Quad-A-B Preamp 2 (QAB2)

For Homodyne Interferometers

Assembly and Operation Manual

Version 1.02 (26-Nov-21)

Copyright © 1994-2021
Sam Goldwasser
--- All Rights Reserved ---

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:
  1. This notice is included in its entirety at the beginning.
  2. There is no charge except to cover the costs of copying.

Table of Contents


Author and Copyright

Author: Samuel M. Goldwasser

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Copyright © 1994-2021
All Rights Reserved

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

1. This notice is included in its entirety at the beginning.
2. There is no charge except to cover the costs of copying.


SG-AB is intended for use in hobbyist, experimental, research, and other applications where a bug in the hardware will not have a significant impact on the future of the Universe or anything else. We will not be responsible for any consequences of such bugs including but not limited to damage to the wafer FAB you picked up on eBay for $1.98 + shipping, financial loss from the use of 37 spools of ABS due to the office 3-D printer fabricating a part 25.4x too large in all dimensions, or bruising to your pet's ego from any number of causes directly or indirectly related to SGAB2. ;-)


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Homodyne interferometry and optical encoders require conversion from the optical detectors to so-called Quad-A-B signals. At low speed, this is not much more than a photodiode with load resistor and digital comparator. And parts like that are included in the various interferometer and entry-level displacement masurement systems. But such a simple scheme is limited to a few thousand counts per second - perhaps up to a few 10s of thousand of counts per second - but not the performance needed for most real applications. For example, to track motion at 1 cm/s using a plane mirror interferometer would require more than 60,000 counts/second (where 1 c/s is equivalent to 1 Hz).

The purpose of the Quad-A-B Preamp (henceforth referred to as QAB2 or simply AB2) is to provide a simple solution that accepts photodiode inputs and generates differential RS422 A and B signals that can be input to µMD0, µMD1, µMD2, or another compatible displacement measuring system. This is not the ultimate device as it is strictly digital Quad-A-B not analog Quad-Sin-Cos. Nor does it have support for an intensity channel to accomodate a varying input amplitude. Those capabilities may come in the future, but don't hold your breath in anticipation. ;-)

QAB2 is on a 1.6 inch by 2.25 inch PCB and runson 12 to 15 VDC. (The PCB itself is called SG-AB2.) The optical input is a beam up to ~3 mm in diameter (using the default photodiode) with an optical power from <25 µW to >1 mW. The actual photodiodes may be mounted up to a few inches from their pads on the PCB but coax or twisted pair should be used beyond that where noise pickup may be a concern. QAB2 has >3 MHz bandwidth (full cycle) which is more than adequate for systems using the kit lasers as well as for many real applications. With a Linear Interferometer which has a full cycle of ~316 nm, the slew rate can be greater than 1 meter per second, which is a fairly nutty velocity. ;-). And it is expected that the bandwidth limit can be extended with trivial changes to only a few part values. This is left as an exercise for the student. ;-)

The actual SG-AB2 PCB is four layers with internal power and ground planes.

Typical Populated AB2 V1.00 PCB


Parts List

Note: There are gaps in some part number sequences by design. ;-)

 Prt  Description                  Comments
  -   PCB, SG-AB2 V1.00            First released version

  D0  Diode, 1N4007                Reverse polarity power protection

  C1  Capacitor, 10-22 µF          Input power bypass
  C2  Capacitor, 0.1 µF            Input power bypass
  C3  Capacitor, 10-22 µF          +9 Vaa bypass
  C4  Capacitor, 0.1 µF            +9 Vaa bypass
  C5  Capacitor, 10-22 µF          +5 Vcc bypass
  C6  Capacitor, 0.1 µF            +5 Vcc bypass
  C7  Capacitor, 0.1 µF            +9 Vaa bypass
  C8  Capacitor, 0.1 µF            +9 Vaa bypass
  C9  Capacitor, 0.1 µF            +5 Vcc bypass
 C10  Capacitor, 0.1 µF            +5 Vcc bypass
 C11  Capacitor, 0.1 µF            PDA feedback or bypass
 C12  Capacitor, 1 nF              1st stage A frequency compensation
 C13  Capacitor, 1 nF              2nd stage A frequency compensation

 C21  Capacitor, 0.1 µF            PDB feedback or bypass
 C22  Capacitor, 1 nF              1st stage B frequency compensation
 C23  Capacitor, 1 nF              2nd stage B frequency compensation

  R0  Resistor, 36K, 1/8 W         PWR LED current limiting
  R1  Resistor, 2.2K, 1/8 W        1st stage A power
  R2  Resistor, 10K, 1/8 W         1st stage A bias
  R3  Resistor, 1.6K, 1/8 W        PDA load
  R4  Trim-pot, 500                PDA load trim
  R5  Resistor, 2.2K, 1/8 W        1st stage A collector load
  R6  Resistor, 220, 1/8 W         1st stage A emitter load
  R7  Resistor, 2.2K, 1/8 W        2nd stage A collector load
  R8  Resistor, 220, 1/8 W         2nd stage A emitter load

  R9  Resistor, 2.2K, 1/8 W        RS422 receiver threshold
 R10  Resistor, 2.2K, 1/8 W        RS422 receiver threshold

 R11  Resistor, 220, 1/8 W         Red LED A current limiting 
 R21  Resistor, 2.2K, 1/8 W        1st stage B power
 R22  Resistor, 10K, 1/8 W         1st stage B bias
 R23  Resistor, 1.6K, 1/8 W        PDB load
 R24  Trim-pot, 500                PDB load trim
 R25  Resistor, 2.2K, 1/8 W        1st stage B collector load
 R26  Resistor, 220, 1/8 W         1st stage B emitter load
 R27  Resistor, 2.2K, 1/8 W        2nd stage B collector load
 R28  Resistor, 220, 1/8 W         2nd stage B emitter load

 R31  Resistor, 2.2K, 1/8 W        Green LED B current limiting

  U1  LM78M09, IC, Regulator, 9V   Vaa 9 V regulator (SMT)
  U2  LM78M05, IC, Regulator, 5V   Vcc 5 V regulator (SMT)
  U3  IC, UA9637                   RS422 receiver
  U4  IC, UA9638                   RS422 driver

  J1  Header/shell/pins or Screw   Power input
       terminal block, 2 pin

  J2  Header/shell/pins or screw   Signal output
       terminal block, 6 pin

 JP1  Jumper block, 3 pin          Gain select A
 JP2  Jumper block, 3 pin          Gain select B

 PDA  Silicon photodiode           Optical sensor A
 PDB  Silicon photodiode           Optical sensor B

 PWR  LED, blue                    Power LED
  A   LED, red                     A LED
  B   LED, green                   B LED

  Q1  Transistor, 2N3904           1st stage A (impedance matching)
  Q2  Transistor, 2N3904           2nd stage A (gain)
  Q3  Transistor, 2N3904           1st stage B (impedance matching)
  Q4  Transistor, 2N3904           2nd stage B (gain)

 SKT1 Socket, 8 pin                For UA9637
 SKT2 Socket, 8 pin                For UA9638
 SKT3 Socket, 2 pin                For photodiode A
 SKT4 Socket, 2 pin                For photodiode B

For those not familiar with the common resistor color code (Black/0, Blown/1, Red/2, Orange/3, Yellow/4, Green/5, Blue/6, Violet/7, Gray/8, White/9), the resistors shown above are 150 ohms (brown-green-brown or 15 with 1 zero) ohms and 330 ohms (33 with 1 zero) ohms. The gold stripe indicates 5 percent tolerance on the value but for the use here, tolerance doesn't matter. (It's possible the resistors you use will have 4 stripes where 3 of them are the value and the 4th is the multiplier, along with one for tolerance. If in doubt confirm the value with a multimeter.) The chart below is from Digikey. (If the link decays, a Web search will readily find another one.)

Resistor Color Code Chart (from the Digikey Web site)

All of these resistors are 1/8 watt which are a bit tiny. So, use a bright light and magnifying glass if necessary as it's easy to confuse locations and color of the bands. If in doubt, measure the resistance with a DMM. As they say in woodworking: "Measure twice and cut once". Replacing a part is much more difficult and risky than installing the correct one in the first place!

The direction of the resistors doesn't matter though it is good practive to have them line up with the labels on the PCB. The polarity of the diodes, electrolytic (large value) capacitors, and the photodiode IS critical. Refer to the layout diagram, above.

The yellow ceramic capacitors are labeled on one side with two digits (always "10" for the values used in QAB2) and a multiplier as power of 10: 102 (1,000 pF, 1 nF), 103 (10,000 pF, or 10 nm), or 104 (100,000 pF, 100 nm, 0.1 µF). The diodes are labeled in itty-bitty print.

Schematic for the QAB2 Version 1.0

The schematic for QAB2 is shown below. (Schematic version numbers are not the same as PCB version numbers.)


QAB2 Version 1.0 Schematic

The graphic below shows the general appearance of the PCB with most of the parts installed:


SG-AB2 PCB Version 1.00: Location of most Components

Printing out the schematic and having it available for reference while assembling the PCB may be helpful.


As promised, here are the detailed "Heathkit™-style" instructions for assembling the SG-AB2 V1.00 PCB.

All components are through-hole except for the voltage regulators (U1 and U1), and except as noted in the detailed assembly procedure, should seat flush on the PCB. They shouldn't be suspended in mid-air swinging in the breeze. :) The resistors in particular like to not stay flat on the PCB unless their leads are bent at a steep angle. Most components are identified on the silk-screen and with only a few exceptions, the label won't be obscured when the part is installed.

A low power soldering iron with narrow tip and thin (e.g., #22 AWG) rosin-core solder will be required. DO NOT even think about attempting this without suitable soldering equipment. It's well worth the investment. A Weller soldering gun or propane torch will not work. :) Rosin core solder is also essential. And while I'm quite confident that you never make mistakes, a means of component removal such as a de-soldering pump (e.g., a full size SoldaPullt™) will be highly desirable. Screwing up component removal can easily ruin the PCB and is not covered under the limited unlimited warranty. :-)

Proper soldering technique will be such that the exposed solder on each pad should be shiny with a concave profile. It should not be a blob and just needs to fill the hole. Solder is not glue. Some excess solder doesn't hurt anything but looks unprofessional. A 10X magnifier may come in handy for inspection. Residual rosin can be cleaned off with isopropyl alcohol or an environmentally-friendly electronic solvent. However, leaving the rosin alone is also acceptable (if ugly).

Total assembly time should be well under 1 hour for someone proficient in fine soldering. Cutting component leads to 1/4 to 3/8 inch before installation will simplify soldering as the long leads won't be poking you in your one good eye. :( :) Then trim flush after soldering.

Print out this document so each step can be checked off ( ) as it is completed.

The parts list below assumes populating with with all components. Exceptions will be noted.

  1. ( ) Confirm that all parts are present and undamaged:

    Inspect the parts closely, especially the (yellow) ceramic capacitors as they may all be physically identical. The labeling is TINY and easy to read incorrectly. It's also easy to misread the itty-bitty 1/8th watt resistor color bands. In some cases, slighlty different values for resistors may be included such as 30K in place of 36K, but these should be intuitively obvious. ;-)

    Testing of the LEDs inserted into the PCB but prior to soldering is recommended. They are very fragile the leads are stressed while soldering. Bend the leads out at a small angle so the LED stays in place and cut them short but DO NOT solder until thee LED has been confirmed to work. Then without stressing the leads, solder quickly and retest.

    Double check the part value before soldering. Use a magnifying glass if necessary. As they say in carpentry: "Measure twice and cut once.". Even with proper desoldering equipment, removing a part without damage to either the part or PCB can be dicey.

  2. ( ) Install U1 (LM78M09 voltage regulator). This is most easily done by adding a tiny bit of solder to the lower-right pad and then holding the chip in place while heating that leg on the IC. Then solder the upper right leg, and finally the tab with just enough solder to form a continuous bead along its length.

  3. ( ) Install U2 (LM78M05 voltage regulator) in a similar manner.

  4. ( ) Install J1 (2-pin header or screw terminal block). For the header, the plastic tab faces toward the center of the PCB. For the terminal block, the wire access holes face out.

  5. ( ) Install PWR (blue LED). The longer lead goes toward the center of the PCB and the flat goes toward the edge. See note above with respect to soldering LEDs.

  6. ( ) Install D0 (1N4007 diode). Pay attention to the polarity.

  7. ( ) Install R0 (36K ohms, oranage-blue-orange).

  8. ( ) Install C1, C3, C5 (10 µF). Note polarity: Positive is marked on the PCB and is also the square pad. Negative on the capacitor is the line.
  9. ( ) Install C2, C4, C6 (0.1 µF).

  10. ( ) Carefully inspect for solder and component lead shorts and unsoldered leads. Correct as needed.

  11. ( ) Smoke test #1. :) Connect a source of 12 to 15 VDC to J1. Pay attention to polarity. The blue LED should come on and the far right legs of U1 and U2 should have voltages very close to +9 VDC and +5 VDC on them, respectively. And nothing should smoke. ;-)

  12. ( ) Install C7, C8, C9, C10 (0.1 µF).

  13. ( ) (Optional) Install C11, C21 (0.1 µF). When installed in the default location (F on the schematic), these provide positive feedback to partially cancel out the photodiode capacitance and may help with high frequency signals. Since the feedback gain is less than 1, the system is still unconditionally stable. They can also be installed as a decoupling (bypass) capacitor to GND (D on the schematic) to an unmarked nearby GND pad (at a 45 degree angle, sorry). If none of this makes sense, just ignore it. ;-) Or try them and see if either is beneficial. Then report your findings. ;-)

  14. ( ) Install C12, C13, C22, C23 (1 nF).
  15. ( ) Install C14 C24 (100 pF).

  16. ( ) Install R1, R5, R9, R10, R21, R25 (2.2K ohms, red-red-red).
  17. ( ) Install R2, R22 (10K ohms, brown-black-orange).
  18. ( ) Install R3, R23 (1.6K ohms, brown-blue-red)
  19. ( ) Install R4, R24 (500 ohms trimpot, marked 501).
  20. ( ) Install R6, R8, R11, R26, R28 (220 ohms, red-red-brown).
  21. ( ) Install R7, R27 (1K ohms, brown-black-red).
  22. ( ) Install R31 (2.2K, red-red-red).

  23. ( ) Install A and B (red and green LEDs. The longer lead goes to the left and the flat goes to the right. See note above with respect to soldering LEDs.

  24. ( ) Install JP1, JP2 (3-pin jumper block). The jumper shold be placed on the top position for high gain (less than around 100 µW of beam power) and bottom position for low gain (greater than around 100 µW of beam power). These may be omitted and replaced with a jumper wire if the desired position is known.

  25. ( ) Install Q1, Q2, Q3, Q4 (2N3904 transistor). Pay attention to the outline on the PCB. These will not seat flush but 1/8-1/4" off the PCB.

  26. ( ) Install SKT3, SKT4 (2 pin male-female socket strip). Although the PDs can be soldered, the use of the socket strips is recommended as the photodiodes are nearly as sensitive to failure due to heat from soldering as the LEDs. Note that there are two possible adjacent locations depending on the polarity of the PD. The one toward the center of the PCB can be used with the face of the PD pointing away from the PCB. Or the other one can be used if the PD is folded over. The anode of the PDs included in the kit is the left pin facing the front with its legs down.

  27. ( ) Prepare the photodiodes. Depending on the method used to convert the output of the interferometer to quadrature signals, Linear Polarizer (LP) or Circular Polarizer (CP - QWP+LP) film may be attached directly to the faces of the PDs. If installing the PD in the socket strip, the leads can be cut to a length of ~1/8 inch.

  28. ( ) Install J2 (6-pin header or screw terminal block). For the header, the plastic tab faces toward the center of the PCB. For the terminal block, the wiring access holes face out.

  29. ( ) Install SKT1, SKT2 (8 pin socket). Pay attention to orientation.

  30. ( ) Plug U3 (UA9637) and U4 (UA9638) into their respective 8 pin sockets. Pay attention to notch or dot. Confirm that all pins are seated properly in the socket. These parts can be damaged if installed incorrectly.

  31. ( ) Carefully inspect for solder and component lead shorts and unsoldered leads correct as needed.

  32. ( ) Smoke test #2: Apply power. Due to the hysteresis of the RS422 receivers, with no siganl the A and/or B LEDs may be on. And nothing should smoke. ;-)

  33. ( ) Initial Operational test: A constant source of light will be required. This can be a non-Zeeman laser or flashlight on max. ;-)

    1. As a rough guide for installing the jumper on JP1 and JP2, if the laser power incident on the photodiodes is expected to have a power greater than 100 µW, use the lower position. Otherwise use the upper position.

    2. Apply power. ;-)

      • Upper position: Adjust the Threshold trim-pots so their associated A and B LEDs just go out. Apply your light source to each of the photodiodes. The associated LED should go on.

      • Lower position: Adjust the Threshold trim-pots so their associated A and B LEDs just go on. Apply your light source to each of the photodiodes. The associated LED should go off.

    For an AC test, an LED flashlight with multiple brightness settings may suffice as they usually use PWM to control brightness. An oscilloscope is highly desirable as well since the A and B LEDs can only show that something is there but not what it looks like. The signal could be from a local AM radio station! ;( :)

    The idaal threshold settings will be at the mid-point of the sinusoidal optical signal from the interferometer.

Theory of Operation

For each channel, QAB2 consists of an impedance matching stage, 1 stage of amplification, a differential-to-TTL converter, TTL-to-RS422 converter, and LED driver circuit as follows:


Troubleshooting? What troubleshooting? ;-) Check for solder bridges and unsoldered leads, that the correct parts are installed, and for those with polarity, that they in the right way around. The LEDs die easily damaged from stress during soldering as noted above - at least twice! So, if the Power or Signal LED doesn't light, it may just be a bad LED. The most likely cause of low or no sensitivity is an incorrect resistor value somewhere - it's easy to accidentally misread the color bands. The parts are reliable, though the UA9637 and UA9638 may fail if plugged in backwards or if the 5 V regulator isn't regulating.

Beyond this, an oscilloscope will be desirable to be able to trace the signal. There are several strategically placed test-points for this purpose.

For friendly tech support, feel free to contact me via the link at the top of this page. ;-)