Reverse-engineering a 1960s cordwood flip flop module with X-ray CT scans

How can you find out what's inside a sealed electronics module from the 1960s? In this blog post, I reverse-engineer an encapsulated flip flop module that was used for ground-testing of equipment from the Apollo space program. These modules are undocumented1, so their internal circuitry is a mystery. Thanks to Lumafield, I obtained a three-dimensional CT scan of the module that clearly shows the wiring and components: transistors, diodes, resistors, and capacitors. From these images, I could determine the circuitry of this flip flop module.

A 3-D scan of the module showing the circuitry inside the compact package. This image uses the blue color map. Click this image (or any other) for a larger version.

The photo below shows the module, a block of plastic 1.5 inches long with 13 pins. I could determine most of its functionality by probing it on a breadboard—conveniently, the pin spacing is compatible with standard solderless breadboards. The module is a flip flop (as the FF label suggests) but some questions remained. Last month, I reverse-engineered a simpler Motorola module (post) using 2-D X-rays. However, this flip flop module was much more complex and I couldn't reverse-engineer it from standard X-rays.

The Motorola LP FF module. It is a 13-pin block.

Fortunately, a company called Lumafield offered to take 3-D X-rays with their Neptune CT scanner. This 6-foot wide unit has a turntable and an X-Y-Z positioning mechanism inside. You put an item on the turntable and the unit automatically takes X-rays from hundreds of different angles. Cloud software then generates a 3-D representation from the X-rays. This industrial system is aimed at product development, product analysis, quality checking, and so forth. It handles metal components, soft goods such as shoes, plastic items, and complex assemblies. I think this is the first time it's been used for 1960s electronics, though.

The Lumafield CT X-ray machine. Photo courtesy of Lumafield.

A simple web-based interface (below) lets you manipulate the representation by rotating and slicing it with your touchpad or mouse. In this screenshot, I'm adjusting the clipping box by sliding the red, green, and blue squares. This yields a cross-section of the module (purple). You can look at the flip flop module yourself at this link; give it a minute to load.

Screenshot of the Lumafield web interface.

Background on the module

To ensure a successful Moon mission, all the systems of Apollo were thoroughly tested on the ground before flight. These Motorola modules were used in test equipment for One box onboard the spacecraft was the Up-Data Link,2 tested by the "Up-Data Link Confidence Test Set" shown below. Unfortunately the test box had no documentation, so I had to reverse-engineer its functionality.

The up-data test box is a heavy rack-mounted box full of circuitry. The wiring on top is for our reverse-engineering, plugged into the box's numerous test points.

The test box was constructed from 25 printed-circuit boards, with the boards connected by a tangled backplane of point-to-point wiring. Each board held up to 15 tan Motorola modules, blocks that look a bit like relays but contain electronic circuitry. The photo below shows one of the boards.

One circuit board from the test box. It has 15 modules including four LP FF modules.

You might wonder why complex electronics would be built from modules instead of integrated circuits. The invention of the integrated circuit in 1958 led to an electronic revolution, but in the mid-1960s integrated circuits were still expensive and rare. An alternative was small hybrid modules that functioned as building blocks: logic gates, flip flops, op-amps, and other circuits. Instead of a silicon chip, these hybrid modules contained discrete transistors, resistors, capacitors, and other components.

The components inside the module

The CT scan (below) provides a high-resolution module of the module, its components, and the wiring. The scan reveals that the module is constructed from two boards, one at the top and one at the bottom, with components mounted vertically, a technique known as cordwood construction. This technique was used in the 1960s when dense packing of components was required, with the cylindrical components stacked together like wooden logs. Unexpectedly, the wiring isn't a printed circuit board (like the previous module that I examined), but spot-welded ribbon wiring. (Note that the wire contacts the side of each pin or lead.) The 13 pins pass vertically through the module, with connections at the top and bottom; the scan shows the shape of each pin in detail.

CT scan of the Motorola LP FF module. In this image, I've used the grayscale color scheme.

The module contains two NPN transistors, mounted upside down with wires attached to the pins. The transistors are in metal cans, which show up clearly in the X-rays. The small square tab sticking out from a transistor indicates the emitter pin. For the transistor on the right, the tiny silicon die is visible between the pins. The die is connected to the pins by bond wires, but the bond wires are too small to be visible in the X-ray.

Two transistors in the module.

Some components aren't as easy to recognize, such as resistors. A carbon composition resistor is constructed from a resistive carbon cylinder, as shown in the cross section. A metal pin sticks into each end of the cylinder, providing the resistor's leads. The carbon doesn't block X-rays, so it is invisible. Thus, a resistor looks like two dangling metal pins in the scan.

X-ray of a carbon composition resistor and a cross-section of a similar (but not identical) resistor. Photo from the book Open Circuits, Copyright Eric Schlaepfer and Windell Oskay; used with permission of the authors.

A carbon film resistor, in contrast, is constructed from a spiral of carbon film on a ceramic rod. The carbon and ceramic don't show up in the scan, but the resistor's end-caps are visible. Thus, the two types of resistors appear different in the images. The module uses both types of resistors; I'm not sure why.

X-ray of a carbon film resistor and a photograph of a similar resistor. The spiral cut in the carbon film controls the resistance. Photo from the book Open Circuits, Copyright Eric Schlaepfer and Windell Oskay; used with permission of the authors.

The module contains many diodes and the internal structure of the diode is visible on the scan. A diode is constructed from a semiconductor die, with a metal S-shaped spring making contact with one side of the die. For some reason, the spring is much more visible in Zener diodes; I assume the spring happens to be made from a more radio-opaque metal.

X-ray slice through a diode, a Zener diode, and a cross-section of a diode. Photo from the book Open Circuits, Copyright Eric Schlaepfer and Windell Oskay; used with permission of the authors.

With careful examination, the diode's die can be seen in the scan as a bright spot at one side of the spring. This reveals the orientations of the diode, which is important for creating a schematic. The two diodes below have opposite orientations: the left one has the die on the top, while the right one has the die on the bottom.

Two diodes in the scan. The first diode has the die at the top, while the second has the die at the bottom.

The module's final components are capacitors, probably silver-mica capacitors. As shown in the cross-section, the capacitor consists of layers of foil and mica. These layers are too thin to show up on X-ray, but the rectangular connections to the leads are visible. Thus, a capacitor looks like rectangles attached to pins.

X-ray of a silver-mica capacitor and a cross-section of a similar capacitor. Photo from the book Open Circuits, Copyright Eric Schlaepfer and Windell Oskay; used with permission of the authors.

The cross-section image below shows a horizontal slice through the module. Since the components are mounted vertically as cordwood, this cuts through the components. The pins at the top and bottom are bright cyan. The blue circles are diodes. The more ghostly circles are resistors. The large hollow circles in the center are the transistors, on top of the capacitors.

A cross-section through the components.

It is easy to extract the wiring from the reconstruction.3 By defining a bounding box in the user interface, I obtained the top wiring layer as a slice, separated from the other circuitry. This view also makes it clear that the wiring is spot-welded to the sides of the pins, and not a printed-circuit board. At the bottom left, you can see where two wires have been welded together.

The top wiring layer in the module.

The wiring on the bottom of the module can be extracted similarly by changing the slice bounds in the user interface. I used a different color map for this image.

The bottom wiring of the board.

By studying the CT scan, I could reverse-engineer the circuitry. The hardest part was examining the diodes closely to determine their orientation. The resulting schematic is shown below (click for a larger version).

Schematic of the flip-flop module.

The core of the flip flop is the two cross-coupled transistors in the center: the output of one transistor is connected (through diodes) to the input (base) of the other. If one transistor is on, it forces the other transistor off. Thus, the flip flop has two stable states with one transistor on and one transistor off. In the remainder of the post, I'll explain the circuit in more detail.

How a J-K flip flop works

A flip flop is a circuit that can be put into two states, outputting a 0 or a 1. A flip flop has many uses, such as storing a bit, providing a delay, implementing a counter, or dividing a frequency by 2. A flip flop is controlled by a clock signal, changing state at the moment when the clock signal switches. (Flip flops often also have asynchronous inputs: Set and Reset inputs that act immediately, regardless of the clock.)

Several different types of flip flops are used for different purposes. A T (toggle) flip flops simply switches from 0 to 1, or 1 to 0, on each clock pulse, dividing the clock frequency by 2. A D (data) flip flop takes a data bit as input, storing it when the clock pulses. The J-K flip flop, however, is a general-purpose flip flop, with its function selected by the J and K control inputs. Its action is defined by the following table.

J	K	Output on clock pulse
0	0	Q (no change)
0	1	0 (clear)
1	0	1 (set)
1	1	Q' (toggle)

Diode-transistor logic NAND gate

The flip flop is constructed from diode-transistor logic NAND gates. The NAND gate has two inputs, isolated from each other by diodes. If both inputs are high, the transistor's base is pulled high by the first resistor. This turns on the transistor, pulling the output low.

With a 1 for both inputs, the transistor turns on, producing a 0 output.

Conversely, if one input (or both) is low, the current passes through the diode and the transistor's base is pulled low. The transistor turns off and the output resistor pulls the output high. Thus, the output is low when both inputs are high, and otherwise high, so the circuit implements a NAND gate.4

With a 0 input, the transistor is turned off, producing a 1 output.

Since this gate uses diodes and a transistor, it is called diode-transistor logic. This logic family was popular in the 1960s, until it was replaced by transistor-transistor logic (TTL). TTL uses a transistor in place of the input diodes, providing better performance.

Cross-coupling two NAND gates produces a simple latch, the Set-Reset latch. When one NAND gate is off, it forces the other gate on. Thus, the circuit has two stable states. Pulling the set' line low forces the output low, while pulling reset' low forces the output high. NAND-gate latches are very common circuits, storing one bit.

Cross-coupling two NAND gates creates a latch.

Understanding the flip flop circuit

The difference between a flip flop and a latch (by a common definition) is that a latch changes state as soon as an input changes, but a flip flop only changes state when triggered by a clock signal. In this section, I'll explain how the clock is implemented in the flip flop module, controlled by the J-K functionality.

The underlying idea is that the clock input is connected through capacitors, so a sharp negative edge on the clock briefly pulls a transistor's base low, turning off the transistor and switching that output high. This makes the flip flop edge-sensitive.

The schematic below shows one-half of the flip flop, omitting the earlier cross-coupled latch circuitry (shown as "feedback"). If the capacitor is charged as shown, then a negative clock pulse (arrow) will pull the capacitor negative, briefly shutting off the transistor and turning on the output Q.5 The latch circuitry will then keep the flip flop in the new state.

When the clock goes low, this can pull the transistor base low, turning the transistor off.

The conditions for the capacitor to charge are that J must be high and Q must be low. Otherwise the capacitor will block the clock pulse.6 In other words, if J is high and Q is low, the output will toggle high on the clock pulse. In the mirror-image circuit (not shown), if K is high and Q' is low, the complemented output will toggle high on the clock pulse. This is the desired behavior for a J-K flip flop.7

The reverse-engineering solves one mystery about the flip flop. When I probed the module on a breadboard, touching a ground wire to the J pin immediately set the flip flop. This is very strange behavior because the J and K inputs are supposed to be controlled by the clock. Moreover, a high (not low) J input should set the output. (And conversely with K.) Looking at the reverse-engineered schematic, though, explains that a sharp pulse on the J pin will act like the clock, sending a pulse through the capacitor, turning off the transistor, and causing a high output. I assume this behavior is not intentional, and J inputs are expected not to transition as sharply as when I touched it with a ground wire.8

Conclusion

I was impressed by the quality of the CT scan. It not only provided a crystal-clear view of the components and wiring, but even showed the internal structure of the components. Being able to see inside a module is like having X-ray vision. (That sounds redundant since it literally is X-rays, but I don't know a better way to describe it.) If you have an application that requires looking inside, I give Lumafield a thumbs-up.

For more background on the Up-data Test Box, I have some Twitter threads: power-up, modules, paper tape reader, and clock circuit. Also see CuriousMarc's video on the box:

I announce my latest blog posts on Twitter, so follow me @kenshirriff for updates. I also have an RSS feed. Many thanks to Lumafield and especially Jon Bruner for performing the CT scan of the module. Thanks to Marcel for providing the Up-Data Link Test Box, which contains the modules, and thanks to John McMaster for earlier X-rays. Cross-section photos copyright Windell Oskay and Eric Schlaepfer, from the upcoming book Open Circuits, which you should check out.

Notes and references

1. Presumably the Motorola modules have documentation somewhere, but we have been unable to find anything. I haven't been able to find even a mention of these modules, let alone details. ↩

2. NASA could send digital messages to the spacecraft from the ground. These data messages could perform specific tasks: control spacecraft equipment by activating relays, send commands directly to the Apollo Guidance Computer, or even set the spacecraft's clock. Onboard the Command Module, these messages were decoded by the Up-Data Link, a drab bluish box (below) mounted in the equipment bay.

   

   The Up-Data Link (UDL) was installed on the Apollo Command Module.

    ↩

3. For the simpler -3.9V module, I extracted the wiring from traditional 2-dimensional X-rays and it was a pain. Cordwood construction has two layers of wiring, at the top and the bottom, so an X-ray from the top merges the two wiring layers together. The side views are even worse, since you can't see the wiring at all. You need to take X-rays of the module at an angle to separate the wiring layers, but there's still overlap, not to mention obstruction from the components. ↩

4. The use of a Zener diode in the gate is a bit unusual. It acts as a level-shifter, raising the input voltage threshold that switches between off and on. (Otherwise the threshold is close to 0 volts, making the inputs too sensitive to noise.) I've found a patent that uses Zener-Coupled Diode Transistor Logic, which is somewhat similar. High Threshold Logic also uses Zener diodes to raise the threshold voltage. ↩

5. You might wonder how the flip flop ends up in the right state during a clock pulse, because there will be a moment when both transistors are turned off and both outputs go high. This seems like a metastable race condition. However, the key is that the feedback path is weaker than the clock pulse. Thus, the transistor on the side without the clock pulse will get turned on by the feedback, while the transistor on the side with the clock pulse remains off. This immediately breaks the symmetry, putting the flip flop into the right state. ↩

6. For the clock pulse to pass through the capacitor, the capacitor must be charged with the input side positive and the base side negative. Then, a negative clock pulse will pull the capacitor negative. However, if both sides of the capacitor are negative, the clock pulse will have no effect. Conversely, if both sides of the capacitor are positive, the clock pulse will pull the capacitor down, but not far enough to turn off the transistor. ↩

7. To understand the J-K action of the flip flop, I've reorganized the standard J-K function table to highlight the state changes.

   J	K	Output if Q is low	Output if Q is high
   0	0	0 (no change)	1 (no change)
   0	1	0 (no change)	0 (clear)
   1	0	1 (set)	1 (no change)
   1	1	1 (set)	0 (clear)

   In other words, if Q is low and J is 1, the flip flop is set. If Q is high and K is 1, the flip flop is cleared. Otherwise, the state remains unchanged. The implementation of the flip flop directly matches this logic. ↩

8. I found that the clock pulse must have a very sharp transition in order to work; my cheap pulse generator wasn't sufficient to act as the clock until I added a buffer transistor. The clock pulse needs to have enough drive current to rapidly discharge the capacitor. If it's too slow, the pulse won't be enough to turn off the transistor. ↩

Reverse-engineering a vintage power supply chip from die photos

I recently did a PC power supply teardown so I figured it would be interesting to go deeper and see what happens inside the power supply's control IC. The die photo below shows the UC3842 chip, which was very popular in older PC power supplies.1 (The chip was introduced in 1984 but this die has a date of 2000.) The tiny silicon die is patterned to create the transistors, resistors and capacitors that make up the circuit. The lighter-colored lines are the metal layer on top of the silicon, forming the chip's wiring. Around the edges, square pads provide the connections from the die to the IC's external pins; tiny bond wires connect the pads to the chip's external pins.

The UC3842 die. Around the outside, the pins are labeled. (Click this image, or any other, for a larger version.)

The photo below shows the chip mounted on the power supply board. For the die photos, I extracted the die from the epoxy package by heating it and then cleaned up the die with a few drops of sulfuric acid. I took photos with a microscope and stitched them together to create a high-resolution image.

The UC3842 chip mounted on the power supply's circuit board. The white glob is silicone, which held many of the power supply components in place.

The chip is from the PC power supply below. This is a switching power supply so it uses several steps to produce the output voltages. On the primary side, the input AC is filtered and then converted to high-voltage DC (roughly 170 to 340 volts) by the bridge rectifier, and the large capacitors smooth it out. Next, the DC is chopped into pulses thousands of times a second by the switching transistor. The control IC constantly adjusts the width of the pulses to regulate the output voltage. These pulses go into the transformer, which converts the high-voltage pulses into low-voltage, high-current. The diodes on the secondary side produce the multiple DC outputs, which are smoothed by the inductors and capacitors.

An ATX power supply with the main components labeled. I removed the heat sinks and capacitors to improve visibility.

This process may seem complex, but it has several advantages over putting the AC from the wall directly into a transformer. First, because the transformer operates at thousands of hertz instead of 60 hertz, a much smaller transformer can be used. Second, chopping the DC into pulses wastes very little energy, compared to a "linear regulator" that converts excess voltage into heat. The result is a power supply that is inexpensive, lightweight, and efficient.

In this blog post, I'll explain the construction of the controller IC, the building blocks of its circuitry, and how it operates. This may be a lot for one blog post, but we'll see how it goes.

Some silicon components

This IC is built from a type of transistor known as bipolar, rather than the MOS transistors that are typically used in modern ICs. The highly-magnified photo below shows an NPN transistor as it appears on the chip, with a cross-section drawing underneath. The metal wiring on top of the transistor is visible as the wide light-colored lines. Different regions of the silicon are doped with impurities to change its electrical properties, yielding N-type and P-type silicon. These regions are faintly visible in the photo. An oxide layer on top of the silicon provides insulation from the metal, except where a contact (black circle or oval) provides a connection between the metal and silicon.

Diagram illustrating the construction of an NPN transistor.

The chip also uses many PNP transistors. Although you might expect a PNP transistor to simply be the reverse of an NPN transistor, it has a different structure, with the regions arranged laterally instead of vertically. The collector and base form concentric square rings around the emitter. The base wire is not connected to the base region directly. Instead, the wire is at a distance, and the base signal travels underneath through the N layer.

Diagram illustrating the construction of a PNP transistor. The dotted lines represent how the collector and base surround the emitter.

Because this chip consists of mostly analog circuitry, it uses a lot of resistors. The photo below shows several typical resistors, the thin grayish-green lines. The resistors are connected to metal wires at either end, the wider metallic-looking traces. Some resistors are straight lines, while others zig-zag to fit a longer resistor (i.e. higher resistance) into the available space.

Resistors on the die.

Resistors are an inconvenient component for integrated circuits. First, they take up a relatively large amount of room, especially long, high-value resistors. Second, they are inaccurate; their value can vary unpredictably from chip to chip, or even on a single chip. For this reason, circuits are typically designed so they depend on the ratio between two resistors, which is much more stable.

Capacitors are also bulky so the chip uses only a few, to stabilize its amplifiers. A capacitor can be formed by using the underlying silicon as one plate, and then putting a layer of polysilicon on top to form the second plate, separated by a thin layer of insulating oxide. Polysilicon is a special type of silicon, and appears green in the photo.

A capacitor on the die.

Architecture of the chip

To summarize the chip, it generates pulses to control the switching transistor that feeds the transformer. These pulses are at a fixed frequency (e.g. 52 kHz), but the width of the pulses increases if more power is needed to keep the output voltage constant. The chip constantly adjusts the pulse width based on voltage and current feedback from the power supply, keeping the output voltages stable even as the load changes.

The UC3842 die. Main functional blocks of the die are labeled.

The die image above has been labeled with the main functional blocks of the chip. It can be compared with the block diagram (below) from the datasheet. I'll describe the main functional blocks before explaining how they are implemented.

Block diagram of the UC3842 chip with annotation. Original from the datasheet.

The power supply's pulses start with the chip's oscillator, which generates pulses at a frequency controlled by an external resistor and capacitor. Below the oscillator is the feedback circuitry that adjusts the pulse width based on voltage and current feedback. The PWM latch (Pulse Width Modulation) combines the oscillator signal and the feedback to generate pulses of the right duration. These pulses go to the high-current output stage, which drives the external switching transistor.

The chip itself is powered by an auxiliary winding on the transformer that provides 15 to 30 volts. The chip regulates this down to an internal 5-volt supply, using a special circuit called a bandgap regulator to keep this voltage stable within 2%, even with changing temperature. (This regulated reference voltage is also provided externally as Vref for external circuitry that needs a stable voltage.)

A potential problem is that if the power supply is unplugged (for example), the chip may behave unpredictably as the input voltage drops. To guard against this, an Under-Voltage Lock Out (UVLO) feature shuts the chip down cleanly if the input drops too low.

A final interesting feature of the chip is how it starts up. As described above, the chip is powered by the transformer, but the chip generates the pulses that feed the transformer. This seems like a chicken-and-egg problem, since the chip won't receive any power until it is already driving the transformer. The solution is a connection to the rectified line voltage through a very large resistor, so the chip receives hundreds of volts but just microamps of current. A Zener diode (below) drops this startup voltage down to 34 volts, enough for the chip to start generating pulses, at which point the transformer takes over.2

The Zener diode on the chip. It limits the startup voltage to 34 volts. It consists of five diodes in series.

The oscillator

The simplified diagram below shows how the oscillator works. In the first phase (A), the external capacitor is charged through the resistor. When the voltage on the capacitor reaches a fixed level, the comparator (triangle) turns on, energizing the discharge transistor. In the next phase (B), the capacitor discharges through an internal resistor, and then the cycle starts again.3 Thus, by choosing particular values for the external resistor and capacitor, the power supply designer can select the oscillator frequency.

This diagram shows how the oscillator is controlled by an external resistor and capacitor.

As mentioned earlier, resistors inside an IC are inaccurate. This poses a problem for the oscillator, since the discharge voltage level is set by resistors. The solution is to tune the resistances by putting fuses in parallel with small resistors and selectively blowing fuses to add the resistors to the circuit.4 Specifically, before the chip is packaged, its performance is measured. To blow a fuse, probes are pressed against the circular contacts and a large current is applied. The additional step of blowing fuses increases the manufacturing cost of the chip, but it provides more precise performance.

Fuses to adjust resistance.

The oscillator has a second set of fuses to tune the discharge resistance (below). These fuses use a different principle: they are "antifuses", which act like fuses in reverse. An antifuse starts off non-conducting, but passing a high current through it creates a conductive metal spike in the antifuse.5

The discharge circuitry of the oscillator. The antifuses adjust resistance in the oscillator.

Current mirrors

The current mirror is a fundamental building block in analog circuits. This chip, like many analog chips, needs small, steady currents to drive amplifiers, bias circuits, pull signals up, and perform other tasks. Rather than using separate resistors to generate each current, a common solution is the current mirror: you control one current with resistors, and then use transistors to make copies of this current. The schematic below shows a simple current mirror where the fixed current through the transistor on the left is mirrored into three identical copies.

A basic current mirror circuit. The current on the left is mirrored into three current sinks.

The diagram above shows the main current mirrors for the chip. The large resistor in the lower-right controls the current through the main transistor, and the other transistors copy this current.6 Small emitter resistors improve the performance.

The current-mirror circuitry on the die.

The feedback or error amplifier

Next, I'll look at the voltage feedback circuit, which lets the chip know if the output voltage is too high or too low. The chip receives the output voltage, scaled to form a feedback signal. The error amplifier compares the feedback to a reference voltage to determine if the voltage is too high or too low.

The error amplifier is based on a differential amplifier, which amplifies the difference between its two inputs. This circuit is common in analog circuits, forming the heart of an op-amp or a comparator. The basic idea is that a current mirror (the circle at the top) generates a fixed current I. This current gets split between the left path (I1) and the right path (I2). If the transistor on the left has a higher input voltage than the transistor on the right, most of the current will go to the left. But if the transistor on the right has a higher input, most of the current will go to the right. This circuit amplifies the voltage difference: even a small difference between the two inputs will switch most of the current from one side to the other.

A differential pair amplifies the difference between the two inputs.

The error amplifier extends this circuit with about a dozen transistors in total. These transistors add buffering to the inputs, control various currents, and provide a second amplification stage. The photo below shows the key components of the error amplifier. The green capacitor on the right stabilizes the amplifier.

The error feedback amplifier as it appears on the die with key components indicated.

The current comparator

The power supply uses voltage feedback to adjust the pulse width, but it also monitors the current through the transformer so the power supply can respond faster to changes in the load. The current feedback is implemented by the "current sense comparator". This is similar to the feedback amplifier, amplifying the difference between the inputs. (Since it is a comparator, not an amplifier, it is designed to output a binary signal instead of an analog level, but the basic principle is the same.) The diagram below shows the key circuitry for the current comparator on the die and how it relates to the block diagram. The output from the error amplifier goes through some circuitry to adjust the voltage levels before entering the comparator.7

How the current sense circuit maps onto the die components.

Under-voltage lockout

Another interesting circuit is the under-voltage lockout (UVLO), in the upper-left of the die. The purpose of this circuit is to shut down the chip cleanly if the input voltage falls too low. (This could happen if there is a power failure or even from unplugging the power supply.)

The heart of the UVLO circuit is a bandgap regulator, which provides a voltage reference that will be stable even if the temperature changes. This is surprisingly difficult in an integrated circuit, since the properties of transistors change with temperature. The bandgap regulator uses two transistors of different sizes so they are affected by temperature differently. In the die photo below, Q2 is six times the size of Q1.

The bandgap circuit for the under-voltage lockout.

The schematic below shows how the bandgap regulator is constructed. The key factor is the voltage between a transistor's base and its emitter (V_be), which decreases with temperature. However, ΔV_be, the difference between the two V_be increases with temperature. With the right resistors, these two factors cancel out, yielding a stable reference voltage. The circuit compares the input voltage to this reference voltage; see the footnote8 for more details.

Schematic of the bandgap regulator. A current mirror directs the same current through both sides of the circuit.

In the UVLO circuit, the bandgap reference is used to detect if the chip's input voltage falls too low. Since the input voltage is around 30 volts, a network of resistors (below) scales it to the bandgap voltage (about 1.2 volts) for comparison.9

This set of resistors forms voltage dividers to reduce the input voltage for the bandgap comparator. Note the mask date of "00" as well as the ST Microelectronics logo at the bottom.

The bandgap voltage reference

The chip uses a second bandgap reference to create an internally-regulated 5 volt supply to power the chip's circuitry. This voltage is also made available to external circuitry that may need an accurate voltage.

At a high level, this voltage reference is a linear power supply, with a power transistor controlling how much of the input voltage passes through to the regulated Vref. The control signal comes from the bandgap regulator, which I'll explain below. The output circuit also has a current-sense resistor to measure the output current. This limits the output current to 50 mA in case of a short circuit. A diode clamps the output if the input voltage suddenly drops.

Schematic of the Vref output circuit. The transistor limits the voltage.

The photo below shows how this circuit is implemented on the die. The power transistor is much larger than the other transistors, so it can support a high-current output. The construction of the diode is similar to the power transistor, but without a collector. The current-sense resistor is short and wide, giving it a low resistance.

Vref output circuit on the die.

The heart of the circuit is the bandgap voltage reference below. The circuit is similar to the bandgap voltage reference for the under-voltage lockout circuit, using two transistors, one with six times the area of the other. However, the six-way transistor has been split into two and surrounds the single transistor. With this layout, even if there is a temperature gradient across the die, the single-transistor and the six-transistor will be at the same average temperature.

The transistors at the heart of the bandgap reference.

The accuracy of the bandgap regulator depends on the accuracy of its resistors. During manufacturing, fuses are blown to tune the resistance, as with the oscillator's resistors. The photo below also shows the resistors that form a voltage divider to reduce the 5-volt output to the 1.2-volt bandgap voltage. In contrast to the thin meandering resistors used elsewhere, these resistors are thick and uniform length to improve their accuracy.

Resistors that control the bandgap reference.

Output

At this point, I'll step back and review the chip's function in the power supply. It controls the switching transistor, causing the transistor to send high-voltage pulses through the transformer. The chip does this by producing control pulses on its output pin. Since the switching transistor is fairly large, the chip outputs a relatively high current (200 milliamps) control signal. This requires fairly large output transistors inside the IC.

The controller chip directs the switching transistor to send pulses through the transformer.

The die photo below shows the IC's two output transistors: the upper one pulls the output high, and the lower one pulls the output to ground. One interesting feature of the chip is that it has two pads on the die for Vin and two pads for ground. The purpose of this is that the output transistors draw a lot of current, which could cause noise fluctuations on the power and ground lines, interfering with the rest of the chip. By providing separate pads, the output transistor is somewhat isolated from the rest of the circuitry.10

Two large transistors drive the output pin.

Variants

One interesting thing about this chip is that four different chips are manufactured from the same silicon. The UC3842 has a 16-volt UVLO threshold, while the UC3843 has an 8.5-volt threshold for lower-voltage applications. Other variants of the chip (UC3844 and UC3845) have a flip flop to reduce the pulse duty cycle. These different chips use slightly different metal wiring over the same silicon base. (It's easier to customize the metal layer than the silicon.) The photo below shows some places where the metal wiring has been severed in the UC3842 to change the wiring.

Closeup of the die with some broken connections indicated with arrows.

Conclusion

Power supplies are usually taken for granted, but they contain a lot of interesting technology. The invention of the power supply control chip in 1975 is a key step in the history of power supply improvements. Modern power supply chips are much more complex, with features to improve efficiency and reduce interference, but the chip that I examined uses the same basic principles.11 Analog chips are built from several important building blocks such as differential amplifiers, current sources, current mirrors, and bandgap voltage references. The UC3842 chip illustrates all of these building blocks, and how they are combined to build complex circuits.

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Notes and references

1. For schematics of power supplies using this UC3842 chip, see this site, near the bottom of the page. ↩

2. The idea of a Zener diode is that it blocks current like a normal diode until it reaches the "breakdown voltage", where it starts conducting. Zener diodes are often formed on chips from the emitter-base junction of NPN transistors, which commonly results in a 6.8-volt breakdown voltage. Looking at the photo, you can see 5 transistor-like structures in series. At 6.8 volts each, this generates the 34-volt breakdown voltage shown in the block diagram. ↩

3. The oscillator's comparator is set to turn off about 1.6 volts below the level at which it turns on, that is it has hysteresis. This ensures that the capacitor discharges significantly rather than settling around the discharge level. The oscillator design is a bit like the 555 timer, with discharge and charge phases triggered by the capacitor voltage. ↩

4. Many of the resistors in the fuse network are made of fixed-length resistors in various combinations. For example, two in parallel gives twice the resistance, while two in series give half the resistance. The advantage of combining fixed-length resistors is that the resistances are more predictable than making resistors of different lengths. The different resistors have roughly binary values, so different combinations of blown fuses select a variety of resistances. ↩

5. I think that the chip uses Zener antifuses, since they look similar to NPN transistors without a collector. The process of blowing the antifuse to make it conductive is called a "Zener zap." ↩

6. The current mirror uses a buffered-feedback design with emitter degeneration resistors (details). The small emitter resistors improve the output impedance. Three of the transistors in the current mirror are set up to split current, so each sinks one-third of the regular current. Another transistor has a larger emitter resistor, reducing the current; a small change in resistance yields a large change in the current. This illustrates the flexibility of a current mirror to produce different currents. ↩

7. The block diagram shows a resistor-diode network between the error amplifier and the current sense comparator. This network scales and clips the error amplifier output to make its levels more useful. The circuitry isn't particularly interesting, so I won't discuss it in detail. I'll mention, though, that the block diagram shows the error amplifier output uses two diodes to drop its voltage. The circuit, on the other hand, raises the other signals by two diode levels instead, which works out the same mathematically. (Transistors are used to implement the diode drops as well as the 1-volt Zener.) ↩

8. The details of a bandgap reference are too complex to explain here, but I'll give a brief overview in this footnote. The basis is that the voltage between a transistor's base and emitter scales drops linearly with the temperature (in Kelvin). But since the two transistors have different areas, the two transistors have different scale factors. The difference between the two transistors' base-emitter voltages increases linearly with temperature. By combining a voltage that decreases linearly with temperature and a voltage that increases linearly with temperature, you can create a voltage that remains almost constant with temperature. This voltage turns out to be the bandgap voltage of silicon, about 1.2 volts.

   Scaling and combining these voltages is done by two resistors, so it is important that temperature doesn't affect the resistances. The circuit is designed so that only the ratio between resistances matters, so if temperature affects both resistors equally, the circuit is unaffected. A problem is that a temperature gradient on the chip could affect some resistors more than others, but the chip uses a clever layout technique to avoid this. There are seven resistor segments: one forms a resistor and six are in series to form a resistor with six times the resistance. The one-unit resistor is put in the middle with three segments above and three segments below. If a temperature gradient, for instance, increases the upper resistances, the resistor in the middle will have an "average" increase, while the 6-unit resistor will have three resistor segments with a large increase and three with a small increase, which will cancel out.

   The bandgap circuit doesn't explicitly generate a 1.2-volt output. Instead, it implicitly compares the input voltage with 1.2 volts. The circuit is set up so a 1.2-volt input balances the currents through both transistors. If the voltage increases, the single transistor passes more current than the six-unit transistor. A current mirror forces each side of the circuit to have the same current, with the result that the "extra" current flows through the output. Thus, if the input voltage is high enough, the circuit produces an output current, activating the chip. But if the input voltage is too low, the circuit doesn't produce an output current, shutting down the chip.

   For more information, see the optimistically-titled How to make a bandgap voltage reference in one easy lesson. ↩

9. Another feature of the under-voltage lockout circuit is hysteresis; it has a higher voltage to turn on than to shut off. The purpose of this is to make sure the power supply doesn't oscillate on and off if the input voltage is near the threshold. Hysteresis is implemented through the input voltage divider, which uses three resistors. If the chip is activated, a transistor feeds the supply voltage into the second resistor, increasing the divider's output voltage. The result is that once the chip is active, the supply voltage must drop more to turn the chip off. ↩

10. Surprisingly, the chip has two pads for power and two pads for ground, but only single power and ground pins. Instead, two bond wires go from the pads to each external power and ground pin. Although this doesn't provide complete separation between the chip's power and the output circuit's power, it is still beneficial since the bond wires are thicker than the metal traces and have lower resistance.

    Although this IC is usually packaged in an 8-pin package, some manufacturers, such as Fairchild, make versions of the UC3842 in 14-pin packages. The extra pins allow separate pins to be used for the circuitry and output power and grounds. ↩

11. While the UC3842 chip was introduced in 1984, the one I examined has a mask date of "00", so this design is from 2000. The power supply itself was from 2005. ↩

Inside a 20-Watt Traveling Wave Tube Amplifier from Apollo

How did the Apollo astronauts communicate on their trip to the Moon, 240,000 miles back to Earth? They used a 32-pound amplifier, built around a special kind of vacuum tube called a traveling-wave tube. In this blog post, I look inside this amplifier and explain how the traveling-wave tube works.

The Collins Radio traveling-wave tube amplifier. The label says "Not for flight" so this amplifier was only used on the ground. Click this photo (or any other) for a larger version.

Surprisingly, this amplifier only produced 20 watts of power, not much more than a handheld walkie-talkie.1 You might wonder how a 20-watt signal could be received all the way from the Moon. To pick up the weak signal, NASA built a network of 26-meter (85-foot) dish antennas that spanned the globe, with ground stations in Spain, Australia, and California. For the signal to the spacecraft, the ground stations broadcast a strong, focused 10,000-watt signal that could be picked up by the spacecraft's small antennas. Additional ground stations with smaller 9-meter (30 foot) antennas filled in coverage gaps, along with tracking ships and airplanes.2

NASA's 26-meter antenna at Honeysuckle Creek, Australia. Photo from NASA.

The communication system on Apollo was very complex, as shown in the diagram below. The amplifier, highlighted in yellow, was just one component of this system (which I'm not going to try to explain here). Most communication went over the "Unified S-Band", which sent voice, data, telemetry, TV, control, and ranging through one unified system. In comparison, the Gemini missions used separate systems for different purposes. (S-band refers to the microwave frequency band used by this system.)

Diagram of the Apollo Block II Telecommunications System. (Click for a larger version.) From "Apollo Logistics Training", courtesy of Spaceaholic.

Inside the amplifier

The amplifier was built by Collins Radio, a company that had a large role in the space program.3 (Collins claims that from Mercury and Gemini to Apollo, every American voice transmitted from space was via Collins Radio equipment.) The photo below shows the amplifier with the cover removed, showing the circuitry inside. Note the tangles of coaxial cables for the high-frequency RF signals. The "Danger High Voltage" warning is due to the thousands of volts required by the traveling-wave tubes.

Inside the amplifier, many coaxial cables connect the RF circuitry.

The block diagram below shows the structure of the amplifier,4 centered on the two traveling-wave tubes that perform the amplification. The amplifier takes two inputs: voice/data and the TV signal. In normal use, one tube amplifies the voice/data signal and the other amplifies the TV signal. An important feature is that either signal can be sent to either tube, or the amplifier can be bypassed entirely. This allows communication if a tube fails, or even if the amplifier entirely fails. The signals are directed by RF relays, electrically-controlled switches. The triplexer sends the two amplified signals to the antenna, and directs the signal from the antenna to the receiver.)

Simplified block diagram of the amplifier. From CSM Functional Integrated System Schematics.

The photo below shows the amplifier with the case removed. (We were unable to disassemble the amplifier completely so this photo is from the documentation.5) The traveling-wave tube is the black cylinder at bottom right, about 10 inches long. The second tube is in the same position on the back of the amplifier.

Photo of the traveling-wave tube amplifier used in Apollo. Photo from Collins S-Band Power Amplifier.

How a traveling-wave tube works

The traveling-wave tube (TWT) is the heart of the amplifier. TWT systems have been popular for satellites because they are compact and provide high amplification with very wide bandwidth.7 They are still widely used in satellites, radar, and other systems.

A traveling-wave tube uses an interesting technique to amplify the input RF signal, different from typical vacuum tubes. It creates a beam of electrons and transfers energy from this beam to the RF signal. In more detail, an electron gun shoots electrons down the tube, (a bit like a CRT). As these electrons travel through the tube, they interact with the RF signal and bunch together, transferring energy to the RF signal.6

The problem is that the electron beam and the RF signal need to travel at approximately the same speed in order to interact, but the electron beam travels at about 10% the speed of light,8 while the RF signal travels at the speed of light. The trick is to put the RF signal through a helix, wrapped around the beam. Because the RF signal travels through the long helix rather than in a straight line, its path through the tube is slowed. With the proper helix design, the RF signal and the electron beam travel at approximately the same net speed down the tube, allowing them to interact.

Diagram of the TWT amplifier. From "Apollo Logistics Training", courtesy of Spaceaholic.

The diagram above shows the components of the traveling wave tube in detail. The heart of the TWT is the drift tube that holds the electron beam, wrapped in the helix for the RF signal. At the left, the electron beam is created by the components of the electron gun (heater, cathode, and electrodes). The RF input and output provide the connections to the helix for the signal that is being amplified. The collector absorbs the weakened electron beam after it has passed through the tube. Finally, the permanent magnets keep the electron beam focused through the tube.

It's hard to see the traveling-wave tube inside the amplifier, since it is mounted at the bottom under a bunch of coaxial cables; the photo below is the best I could do. The tube looks like a black cylinder, but you can see the coaxial cables attached at the left and right.

The traveling-wave tube inside the amplifier.

Other parts of the amplifier

Next, I'll briefly describe the other circuitry inside the amplifier. A traveling wave tube requires high voltage to accelerate the electron beam. The photo below shows two of the power supply transformers. The amplifier was powered with 115 volts AC, 3-phase at 400 cycles per second. It also used 28 volts DC for the control circuitry. Note the circuitry encased in plastic at the bottom of the photo.

The amplifier uses high-voltage transformers to power the traveling-wave tubes.

As described earlier, the RF relays switch signals between the two tubes to provide redundancy. The relays (below) are the fairly large square units with coaxial cables attached. These relays are more complex than typical relays because they must transfer gigahertz RF signals. The internal wiring is constructed from metal strips between double ground planes along with waveguides.

The relays with coaxial cables attached.

Another interesting component of the amplifier is the triplexer, a special RF component that connects the antenna to the amplifier. The idea of the triplexer is that it has three ports, each for a different frequency, and keeps the signals on each port separate from each other. Specifically, it combines the main 2287.5 megahertz signal with the TV's 2272.5 megahertz signal and sends these to the antenna. The signal from the ground is at 2106.4 megahertz; the triplexer directs this signal from the antenna to the receiver. Internally, the triplexer has band-pass filters for each frequency, providing a large amount of isolation (60 dB) between its three ports.

The triplexer.

The triplexer is the metal box in the photo above. Note the coax connections with the antenna connection labeled. Although the triplexer says "Danger High Voltage" on top, this refers to the surrounding power supply circuitry, not the triplexer itself.

Controlling the amplifier

The astronauts had control switches in the Command Module to turn the power amplifier on and off, and switch between the primary and secondary tubes. The diagram below shows the location of these switches, marked PWR AMPL. The PRIM/SEC selects which tube was used for the main signal and which was used for the TV signal. The HIGH/OFF/LOW switch selected the power output level for the amplifier. When the amplifier was off, the input signal was connected directly to the antenna, bypassing the amplifier.

Astronauts controlled the amplifier through switches on the console. Diagram from Command/Service Module Systems Handbook p208.

Conclusion

This power amplifier illustrates the complexity of the communication systems for Apollo.9 Even though the amplifier is complex internally with redundant traveling-wave tubes, it is just one of many pieces of hardware. The diagram below shows the Command Module's equipment bay, with the amplifier highlighted in yellow. (The Apollo Guidance Computer was directly above the amplifier, two rows up.)

Diagram of the Apollo Command Module's equipment bay with the S-band power amplifier highlighted. From Command/Service Module Systems Handbook p212.

We are currently investigating the possibility of powering up this amplifier to see if it still operates. I announce my latest blog posts on Twitter, so follow me @kenshirriff for updates. I also have an RSS feed. Thanks to Steve Jurvetson for loaning me this amplifier. Thanks to Spaceaholic and Mike Stewart for providing diagrams and the Collins Aerospace Museum for additional information.

Notes and references

1. Walkie-talkies typically use 0.5 to 5 watts of power, with some models providing 8 watts, mostly limited by FCC regulations. There are a few 20-watt or even 25-watt handheld radios. ↩

2. This photo shows the Vanguard tracking ship. This ship was a surplus tanker from World War II that was repurposed as a missile tracking ship by covering it with antennas. NASA used the ship for communication with Apollo, and the ship was scrapped in 2013.

   

   The Vanguard ship, from Wikimedia.

    ↩

3. The document Collins S-Band Power Amplifier has technical specifications for the amplifier. The presentation Collins Role in Space Communications describes the Collins equipment used in Mercury, Gemini, and Apollo. Collins built equipment for the spacecraft and transmitting and receiving equipment on the ground. ↩

4. Here's a more detailed diagram of the power amplifier circuitry. This diagram shows the power supply and control circuitry in more detail. In particular, the circuitry lets the tubes heat up for 90 seconds before use. Circuitry also shuts down the power if there is a fault or loss of a power phase.

   

   The power amplifier diagram. From "Apollo Logistics Training", courtesy of Spaceaholic.

    ↩

5. The Collins photo makes it look like you can simply remove the case from the amplifier. However, the photo is misleading since the amplifier doesn't come apart like that. We attempted to remove the amplifier from the case, but it is fastened with many inaccessible screws and some components are glued down. We suspect that the amplifier was assembled inside the case, making it very difficult to perform any pre-launch maintenance. We gave up on disassembling the amplifier completely, which is why all our photos show the view from the top. ↩

6. The interaction between the electron beam and the RF signal in the helix is complex, but the net result is that energy is transferred from the beam to the signal. Specifically, the electric field from the RF signal produces positive and negative waves. These accelerate and decelerate the electrons, causing them to bunch together. (On the whole, the electron beam decelerates more than it accelerates, so it loses energy.) The moving bunches of electrons induce more current in the helix, strengthening the RF signal. The result is a feedback loop, causing the RF signal to grow exponentially as it travels through the tube.

   For more information on how traveling-wave tubes work, see Traveling Wave Tube, Recent theory of traveling-wave tubes, or this long presentation. ↩

7. A traveling-wave tube can amplify a large range of frequencies (i.e. it has a high bandwidth) because it doesn't have any resonant elements (unlike a klystron, for instance). Thus, it doesn't need to be tuned to a particular frequency. ↩

8. Ignoring relativistic effects, the speed of an electron beam accelerated by a voltage is given by

   

   Equation for electron beam speed.

   where v₀ is the velocity, V_b is the voltage, e is the charge of an electron, and m_e is the mass of an electron. For example, applying 6000 volts yields an electron speed of 46,000 km/second, about 15% the speed of light.

   This equation is a rearrangement of the kinetic energy from the velocity and the energy from the voltage potential difference.

   

   Kinetic energy equation.

   In the traveling-wave tube, the electron beam must be slightly faster than the (net) RF signal speed so the beam will transfer energy to the RF signal as the beam is slowed.

    ↩

9. For more information on Apollo communication, see Apollo Experience Report - S-Band System Signal Design and Analysis. See also CSM Functional Integrated System Schematics and Command/Service Module Systems Handbook. ↩