distinguish a 0 from a 1.

Diagram of the normal bi-phase module. From Command/Service Module Systems Handbook p63.

The premoduation processor contains two modulator modules: the "normal" module and the "auxiliary" module. The normal module transmits real-time data over PM, while the auxiliary module is more flexible. The normal bi-phase modulator module (below) contains a complex tangle of circuitry. The 1.024 MHz bandpass filter is the large metal package at the right, limiting the output signal to a narrow frequency range around 1.024 Mhz.

Inside the bi-phase modulator module.

The auxiliary bi-phase modulator (below) is roughly the same as the normal modulator, but with a bit more circuitry to switch between modes, transmitting either recorded PCM data from tape or real-time PCM data, using the PM or FM transmitters. Curiously, the different modes are selected by switching the power supply between NORM and AUX. In NORM mode, the auxiliary module transmits recorded data over FM. In AUX mode, the auxiliary module transmits real-time data over both PM and FM, providing a backup in case the normal module fails.

Diagram of the auxiliary bi-phase module. From Command/Service Module Systems Handbook p63.

The output signals from the bi-phase modulators are processed either by the FM mixer / LM PCM limiter module or the PM mixer / key / TV module; these are discussed later.

Underside of the auxiliary bi-phase modulator. The bandpass filter is at the left.

Voice clipper module

Voice communication from astronauts to the ground ("downlink voice") went through multiple stages of processing to improve quality. The design standard for the Apollo audio system was 90% word intelligibility for the main links and 70% for the backup links.9 This standard seems surprisingly poor, with one out of 10 words unintelligible, but achieving this standard was challenging due to the extreme distance to the Moon. Moreover, the spacecraft had a lot of ambient noise that interfered with communication. To maximize voice intelligibility over the available radio link, the voice signal was pre-emphasized and clipped. The voice clipper module (below) implemented the pre-emphasis and clipping of the voice signal.

Diagram of the voice clipper module. From Command/Service Module Systems Handbook p63.

The photo below shows the voice clipper module. It has two gray audio transformers at the left. The remainder of the module is filled with circuitry.

The voice clipper module.

The voice signal next goes to the voice relay module, discussed below. (The backup voice signal, however, went directly to the S-band transceiver for transmission to ground.)10

Voice relay module

The voice relay module permitted voice communication from the Lunar Module to be relayed through the Command Module instead of being transmitted directly to Earth from the Lunar Module. If the S-band mode switch was set to "RELAY", the voice and biomedical data from the Lunar Module would be mixed in with the Command Module's voice signal and sent to Earth. This module also optionally applied a low-pass filter to the Command Module's voice signal, under the control of the VHF duplex switch. (I think this is so voice and biomed data can be sent over the same channel without frequency conflict.)

Diagram of the voice relay module. From Command/Service Module Systems Handbook p63.

The photo below shows the voice relay module circuitry. There are three audio transformers, along with circuitry liberally encased in goo.

The voice relay module.

Flipping the module over, the upper right corner is completely covered in plastic. The reason for this is unclear. That corner holds one of the transformers, but I don't see a reason why this one in particular would be covered.

The other side of the voice relay module.

Voice modulator module

Next, the voice signal went to the voice modulator module, which used a complicated circuit to apply frequency modulation. First, the voice signal controls a 113-kilohertz voltage-controlled oscillator (VCO), yielding an FM signal at 113 kilohertz. Next, this signal is mixed with a 512-kilohertz signal from the central timing equipment (CTE), yielding signals at the sum and difference frequencies (399 kHz and 625 kHz). The bandpass filter passes the 625-kilohertz FM signal. The signal frequency is doubled and filtered to produce the final 1.25 MHz FM signal.

Diagram of the voice modulator module. From Command/Service Module Systems Handbook p63.

Three large modules are visible inside: the voltage-controlled oscillator and the two bandpass filters.

Inside the voice modulator module.

The other side of the module has the circuitry, wired to the larger modules. The frequency-doubler may be implemented by a varactor diode, but I haven't located it. From the voice modulator, the voice signal passed to the PM mixer / key / TV module.

Another view of the voice modulator module.

PM mixer/key/TV module

As the name suggests, the PM mixer/key/TV module had multiple functions. In the top part of the diagram below, the mixer combines three data sources: voice with data, emergency keying, and voice. The voice and data combination consists of PCM data at 1.024 megahertz with voice data at 1.25 megahertz; the PCM data is provided by one of the bi-phase modulator modules, while the voice data is provided by the voice modulator module. The next mixer input is the emergency key signal. The purpose of emergency key is that if voice communication failed, an astronaut could send Morse code by using the XMIT key on their communication cable. This key signal might be able to get through to Earth even if voice communication fails or is unintelligible. This module produces the emergency key signal at 512 kHz along with a 400 Hz feedback tone for the astronauts. The final mixer source is voice. The sum of these signals is sent to the S-band PM transponder for phase modulation and transmission. This module also includes a TV isolation amplifier to supply a TV signal to ground support equipment (GSE) before launch.

Diagram of the PM mixer / key / TV module. From Command/Service Module Systems Handbook p63.

The photo below shows this module. On the front right is a component that looks a bit like a power transistor. However, it is an adjustable component (note the screw in the middle), probably a variable resistor.

The PM mixer / key / TV module.

SCO (subcarrier oscillator) and differential amplifier

This module is used for transmitting three channels of analog scientific data. (This is in contrast to most of the data, which was transmitted digitally, using pulse-code modulation (PCM).) Each of the three scientific signals modulates a subcarrier oscillator on a different frequency: 95 kHz, 125 kHz, and 165 kHz. These signals are sent to the FM mixer / LM PCM limiter module, which will be discussed in the next section.

This module also contains relays so the real-time scientific data could be directed to tape for storage. The recorded data could be played back for transmission, amplified by the differential amplifiers. The mode was controlled by the S-band Aux TV/SCI switch. If set to SCI, real-time scientific data was transmitted. If the transmitter was used for TV, the scientific data was recorded to tape for later playback. The tape recorder switch was set to PCM/ANLG to play back the analog data.

Diagram of the SCO differential amplifier module. From Command/Service Module Systems Handbook p63.

Inside, the three large tan oscillator modules are visible. The three relays are the smaller grayish boxes. This module has the D-Sub connector attached with wires and rotated 90°, unlike the other modules that have the connector mounted to the end of the module.

The SCO & differential amplifier module.

On the other side of the module, the circuitry is visible. Note the 6-pin transistors (gold and green circles). These probably contain two carefully-matched transistors for the differential amplifiers. The performance of a differential amplifier strongly depends on its two input transistors; by putting the transistors in the same package, the effects of temperature are minimized.

Another view of the SCO & differential amplifier module.

FM mixer / LM PCM limiter module

The final module is the FM mixer / LM PCM limiter. Like the PM mixer module, this module combines multiple signals for transmission. But this module prepares signals for FM transmission rather than PM transmission. Specifically, the module combines the three analog scientific data inputs, digital PCM data from the Lunar Module, intercom voice from the Lunar module, and PCM data modulated at 1.024 MHz. Various switches on the console control the different modes.

Diagram of the FM mixer / LM PCM limiter module. From Command/Service Module Systems Handbook p63.

The photo below shows the module's circuitry. It has four gray transformers along with the typical transistors, diodes, resistors, and capacitors.

The FM mixer / LM PCM limiter module.

The unusual feature of this module is the encapsulated module in the upper left. This module appears to contain three transistors and five capacitors. It's unclear why these components are encased in plastic. The block diagram for this module doesn't show any special circuitry that would motivate encapsulation. I hope to reverse-engineer this module to figure this out.

The encapsulated block appears to contain three transistors and five capacitors.

Conclusion

Well, I had planned to write a quick description of what we saw inside the premodulation processor but it turned out to be much more complicated than I expected. Congratulations on making it to the end of this blog post.

The premodulation processor illustrates how analog electronics were very bulky before integrated circuits became popular. In the modules, amplifiers and other functional blocks were built from discrete components. The result was a 14.5-pound box to perform a few signal processing tasks. A decade later, many of the circuits could have been replaced with compact ICs.

The premodulation processor also shows how complex everything was in Apollo. You might think that transmitting voice, data, and TV wouldn't be too tricky, just three signals. But everything needed to be redundant. Then there were special cases, such as recording data when you're on the far side of the Moon. Or communicating between astronauts in the Command Module, in the Lunar Module, and walking on the Moon. All these cases required circuitry to switch signals and optimize the radio link for each mode. And the premodulation processor is just one of many boxes in the Apollo communications system! Apollo is like a fractal, where you find successive layers of complexity as you look closer at any system.

We haven't been able to find detailed schematics of the premodulation processor modules, so I plan to reverse-engineer some of the circuitry. I announce my latest blog posts on Twitter, so follow me @kenshirriff for updates. I also have an RSS feed. Thanks to Marcel for providing the premodulation processor and letting Mike, CuriousMarc and me disassemble it.

Front view of the premodulation processor with the case removed.

Notes and references

1. The Apollo Operations Handbook: Telecommunications System gives this description: "The premodulation processor (PMP) equipment provides the interface connection between the airborne data-gathering equipment and the RF electronics. The PMP accomplishes signal modulation and demodulation, signal mixing, and the proper switching of signals so that the correct intelligence corresponding to a given mode of operation is transmitted." ↩

2. The premodulation processor was one of many boxes of electronic circuitry packed into the spacecraft and linked by thick cables. The diagram below highlights where it was mounted in the lower equipment bay of the Apollo Command Module.

   

   The premodulation processor was one of many electronic boxes in the Command Module's lower equipment bay. Diagram from Command/Service Module Systems Handbook p212.

    ↩

3. The block diagram below shows the functions of the premodulation processor, along with the switches that control it.

   

   A block diagram of the premodulation processor. From Apollo Operations Handbook: Telecommunications System.

   The block diagram below provides a more detailed view of the premodulation processor. I split out the sub-module diagrams for the discussion, but the full diagram shows the interconnections between the modules.

   

   Block diagram of the PMP. (Click for a larger version.) From Command/Service Module Systems Handbook p63.

    ↩

4. As shown by the nameplate, the premodulation processor was built by Collins Radio in 1966, two days before Christmas. Collins Radio built much of the communications equipment for the space program from Mercury through Apollo including the Deep Space Network antenna system, microwave links, and ground support equipment (details).4

   

   The nameplate for the premodulation processor shows that it was built by Collins Radio.

    ↩

5. The backplane is a sheet of metal with D-Sub connectors for each module. The round connectors are underneath, wired to the backplane by individual wires.

   

   The premodulation processor's backplane links the modules to the external connectors.

   The four round military-style connectors are shown below. Two connectors have individual pins, while two connectors each have tiny coaxial connections.

   

   The premodulation processor had four connectors for its numerous audio, RF, and control signals.

    ↩

6. We've examined several different Apollo electronics boxes and surprisingly they use completely different manufacturing techniques, even for boxes built by the same manufacturer. Techniques we've seen include printed-circuit boards, surface-mount components, cordwood modules, "dead-bug" components on a ground plane, point-to-point components, and encapsulated hybrid modules. I expected that there would be a standard manufacturing technique (like PCBs are standard now), but everything is different. ↩

7. The story of "Set SCE to Aux" is a well-known Apollo incident where disaster was averted. In brief, Apollo 12 was struck twice by lightning just seconds after launch. Inside the spacecraft, so many warning lights lit up that astronaut Conrad thought "the whole board looks like a Christmas tree". On the ground, consoles started displaying nonsense telemetry. Everyone was mystified until engineer John Aaron recalled seeing similar garbled telemetry during a test. He knew the solution and gave the puzzling command "Try SCE to Auxiliary". This switch was so obscure that astronaut Conrad responded, "What the hell is that?" Fortunately, astronaut Bean flipped the switch, bringing the SCE unit back to operation and restoring telemetry. There were other consequences of the lightning strike, but after the fuel cells were brought back online and the inertial guidance system was realigned, the spacecraft continued uneventfully to the Moon.

   The underlying problem was that the lightning strike caused the spacecraft's fuel cells to go offline. The DC voltage bus was supposed to be at 28 volts, but the loss of the fuel cells caused the voltage to sag to about 18 volts. Within milliseconds, the voltage climbed to 24 volts under battery power, still low. The low voltage caused the primary power supply of the SCE (signal conditioning equipment) to shut down. Since the SCE's role was preparing dozens of analog sensor voltages for telemetry, this caused the telemetry values to Mission Control to be garbled. Flipping the SCE switch to Aux caused the SCE to use its auxiliary power supply, restoring the SCE to operation.

   The published descriptions of this incident are vague on exactly why the auxiliary power supply worked when the primary didn't, so I looked at the SCE diagram (below) to fill in a few details. Power enters at the left and passes through the SCE's famous power switch, which has three positions: NORM, OFF, and AUX. Inside the SCE, there are two power supplies (red) for redundancy, along with some control circuitry at the top. One of the two power supplies is active at a time, unless both power supplies are deactivated for an overvoltage or undervoltage condition.

   

   Diagram of the SCE power supply and the switch. From Command/Service Module Systems Handbook p118.

   The SCE has a flip flop (purple) that selects a power supply by disabling (blue) the unused one. When you switch SCE to AUX, one action is that it toggles the flip flop, switching from supply #1 to #2, or #2 to #1. But I don't think that was important for Apollo 12. AUX mode also blocks the undervoltage signal via an AND gate (green). That is, if the input voltage was still too low, both power supplies would be shut down in NORM mode but either one could function in AUX mode. This, I think, is why "SCE to AUX" powered up the SCE.

   Another interesting feature is the automatic failover (orange). In NORM mode, the SCE will automatically switch power supplies if an internal voltage is bad for 200 ms. However, the failover logic is blocked by the undervoltage detector, so it would not have taken place in Apollo 12. But otherwise, if one of the power supplies failed, the SCE would transparently switch to the other one.

   Curiously, the official NASA report Analysis of Apollo 12 Lightning Incident barely has two sentences on the SCE in its 94 pages. Although the SCE gets all the public attention in this incident, it seems like NASA didn't really care about it since the telemetry wasn't critical to the mission. NASA was much more interested in other effects of the lightning strike: the fuel cell shutdown, the effects on the computer and guidance systems, 9 failed sensors, and potential effects on the pyrotechnics. For more on the Apollo 12 incident, see the transcript, the detailed Scott Manley video, and an Apollo Flight Journal post.

   Note that the SCE's power supply logic is different from other units. Most units (such as the transponder, TWT amplifier, and premodulation processor) have primary and secondary power supplies, with a switch to explicitly select one or the other. However, in the SCE, the Aux switch toggles between power supplies, rather than selecting a specific auxiliary power supply. ↩

8. Astronauts used multiple switches on the control console to control the premodulation processor. These switches were grouped in the lower-right corner of the console with other communications switches. The diagram below shows the relevant switches, highlighted in yellow.

   ↩

   The Command Module console contains switches to control the premodulation processor. These switches are highlighted in yellow. Diagram based on from Command/Service Module Systems Handbook p208.

9. For detailed information on the voice communication system, see Apollo Experience Report - Voice Communications Techniques and Performances. It discusses the performance requirements for the Apollo communications system and how the system was designed to achieve the intelligibility requirements. ↩

10. The idea of backup voice was to provide a voice channel for emergencies that used less power, at the cost of garbling up to 30% of the words. After the explosion, Apollo 13 used the backup voice system so they could turn off the Lunar Module's power amplifier and conserve electrical power. (See Apollo 13 Mission Operations Report pages N-2 and N-7, as well as the transcript.) Backup voice was also used at times during Apollo 16 due to a failure of the Lunar Module's steerable S-band antenna; see Apollo 16 Mission Report page 7-3, which calls this mode "down voice backup". (I should point out that these backup voice incidents involved the Lunar Module, not the Command Module's premodulation processor.) ↩