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. ↩

"Space age electronics": Inside a GE thin-film paperweight from the 1960s

In the early 1960s, General Electric developed a technology called thin-film electronics.1 These circuits were built from thin films of material, much more compact than individual components. For weight-sensitive applications such as satellites and military equipment, thin-film electronics could potentially be revolutionary.

The GE paperweight consists of circuitry and a satellite model encased in thick clear plastic. It is labeled "Light Military Electronics Department, Defense Electronics Division, General Electric. Space Age Electronics, thin film circuits."

GE's Light Military Electronics department1 built the paperweight above to showcase their "Space Age Electronics". In the center is a thin-film circuit, next to a model of an early satellite. However, the paperweight contained a surprise: when picked up, the paperweight emitted a beep-beep-beep noise, sounding just like a satellite.2 In this blog post, I reverse-engineer the "Space Age Electronics" inside this paperweight and explain how it works. In brief, the visible thin-film circuit implements a flip flop. The remaining circuitry is hidden in the compartment on the left: two oscillators that produce the beeps. These oscillators are implemented in another unusual 1960s technique called "cordwood'.

The thin-film module

The most visible part of the paperweight is the thin-film module. The idea behind thin film is to build resistors and capacitors as thin layers on a substrate, rather than using individual components. Resistors are formed from thin strips of resistive material, the vertical reddish-brown lines on the module's surface. For higher resistance, these lines zig-zag back and forth.3 Capacitors are formed from two thin layers of metal (the plates), separated by an insulating dielectric material.

This angle view shows how the semiconductor components are mounted above the thin film circuitry.

Thin-film transistors were not commercially practical in the 1960s, so the module has tiny discrete transistors and diodes mounted on top, connected by golden wires. (This must have been expensive to manufacture.) In the photo above, the shadows show that the semiconductor components (black blobs) are slightly above the surface. You can distinguish the diodes by their green dots. At the left, five metallic strips provide power and signal connections to the module, with golden contacts connecting these strips to the thin-film circuitry.

A closeup of the thin-film module.

Interest in thin-film technology declined in the mid-1960s as integrated circuits became commercially available. Integrated circuits were cheaper, could fit more components into a chip, and could be mass-produced. For these reasons, integrated circuits took over the electronics market. Thin-film circuits are still used, but only for specialized applications.

I traced out the paperweight's thin-film circuit and found that it implements a toggle flip flop, a standard electronic circuit. The flip flop stores either a 1 state or a 0 state, like a single bit of memory. When it gets a negative pulse on the trigger input, it flips to the opposite state. Thus, as it receives input pulses, it goes "on", "off", "on", "off", etc. In the paperweight, the flip flop creates the separate beeps. The paperweight generates a beep while the flip flop is on, and is silent when the flip flop is off.

Schematic of the circuit in the thin-film module.

You can match up the components in the schematic with the components in the photo: two transistors, two diodes, four capacitors, and multiple resistors. Note that the two sides of the circuit are symmetrical, both in the schematic and in the photo. One side of the circuit is on and one side is off. Depending on which side is on, the circuit holds a 0 or a 1. See the footnote4 for more details.

Inside the paperweight

The left side of the paperweight has a compartment with some interesting circuitry inside. The paperweight was powered by a 22½ V battery, which was relatively common back then but is now obsolete. It looks a bit like a 9-volt battery, except it has one contact at each end. Next to the battery is a vintage earphone, the round pink component. It acts as the speaker in this device.

Looking inside the paperweight's compartment reveals more circuitry.

Another unusual component is the tilt switch in the lower right, which turns the paperweight on and off. (I don't know if this tilt switch contains mercury or has a rolling ball inside.) When the paperweight is horizontal, the tilt switch is open. But if the paperweight is picked up, the tilt switch closes. This probably added to the "drama" of the paperweight, since someone will think it is just a decoration until they pick it up and it starts beeping.

The tilt switch turns the paperweight on and off.

In the upper right of the compartment, a block of plastic encases the oscillator circuitry. The module is built with "cordwood" construction, a way of building high-density circuits that was popular in the 1960s. Instead of mounting components flat on a circuit board, cordwood puts components between two boards. (They are stacked together like logs, giving cordwood its name.) The photo below shows the components; it isn't as clear as I'd like because the components are embedded in yellowing plastic.

This view of the module shows three resistors (striped) and two capacitors (silver).

On each side of the module, components are wired with point-to-point wiring, as shown below. This photo also shows how the insulated connection wires are also embedded in the module. The large dark circles are the two transistors.

Closeup of the cordwood module, showing the wiring. The transistors and the ends of the resistors and capacitors are visible.

The oscillators use unijunction transistors, a somewhat unusual type of transistor, different from standard NPN and PNP transistors. Oscillators could be easily created from unijunction transistors due to their nonlinear characteristics. The unijunction transistor was invented by General Electric in 1953, so it's not surprising that General Electric made use of them in this paperweight. The GE logo is visible on top of the transistors.

In this view of the module, the script "GE" logo is visible on top of the transistors. These transistors are part number 2N491

The cordwood block holds two oscillators, to control the duration of each beep and to generate the beep sound itself. The first oscillator generates five pulses per second. These pulses go to the thin-film flip-flop circuit, which will change its state between off and on with each pulse. That is, the flip flop is off for 200 milliseconds, on for 200 milliseconds, and so forth. The output from the flip flop powers the second transistor oscillator, which generates a 3.5-kilohertz tone. The result is the repeating beep-beep-beep output from the paperweight.

Schematic of the unijunction transistor oscillators.

The schematic above shows the two oscillators. The idea behind a unijunction transistor oscillator is that the capacitor slowly charges through the resistor. As the capacitor charges, the voltage on the emitter (symbolized by the arrow) increases. When it reaches the trigger voltage, the transistor turns on and the capacitor discharges to ground. The cycle repeats, generating a sequence of pulses on the output.

Conclusion

I think the paperweight is from approximately 1962, based on GE's thin-film research at the time and the appearance of the paperweight's model satellite.6 The paperweight was produced in the midst of the space race; John Glenn became the first American in orbit in 1962. Satellites were still a new "space-age" thing at the time, so the paperweight was a symbol of General Electric's advanced technology.5 The beeps from the paperweight are similar to those produced by Sputnik (1957). At the time, the paperweight must have been an impressive object, a vision of the future.

Thanks to Peter B. Newman, technology collector and educator for sending me the paperweight for analysis. Thanks to Mikes Electric Stuff for identifying the tilt switch for me.

I announce my latest blog posts on Twitter, so follow me @kenshirriff. I also have an RSS feed.

Notes and references

1. The paperweight was built by GE's Light Military Electronics department. In the early 1960s, this department produced aerospace electronics such as digital guidance computers, flight-control systems, and satellite sensors. These were used in weapons including the F-105 fighter-bomber, Sidewinder missile, and Polaris and Atlas ICBMs. ↩↩

2. Sputnik's beeps were approximately 150-300 ms long at 1.5 kilohertz. (The frequency isn't well-defined because the transmission was just a carrier switched on and off, but this is the frequency in typical recordings.) The paperweight's beeps were approximately 200 ms long at 3.47 kilohertz. The point is that the paperweight's beeps were designed to resemble the beeps from a satellite such as Sputnik, and people would have recognized this at the time. You can hear the beeps of the paperweight here; I had to edit the audio a bit because I discovered too late that the doorbell rang in the middle of the recording. ↩

3. In the module, some of the resistors are connected to the metal layer through structures that have teeth kind of like a comb. I'm not sure what the purpose of these structures is. My hypothesis is that by changing the number of "teeth", the active length of the resistor can be changed, adjusting the resistor. (Modifying the metal layer is easier than modifying the thin-film layers.) ↩

4. The two transistors are cross-connected, so when one transistor is on, it forces the other one off. The trigger capacitors are pre-charged through the corresponding output. The result is that the transistor that is currently on (output low) will be pulled lower than the transistor that is currently off (output high). This turns off the first transistor, flipping the state of the circuit. It's a fairly standard flip-flop circuit; more details are here. ↩

5. In 1960, GE hoped to build a commercial communications satellite network, and formed a subsidiary "Communication Satellites Inc" in 1960. However, GE abandoned that goal in 1961 (probably due to antitrust issues) to focus on manufacturing equipment for space vehicles. ↩

6. The satellite in the paperweight resembles the Ariel 1 (1962) and Ariel 2 (1964) satellites, with its paddle-like solar cells. It's not an exact match, so I don't know if the satellite is an artist's conception, or is a different satellite. If you recognize the satellite, please let me know. ↩

Reverse-engineering a vintage comparator chip

I recently saw an interesting die photo of an unknown chip on Twitter, so I did some analysis of it. Looking at the circuitry inside, the chip appears to be four comparators, probably in the ECL (Emitter Coupled Logic) family. This is a quick blog post to summarize my investigation.

The die photo below shows the chip under the microscope. Regions of the silicon appear pink, blue, or yellow, depending on how the silicon was doped. The speckled regions are the metal layer on top of the silicon, wiring the circuitry together. Around the edges, the black bond wires connect the chip to the external pins. These wires are attached to the square bond pads. The die has four blocks of circuitry, one for each of its four comparators. Much of the die is unused, especially the large metal area in the middle. Because this chip's circuitry is relatively simple, it only uses a fraction of the available space.

The chip with pins labeled. Click this photo (or any other) for a larger version. Photo courtesy of EvilMonkeyDesignz.

The photo below shows the chip with its metal lid removed. Modern chips are usually in a black epoxy package, but this chip has a white ceramic package. The tiny silicon die is visible in the middle, with bond wires connecting the die to the lead frame, the metal connections to the chip's gold-plated pins. The metal layer on top of the die is visible, and can be matched with the die photo above. The semi-circular notch on the left indicates the orientation of the chip; the "P" is pin 1.

The chip with the metal lid removed, showing the tiny silicon die inside. Photo courtesy of EvilMonkeyDesignz.

The chip's components

Transistors are the key components in a chip. This chip uses a type of transistor called the NPN transistor. The photo below shows a transistor as it appears on the chip. The orange and blue colors are regions of silicon that have been doped differently, forming N and P regions. The speckled areas are the metal layer of the chip on top of the silicon—these form the wires connecting to the transistor's collector, emitter, and base.

Underneath the photo is a cross-section drawing showing approximately how the transistor is constructed. There's a lot more than just the N-P-N sandwich you see in books, but if you look carefully at the vertical cross-section below the 'E', you can find the N-P-N that forms the transistor. The emitter (E) wire is connected to N+ silicon. Below that is a P layer connected to the base contact (B). And below that is an N+ layer connected (indirectly) to the collector (C). The emitter, base, and collector can be distinguished on the die with careful examination. The base's region surrounds the emitter, forming a blue rectangle. The collector contact is larger and off to the side.

Structure of an NPN transistor. Top: transistor as it appears on the die. Bottom: cross-section diagram.

The other key components of this chip are the resistors. The photo below shows two resistors as they appear on the die. The resistors are formed from strips of higher-resistance P silicon, which appears blue in the die photos. The two ends of each resistor are connected to the metal layer. A resistor has higher resistance if it is longer and narrower. While one resistor below is a simple rectangle, the other has a complex zig-zag shape to fit more length into the available space. (The blue rectangle in the lower right is a transistor, not a resistor. To distinguish it from a resistor, note that it has three contacts, and the two contacts on the rectangle are not symmetrical.)

Two resistors as they appear on the die.

The circuitry

Once the components can be recognized on the die, the circuit can be traced out and reverse-engineered. But before I describe the complete circuit, I'll explain one important functional block.

The schematic below shows a differential pair, or long-tailed pair, which amplifies the difference between its two inputs. This circuit is common in analog circuits, forming the heart of an op-amp. It is also the basis of Emitter-Coupled Logic (ECL). The basic idea is that a current sink (the two circles at the bottom) 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.

Schematic of a simple differential pair circuit. The current sink sends a fixed current I through the differential pair. If the two inputs are equal, the current is split equally between the two branches. Otherwise, the branch with the higher input voltage gets most of the current.

In this chip, the circuit is used as a comparator, a circuit that compares the two inputs and generates a logic output that indicates which input is higher. The side with the current will get pulled low, while the other side is pulled high by the resistor. Thus, the output can be treated as a logic signal.

The schematic below shows the circuitry for one of the four comparators on the chip. Note that the differential amplifier circuit above is used twice. For better performance, the output from the first differential amplifier is fed into a second differential amplifier. This sharpens the output: if the inputs are close together, the outputs from the first stage may not be fully "0" or "1". The second stage amplifies this difference, providing solid "0" and "1" outputs. The current sinks provide a relatively constant current for the amplifiers, but I won't explain them in detail.

Schematic of one comparator.

The diagram below shows the circuitry on the die, corresponding to the schematic above. The red boxes indicate the transistors for the two amplifier stages and their associated current sinks. The other components are the resistors, which appear as blue rectangles. The wiring in the metal layer connects the components together, as well as the inputs on the left and the output at the top.

One comparator, as it appears on the die.

The diagram below shows how the circuitry of the chip maps onto its 16 pins. Each triangle represents a comparator with its positive and negative inputs. Two of the comparators have a single output, while two also have an inverted output (indicated by a bubble).

Reverse-engineered pinout of the chip.

Conclusion

So what is this chip? Maybe it's simply four comparators, but they could have a specific purpose. The chip could be a converter for four differential input signals, e.g. DCS (Differential Current Switch) logic. Another chip in the family seems to be Emitter-Coupled Logic, so this chip could be four ECL inverters (but it doesn't make sense to have four pins for the reference voltage). It's a bit puzzling that two comparators have inverted and noninverted outputs, while two have single outputs.

Based on the circuitry and ceramic packaging of this chip, I estimate that it is from around 1970. The chip is labeled "OQ104", so I did a bunch of searching through old databooks, but I couldn't find anything that matches it. Since the chip looks like ECL and has the part number 104, it's tempting to think that the chip might be the 10,104 part in the MECL 10,000 series. Unfortunately, the 10,104 is a quad AND gate, and the internal circuitry is different. The "P" on the chip might indicate Philips, but I couldn't find any matching Philips components. For now, the exact identity of this chip is a mystery.

I announce my latest blog posts on Twitter, so follow me @kenshirriff. Many thanks to EvilMonkeyz Designs for providing the photos; follow on Instagram or Twitter for more interesting die photos.

Teardown of a quartz crystal oscillator and the tiny IC inside

The quartz oscillator is an important electronic circuit, providing highly-accurate timing signals at a low cost. A quartz crystal has the special property of piezoelectricity, changing its electrical properties as it vibrates. Since a crystal can be cut to vibrate at a very precise frequency, quartz oscillators are useful for many applications. Quartz oscillators were introduced in the 1920s and provided accurate frequencies for radio stations. Wristwatches were revolutionized in the 1970s by the use of highly-accurate quartz oscillators. Computers use quartz oscillators to generate their clock signals, from ENIAC in the 1940s to modern computers.1

A quartz crystal requires additional circuitry to make it oscillate, and this analog circuitry can be tricky to design. In the 1970s, crystal oscillator modules became popular, combining the quartz crystal, an integrated circuit, and discrete components into a compact, easy-to-use module. Curious about the contents of these modules, I opened one up and reverse-engineered the chip inside. In this blog post, I discuss how the module works and examine the tiny CMOS integrated circuit that runs the oscillator. There's more happening in the module than I expected, so I hope you find it interesting.

The oscillator module

I examined the oscillator module from an IBM PC card.2 The module is packaged in a rectangular 4-pin metal can that protects the circuitry from electrical noise. (It is the "Rasco Plus" rectangular can on the right, not the square IBM integrated circuit.) This module produced a 4.7174 MHz clock signal, as indicated by the text on the package.

The quartz oscillator module is in the lower right, labeled Rasco Plus. 4.7174 MHZ, © Motorola 1987. The square module is an IBM integrated circuit. Click this (or any other image) for a larger version.

I cut open the can to reveal the hybrid circuitry inside. I was expecting a gem-like quartz crystal inside, but found that oscillators use a very thin disk of quartz. (I damaged the crystal while opening the package, so the upper part is missing..) The quartz crystal is visible on the left, with metal electrodes attached to either side of the crystal. The electrodes are attached to small pegs, raising the crystal above the surface so it can oscillate freely.

Inside the oscillator package, showing the components mounted on the ceramic substrate.

On the right side of the module is a tiny CMOS integrated circuit die. It is mounted on the ceramic substrate and connected to the circuitry by tiny golden bond wires. A surface-mount capacitor (3 nF) and a film resistor (10Ω) on the substrate filter out noise from the power pin.

The IC's circuitry

The photo below shows the tiny integrated circuit die under a microscope, with the pads and main functional blocks labeled. The brownish-green regions are the silicon that forms the integrated circuit. A metal layer (yellowish white) wires up the components of the IC. Below the metal, reddish polysilicon implements transistors, but it is mostly obscured by the metal layer. Around the outside of the chip, bond wires are connected to pads, wiring the chip to the rest of the oscillator module. Two pads (select and disable) are left unconnected. The chip was manufactured by Motorola, with a 1986 date. I couldn't find any information on the part number SC380003.

The integrated circuit die with key blocks labeled. "FF" indicates flip-flops. "sel" indicates select pads. "cap" indicates pads connected to the internal capacitors.

The IC has two functions. First, its analog circuitry drives the quartz crystal to produce oscillations. Second, the IC's digital circuitry divides the frequency by 1, 2, 4, or 8, and produces a high-current clock output signal. (The division factor is selected by the two select pins on the IC.)

The oscillator is implemented with a circuit (below) called a Colpitts oscillator, which is more complex than the usual quartz oscillator circuit.43 The basic idea is that the crystal and the two capacitors oscillate at the desired frequency. The oscillations would rapidly die out, however, except for the feedback boost from the drive transistor.

Simplified schematic of the oscillator.

In more detail, as the voltage across the crystal increases, the transistor turns on, feeding current into the capacitors and boosting the voltage across the capacitors (and thus the crystal). But as the voltage across the crystal decreases, the transistor turns off and the current sink (circle with arrow) pulls current out of the capacitors, reducing the voltage across the crystal. Thus, the feedback from the drive transistor strengthens the crystal's oscillations to keep them going.

The bias voltage and current circuits are an important part of this circuit. The bias voltage sets the drive transistor's gate midway between "on" and "off", so the voltage oscillations on the crystal will turn it on and off. The bias current is set midway between the drive transistor's on and off currents so the current flowing in and out of the capacitors balances out.5 (I'm saying "on" and "off" for simplicity; the signal will be a sine wave.)

A large part of the integrated circuit is occupied by five capacitors. One is the upper capacitor in the schematic, three are paralleled to form the lower capacitor in the schematic, and one stabilizes the voltage bias circuit. The die photo below shows one of the capacitors after dissolving the metal layer on top. The red and green region is polysilicon, which forms the upper plate of the capacitor, along with the metal layer. Underneath the polysilicon, the pinkish region is probably silicon nitride, forming the insulating dielectric layer. The doped silicon (not visible underneath) forms the bottom plate of the capacitor.

A capacitor on the die. The large faint square to the left of the capacitor is a pad for connecting a bond wire to the IC. The complex structures on the left are clamp diodes on the pins. The cloverleaf structures on the right are transistors, which will be discussed later.

Curiously, the capacitors aren't connected together on the chip, but are connected to three pads that are wired together by bond wires. Perhaps this provides flexibility; the capacitance in the circuit can be modified by omitting the wire to a capacitor.

The digital circuitry

The right side of the chip contains digital circuitry to divide the crystal's output frequency by 1, 2, 4, or 8. This lets the same crystal provide four different frequencies. The divider is implemented by three flip-flops in series. Each one divides its input pulses by 2. A 4-to-1 multiplexer selects between the original clock pulses, or the output from one of the flip-flops. The choice is made through the wiring to the two select pads on the right side of the die, fixing the ratio at manufacturing time. Four NAND gates (along with inverters) are used to decode these pins and generate four control signals to the multiplexer and flip-flops.

How CMOS logic is implemented

The chip is built with CMOS logic (complementary MOS), which uses two types of transistors, NMOS and PMOS, working together. The diagram below shows how an NMOS transistor is constructed. The transistor can be considered a switch between the source and drain, controlled by the gate. The source and drain (green) consist of regions of silicon doped with impurities to change its semiconductor properties and called N+ silicon. The gate consists of a special type of silicon called polysilicon, separated from the underlying silicon by a very thin insulating oxide layer. The NMOS transistor turns on when the gate is pulled high.

Structure of an NMOS transistor. A PMOS transistor has the same structure, but with N-type and P-type silicon reversed.

A PMOS transistor has the opposite construction from NMOS: the source and drain consist of P+ silicon embedded in N silicon. The operation of a PMOS transistor is also opposite from the NMOS transistor: it turns on when the gate is pulled low. Typically PMOS transistors pull the drain (output) high, while NMOS transistors pull the drain low. In CMOS, the transistors act in a complementary fashion, pulling the output high or low as needed.

The diagram below shows how a NAND gate is implemented in CMOS. If an input is 0, the corresponding PMOS transistor (top) will turn on and pull the output high. But if both inputs are 1, the NMOS transistors (bottom) will turn on and pull the output low. Thus, the circuit implements the NAND function.

A CMOS NAND gate is implemented with two PMOS transistors (top) and two NMOS transistors (bottom).

The diagram below shows how a NAND gate appears on the die. The transistors have complex, meandering shapes, unlike the rectangular layouts that appear in textbooks. The left side holds the PMOS transistors, while the right side holds the NMOS transistors. The polysilicon that forms the gates is the slightly reddish wiring on top of the silicon. Most of the underlying silicon is doped, making it conductive and slightly darker than the non-conductive undoped silicon along the left and right edges and in the center. For this photo, the metal layer was removed with acid to reveal the silicon and polysilicon underneath; the yellow line illustrates where some of the metal wiring was. The circles are connections between the metal layer and the underlying silicon or polysilicon.

A NAND gate as it appears on the die.

The transistors in the die photo can be matched up with the NAND-gate schematic; look at the transistor gates formed by polysilicon and what they separate. There is a path from the +5 region to the output through the large elongated PMOS transistor on the left, and a second path through the small PMOS transistor near the center, indicating the transistors are in parallel. Each gate is controlled by one of the inputs. On the right, a path from ground to the output connection must go through both of the concentric NMOS transistors, indicating they are in series.

This integrated circuit also uses many circle-gate transistors, an unusual layout technique that allows multiple transistors in parallel at high density. The photo below shows 16 circle-gate transistors. The copper-colored cloverleaf patterns are the transistor gates, implemented with polysilicon. The inside of each "leaf" is the transistor drain, while the outside is the source. The metal layer (removed) wires all the sources, gates, and drains together respectively; the parallel transistors act as one larger transistor. Paralleled transistors are used in the output pin drivers to provide high current for the output. In the bias circuitry, different numbers of transistors are wired together (e.g. 6, 16, or 40) to provide the desired current ratios.

Sixteen circle-gate transistors with four gate connections.

Transmission gate

Another key circuit in the chip is the transmission gate. This acts as a switch, either passing a signal through or blocking it. The schematic below shows how a transmission gate is constructed from two transistors, an NMOS transistor and a PMOS transistor. If the enable line is high, both transistors turn on, passing the input signal to the output. If the enable line is low, both transistors turn off, blocking the input signal. The schematic symbol for a transmission gate is shown on the right.

A transmission gate is constructed from two transistors. The transistors and their gates are indicated. The schematic symbol is on the right.

Multiplexer

A multiplexer is used to select one of the four clock signals. The diagram below shows how the multiplexer is implemented from transmission gates. The multiplexer takes four inputs: A, B, C, and D. One of the inputs is selected by activating the corresponding select line and its complement. That input is connected through the transmission gate to the output, while the other inputs are blocked. Although a multiplexer can be built with standard logic gates, the implementation with transmission gates is more efficient.

The 4-to-1 multiplexer is implemented with transmission gates.

The schematic below shows the transistors that make up the multiplexer. Note that inputs B and C have pairs of transistors. I believe the motivation is that a pair of transistors presents half the resistance to the signal. Since inputs B and C are the higher-frequency signals, the pair of transistors allows them to pass through with less distortion and delay.

Schematic of the multiplexer, matching the physical layout on the chip.

The image below shows how the multiplexer is physically implemented on the die. The polysilicon gate wiring is most prominent. The metal layer has been removed; the metal lines ran vertically connecting corresponding transistors segments. Note that the sources and drains of neighboring transistors are merged into single regions between the gates. The top rectangle holds the NMOS transistors while the lower rectangle holds the PMOS transistors; because PMOS transistors are less efficient, the lower rectangle needs to be larger.

Die photo of the multiplexer.

Flip-flop

The chip contains three-flip-flops to divide the clock frequency. The oscillator uses toggle flip-flops, that flip between 0 and 1 each time they receive an input pulse. Since two input pulses result in one output pulse (0→1→0), the flip-flop divides the frequency by 2.

A flip-flop is constructed from transmission gates, inverters, and a NAND gate, as shown in the schematic below. When the input clock is high, the output passes through the inverter and the first transmission gate to point A. When the input clock switches low, the first transmission gate opens, so point A holds its previous value. Meanwhile, the second transmission gate closes, so the signal passes through the second inverter and transmission gate to point B. The NAND gate inverts it again, causing the output to flip from its previous value. A second cycle of the input clock repeats the process, causing the output to return to its initial value. The result is that two cycles of the input clock result in one cycle of the output, so the flip-flop divides the frequency by 2.

Implementation of a toggle flip-flop.

Each flip-flop has an enable input. If a flip-flop is not needed for the selected output, it is disabled. For instance, if the "divide by 2" mode is selected, only the first flip-flop is used, and the other two are disabled. I assume this is done to reduce power consumption. Note that this is independent from the module's disable pin, which blocks the module output entirely. This disable feature is optional; this particular module does not provide the disable feature and the disable pin is not wired to the IC.

The schematic above shows the inverters and transmission gates as separate structures. However, the flip-flop uses an interesting gate structure that combines the inverter and the transmission gate (left) into a single gate (right). The pair of transistors connected to data in function as an inverter. However, if the clock in is low, both power and ground are blocked so the gate will not affect the output and it will hold its previous voltage. This provides the transmission gate functionality.

Implementation of a combination inverter / transmission gate.

The photo below shows how one of these gates appears on the die. This photo includes the metal layer on top; the reddish polysilicon gates are visible underneath. The two PMOS transistors are on the left, as concentric loops, while the NMOS transistors are on the right.

One of the combination inverter / transmission gates, as it appears on the die.

Conclusion

While the oscillator module looks simple from the outside, on the inside there's a lot more complexity than you might expect.6 It contains not just a quartz crystal but also discrete components and a tiny integrated circuit. The integrated circuit combines capacitors, analog circuitry to drive the oscillations, and digital circuitry to choose a frequency. By changing the wiring to the integrated circuit during manufacturing, four different frequencies can be selected.

I'll end with the die photo below showing the chip after removing the metal and oxide layers, showing the silicon and polysilicon underneath. The large pinkish capacitors are the most visible feature in this image, but the transistors can also be seen. (Click the image for a larger version.)

Die photo of the oscillator chip with metal removed to show the polysilicon and silicon underneath.

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Notes and references

1. Modern PCs use quartz crystals, but with a more complex technique to get multi-gigahertz clock frequencies. A PC uses a crystal with a much lower frequency, and multiplies the frequency using a circuit called a phase-locked loop. Computers often used a 14.318 MHz crystal because that frequency was used in old television sets, so crystals with that frequency were common and cheap. ↩

2. Why does the board use a 4.7174 MHz crystal, a somewhat unusual frequency? In the 1970s, the IBM 3270 was a very popular CRT terminal. These terminals were connected with coaxial cable and used the Interface Display System Standard protocol with a 2.3587 MHz bit rate. In the late 1980s, IBM produced interface cards to connect an IBM PC to a 3270 network. I obtained the crystal from one of these interface cards (type 56X4927), and the crystal frequency of 4.7174 MHz is exactly twice the 2.3587 MHz bit rate. ↩

3. The terminology used for crystal oscillators is confusing with "Colpitts oscillator" and "Pierce oscillator" used in contradictory ways. I looked into the history of oscillators to try to sort out the naming, and I'll discuss it in this footnote.

   In 1918, Edwin Colpitts, the head researcher at Western Electric, invented an inductor/capacitor oscillator, now known as the Colpitts Oscillator. The idea is that the inductor and capacitors form a "resonant tank", which oscillates at a frequency set by the component values. (You can think of the electricity in the tank as sloshing back and forth between the inductor and the capacitors.) On their own, the oscillations would rapidly die out, so an amplifier is used to boost the oscillators. In the original Colpitts oscillator, the amplifier was a vacuum tube. Later circuits moved to transistors, but it can also be an op-amp or other type of amplifier. (Other circuits, such as the module I examined, ground an end and provide feedback to the middle. In that case, there is no inversion from the capacitors, so a non-inverting amplifier is used.)

   

   A simplified schematic of a Colpitts oscillator, showing the basic components.

   The key feature of the Colpitts oscillator is the two capacitors, which form a voltage divider. Since the capacitors are grounded in the middle, the two ends will have opposite voltages: when one end goes up, the other goes down. The amplifier takes the signal from one end, amplifies it, and feeds it into the other end. The amplifier inverts the signal and the capacitors provide a second inversion, so the feedback strengthens the original signal (i.e. it has a phase shift of 360°).

   In 1923, George Washington Pierce, a professor of physics at Harvard, replaced the inductor in the Colpitts oscillator with a crystal. The crystal made the oscillator much more accurate (higher Q factor), leading to its heavy use in radio transmission and other applications. Pierce patented his invention and made a lot of money off it from companies such as RCA and AT&T. The patents led to years of litigation, eventually reaching the Supreme Court. (For more information, see this thesis on crystal history.)

   For several decades, the common terminology was that a Pierce oscillator was a Colpitts oscillator that used a crystal. (See Air Force Manual, 1957 and Navy training, 1983 for instance.) The Pierce oscillator often omitted the characteristic voltage-divider capacitors, using the stray capacitance of the vacuum tube instead. But then terminology shifted, with "Colpitts oscillator" and "Pierce oscillator" indicating two different types of crystal oscillator: Colpitts with the capacitors and Pierce without the capacitors. (See, for example, the classic electronics text Horowitz and Hill.)

   Another change in terminology was to describe the Colpitts oscillator, Pierce oscillator, and Clapp oscillator as topologically identical crystal oscillators, just differing in what point in the circuit was considered AC ground (the collector, emitter, or base respectively). (See Frerking's Crystal Oscillator Design and Temperature Compensation (1978, p56) or Maxim's crystal oscillator tutorial.) Alternatively, these oscillators can all be called Colpitts, but common-collector, common-emitter, or common-base (details).

   The point of this history is that oscillator terminology is confusing, with different sources calling oscillators Colpitts or Pierce in contradictory ways. Getting back to the oscillator module I examined, it could be described as a common-drain Colpitts oscillator (analogous to common-collector). It would also be called a Colpitts oscillator using the terminology based on the ground position. Historically, it would be called a Pierce oscillator since it uses a crystal. It's also called a single-pin crystal oscillator since only one pin of the crystal is connected to the circuitry (and the other is grounded). ↩

4. The typical quartz oscillator is built using a simple circuit called the Pierce-gate oscillator, where the crystal forms a feedback loop with an inverter. (The two capacitors grounded in the middle make this very similar to the classical Colpitts oscillator.)

   

   The Pierce oscillator circuit commonly used as a computer clock. Diagram by Omegatron, CC BY-SA 3.0.

   I'm not sure why the module I disassembled uses a more complex oscillator circuit that requires tricky biasing. ↩

5. The voltage bias and current bias circuits are moderately complex analog circuits built with a bunch of transistors and a few resistors. I won't describe them in detail, but they use feedback loops to generate the desired fixed voltage and current. ↩

6. If you want to learn more about quartz oscillators, there are interesting videos at EEVblog, electronupdate, and WizardTim. Colpitts oscillators are explained in videos at Hackaday. ↩