Reverse-engineering an unusual IBM modem board from 1965

The vintage IBM circuit board below has a large metal block on it that caught my attention, so I investigated it in detail. It turns out that the board is part of a modem, and the large metal box is a transformer. This blog post summarizes what I learned about this board, along with a bit of history on modems.

The IBM modem board, type HGB.

This board is a Standardized Modular System (SMS) card, but a very unusual one. In the late 1950s, IBM introduced the Standardized Modular System card, small circuit boards that held a simple circuit, and used these boards to build computers and peripherals into the mid-1960s. The idea was to design a small number of standardized boards that implemented logic functions and other basic circuits. The number of different board designs spiraled out of control, however, with thousands of different types of SMS cards. (I've made an SMS card database describing over 1400 different cards.)

This is a typical SMS card, implementing a triple AND gate.

Most SMS cards look like the one above, so the card with the metal block struck me as very unusual. Although some SMS cards are double-width "twin cards", I'd never seen one with a large metal block sandwiched between two boards, so it got my curiosity.

One suggestion was that the metal box was a oven-controlled crystal oscillator (OCXO). A OCXO is often used when a high-precision frequency source is required. The frequency of a quartz crystal varies with temperature, so by putting the crystal in a temperature-controlled module (like the one below), the frequency remains stable.

A vintage crystal oven that plugged into a tube socket. Photo by Wtshymanski (CC BY-SA 3.0).

However, measurements of the module by Curious Marc and Eric Schlaepfer (TubeTimeUS) determined that the metal box was a large transformer (1:1 ratio, about 8 mH inductance). The photo below shows the four connections to the windings, while the external metal wires grounded the case. The transformer is heavy—the board weighs almost exactly one pound—so it's probably filled with oil.

The transformer on the modem board.

The board shows its age through its germanium transistors, which were used before silicon transistors became popular. Most of the transistors are PNP, apparently because it was easier to produce PNP germanium transistors than NPN. (Silicon transistors are the opposite with NPN transistors much more common than PNP, largely because the electrons in NPN transistors move more easily than the holes in PNP transistors, giving better performance to NPN transistors.)

Closeup of the Texas Instruments transistors. Most of the transistors on the board were PNP type 033.

I found a document1 that gave the board's part number as a transmitter board for an IBM modem, transmitting data across phone lines. The large transformer would have been used to connect the modem to the phone lines while maintaining the necessary isolation. The modem used frequency-shift keying (FSK), using one frequency for a 1 bit and a second frequency for a 0 bit. I reverse-engineered the board by closely studying it, and discovered that the board generates these two frequencies, controlled by a data input line. This confirmed that the board was a modem transmitter board.

The photo below shows the underside of the board, with the traces that connect the components. The board is single-sided, with traces only on the underside, so traces tend to wander around a lot, using jumper wires on the other side to cross over other traces. (It took me a while to realize that the transformer's case was just wired to ground, since the trace wanders all over the board before reaching the ground connection.) At the bottom of the board are the two gold-plated 16-pin connectors that plug into the system's backplane. The connector on the left provides power, while the connector on the right has the signals.

The underside of the printed circuit board for the modem card.

The result of my reverse-engineering is the schematic below. (Click for a larger version.) The circuit seems complicated for a board that just generates a varying frequency, but it took a lot of parts to do anything back then. At the left of the schematic are the board's two inputs: a binary data signal, and an enable signal that turns the oscillator on. Next are the oscillator that produces the signal, and a 13 millisecond delay (both discussed below). The output from the oscillator goes through a filter that makes it somewhat more sine-like. The signal is then amplified to drive the transformer, as well as to produce a direct output.

Reverse-engineered schematic of the IBM modem board. (Click this image, or any other, for a larger version.)

The oscillator

The oscilloscope trace below shows the output that I measured from the board after powering it up. The blue line shows the data input, while the cyan waveform above shows the frequency output. You can see that the output frequency is different for a "1" input and a "0" input, encoding the data. (The height also changes, but I think that's just a side-effect of the circuit.)

Oscilloscope trace showing how the frequency of the output signal varies with the input data.

The modem is supposed to generate frequencies of 1020 Hertz for a "mark" (1) and 2200 Hertz for a "space" (0). However, I measured frequencies of 893 and 1920, about 13% too low. This seems like reasonable accuracy for components that are 55 years old. (I don't know what the expected accuracy was at the time. There aren't any adjustments, so the frequencies probably weren't critical. Also, since the two frequencies differ by more than a factor of two, there's a large margin. Another possibility is that I guessed that the board is powered with ±12V but different voltages might yield more accurate frequencies.)

The modem operated at up to 600 baud. This corresponded to 100 characters per second for 6-bit characters, or 75 characters per second for 8-bit characters. The oscilloscope trace below shows the signal changing at 600 baud. At this rate, one bit is represented by only 1.7 cycles of the slower frequency, so the receiver doesn't have a lot of information to distinguish a 0 or a 1 bit. Also note that the waveform is somewhat distorted, not a clean sine wave.

The output signal when fed bits at 600 baud (i.e. a 300 Hertz square wave).

The heart of this board is the frequency-shift keying oscillator that generates the variable output frequency.2 The input data bit selects one of two control voltages to the oscillator, controlling its output frequency.

The oscillator is a fairly common transistor-pair circuit. The diagram below illustrates how it works. (It uses PNP transistors and runs on -12 volts, so ground is the higher voltage, which may be a bit confusing.) Suppose transistor T1 is on and T2 is off. Capacitor C2 will discharge through resistor R2, as shown. When its voltage reaches about -0.6 volts, T2 will turn on. This will pull the right side of C1 up to ground; it was previously at -12 volts because of R4. This causes the left side of C1 to jump up to about +12 volts, turning off T1.

The process then repeats on the other side, with C1 discharging through R1 until T1 turns off and T2 turns on. The result is that the circuit oscillates. The discharge rate is controlled by the values of R1 and R2, and the control voltage; a lower voltage will cause the capacitors to discharge faster and thus faster oscillations.

Oscilloscope traces of the oscillator, showing the alternating decay cycles.

The traces above show the action of the oscillator, producing the cyan output signal. The yellow curve shows the voltage on the left side of C2, the pink trace shows the voltage on the left side of C1, and the blue trace shows the voltage on the right side of C2. The pink and blue traces show the alternating discharge cycles for the capacitors; the faster discharge yields a higher output frequency.

Schematic of the oscillator at the heart of the board.

The output of the oscillator is essentially a square wave, so it goes through some resistor-capacitor filtering stages that shape it to better approximate a sine wave. The top line (yellow) shows the output of the oscillator, and the lines below show the signal as it progresses through the filter. The result is still fairly distorted, but much smoother than the original square wave.

The square wave signal and the results after filtering.

Delay circuit

Another interesting circuit takes the enable signal and outputs this signal delayed by 13 milliseconds. When I reverse-engineered this circuit (below), I figured it was just buffering the signal but it appeared overly complex for that. I measured its behavior and discovered that it implements a delay.

Reverse-engineered schematic showing the 13ms delay circuit on the modem board.

The circuit contains several buffers, but the heart of it is a resistor-capacitor delay. When the enable line is activated, the capacitor is pulled to -12V slowly through the resistors, creating the delay. The photo below shows the delay capacitor and associated resistors.

The diode (striped glass cylinder), resistors (brown striped components), and capacitor (larger metal cylinder) create the delay.

The oscilloscope trace shows the operation of the delay circuit. When the (inverted) enable line (blue) goes low, the signal output (cyan) immediately turns on. However, the enable outputs (yellow and pink) are delayed by about 13 milliseconds.

Oscilloscope trace of the delay circuit.

I don't know the reason behind this delay circuit. Maybe it gives the oscillator time to settle after being enabled? Maybe the modem protocol uses 13 milliseconds of signal to indicate the start of a new message?

Some background on Teleprocessing

If you used computers in the 1990s, you probably used a dial-up modem like the one below to call a provider such as AOL through your phone line. The name "modem" is short for MOdulator-DEModulator, since it modulates the analog signal to encode the digital bits, as well as demodulating the received signal back to digital. In this way, the modem provided the connection between your computer's digital signals and the analog frequencies transmitted by phone lines.

A Hayes modem from 1982. Photo by Aeroid (CC BY-SA 4.0).

The history of modems goes back much further, though. IBM introduced what they called "Teleprocessing" in the early 1940s, converting punch-card data to paper tape and sending it over telegraph lines for the U.S. Army.1 In the early 1950s, a device called Data Transceiver removed the intermediate paper tape, connecting directly to a telephone line. With the introduction of the IBM System/360 mainframe in 1964, Teleprocessing became widespread, used for many applications such as remote data entry and remote queries. Banking and airline reservations made heavy use of Teleprocessing. Timesharing systems allowed users to access a mainframe computer over remote terminals, kind of like cloud computing. Even the Olympics used Teleprocessing, transmitting data between widely-separated sites and a central computer that computed scores.

Back then, modems were large cabinets. The board that I examined could be used in an IBM 1026 Transmission Control Unit (below).3 This low cost unit was designed to "make a modest start toward satisfying your data communication requirements ... until it is time to step up to more powerful transmission control units". It could connect a computer such as the IBM 1401 to a single communications line.

IBM 1026 Transmission Control Unit. Photo from Computer History Museum.

Larger installations could use the IBM 1448 Transmission Control Unit (below). This refrigerator-sized cabinet was 5 feet high and could support up to 40 communications links.

The IBM 1448 Transmission Control Unit was a large cabinet. Photo from IBM 1448 Transmission Control Unit manual.

Nowadays, people often use a cable modem or DSL modem to connect to the Internet. Fortunately technology has greatly improved and these modems aren't the large cabinets of the 1960s. Speeds have also greatly improved; a modern 180 Mbps network connection is 300,000 times faster than the 600 baud modem board that I examined. At that rate, a web page that now loads in a second would have taken over three days!

Conclusion

This may seem like an overly detailed analysis of a random circuit board. But I was curious about the board due to its unusual transformer. I also figured it would be interesting to reverse-engineer the board to see how IBM built analog circuits back in the 1960s. Hopefully you've enjoyed this look at a vintage modem board.

Side view of the modem SMS card. The transformer is the metal box at the left.

I announce my latest blog posts on Twitter, so follow me at kenshirriff. I also have an RSS feed. Thanks to Nick Bletsch for sending me the board. I discussed this board on a couple of Twitter threads and got a bunch of interesting comments.

Notes and references

1. For more information, see Introduction to Teleprocessing Technical information is in Teleprocessing—General FE Handbook page 7-7 lists part number 373807 (my board) as a Transmitter card Type 2A. Page 7-30 then describes some characteristics of this modem type. IBM Teleprocessing 1940-1960 provides a historical look. ↩↩

2. The oscillator is essentially a voltage-controlled oscillator (VCO). However, since it only takes two different input voltages (about -2.5 and -9 volts), the circuit isn't as challenging as a typical VCO, which takes a wide range of inputs and needs to have a linear response. ↩

3. The modem card I examined could be used with an IBM 1050 or 1060 Data Communications System, which I believe was the remote terminal subsystem. It could also be used with the IBM 1448 and IBM 1026 Transmission Control Units. (The IBM 1448 connected to an IBM 1410 or IBM 7010 computer.) ↩

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.

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

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. ↩