Optical Receiver 3 (OR3)

For Heterodyne Interferometers using Two Frequency Lasers

Assembly and Operation Manual

Version 1.04 (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.


SGOR 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 SGOR3. ;-)


Note: Local links and clickable graphics open in a single new tab or window depending on your browser's settings.

An optical receiver (which may be abbreviated as simply "OR") when used in the context of heterodyne interferometry is the component which converts a laser beam consisting of a pair of orthogonally-polarized components at slightly different optical frequencies to an electrical signal at their difference frequency. (ORs can also be used with any optical input having an AC component such as an amplitude modulated beam.) Traditionally, commercial 10780 ORs from HP/Agilent/Keysight (or less commonly, 1031s from Excel) have been included with the "DIY Interferometer Displacement Measurement System Kits". However, not building the ORs has always been considered as sort of cheating since nearly everything is, well, DIY. And the commercial ORs are becoming harder and more expensive to come by. ;-) In addition, they require 15 VDC and may not work properly with the 12 VDC used in other parts of the kits, necessitating the addition of a voltage boost converter. And there is that unobtanium 4-pin BNC connector which means wiring is somewhat of a kludge.

OR3 is on a 1 inch by 2.75 inch PCB and remedies most of these deficiencies. The optical input is a beam up to ~3 mm in diameter (using the default photodiode) with an optical power from <10 µW to >1 mW. While OR3 as tested doesn't have the frequency response of the commercial ORs, its >3 MHz bandwidth is more than adequate for systems using the kit lasers. 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. ;-)

A case with a copper foil liner or wrap for shielding could easily be fabricated for this using that lonely 3-D printer eagerly awaiting something to 3-D print. ;-)

The actual SG-OR3 PCB is four layers with internal power and ground planes. This does appear to have better sensitivity without stray noise pickup than a two layer PCB with the same component placement but no planes. The gain can be set at the half way point and still be quiet with no signal without shielding. And the sensitivity there is better than 2 µW at 3 MHz. The populated SG-OR3 PCB is shown below. It may look HUGE in your Web browser. Don't be fooled. ;-)

Typical Populated OR3 V1.02 PCB


Parts List

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

 Prt  Description                  Comments
  -   PCB, SG-OR3 V1.02            First released version

  D0  Diode, 1N4007                Reverse polarity power protection
  D1  Diode, 1N4148                Signal LED network
  D2  Diode, 1N4148                Signal LED network

  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            +9 Vaa bypass
 C10  Capacitor, 0.1 µF            +5 Vcc bypass
 C11  Capacitor, 0.1 µF            1st stage feedback
 C12  Capacitor, 10 nF             1st to 2nd stage coupling
 C13  Capacitor, 10 nF             2nd to 3nd stage coupling
 C14  Capacitor, 10 nF             3nd stage to RS422 Rcvr coupling
 C16  Capacitor, 0.1 µF            RS422 receiver inverting input bypass
 C17  Capacitor, 0.1 µF            Output coupling
 C18  Capacitor, 0.1 µF            Output coupling
 C19  Capacitor, 10 nF             SIG LED network coupling
 C20  Capacitor, 1 nF              2nd stage frequency compensation
 C21  Capacitor, 1 nF              3nd stage frequency compensation

  R0  Resistor, 36, 1/8 W          PWR LED current limiting

  R1  Resistor, 2.7K, 1/8 W        1st stage power
  R2  Resistor, 100K, 1/8 W        1st stage bias
  R3  Resistor, 36K, 1/8 W         PD load
  R4  Resistor, 10K, 1/8 W         1st stage load
  R5  Resistor, 100K, 1/8 W        2nd stage bias
  R6  Resistor, 15K, 1/8 W         2nd stage bias
  R7  Resistor, 2.2K, 1/8 W        2nd stage collector load
  R8  Resistor, 220, 1/8 W         2nd stage emitter load
  R9  Trimpot, 10K                 Gain adjust
 R10  Resistor, 100K, 1/8 W        3rd stage bias
 R11  Resistor, 15K, 1/8 W         3rd stage bias
 R12  Resistor, 2.2K, 1/8 W        3rd stage collector load
 R13  Resistor, 220, 1/8 W         3rd stage emitter load
 R18  Resistor, 10K, 1/8 W         RS422 receiver bias
 R20  Resistor, 1K, 1/8 W          LED current limiting

  U1  LM78M09, IC, Regulator, 9V   Vaa 9 V regulator (SMT)
  U2  LM78M05, IC, Regulator, 5V   Vcc 5 V regulator (SMT)
  U3  IC, SN75179B                 RS422 receiver/driver

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

  J2  Header/shell/pins or screw   Signal output
       terminal block, 3 pin

 PD1  Silicon photodiode           Optical sensor

 PWR  LED, blue                    Power LED
 SIG  LED, green                   Signal LED

  Q1  Transistor, 2N3904           1st stage (impedance matching)
  Q2  Transistor, 2N3904           2nd stage (gain)
  Q3  Transistor, 2N3904           3rd stage (gain)

 SKT1 Socket, 8 pin                For SN75179B
 SKT2 Socket, 2 pin                For photodiode

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 OR3) 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 OR3 Version 1.2

The schematic for OR3 is shown below. (Schematic version numbers are not the same as PCB version numbers.)


OR3 Version 1.2 Schmatic

The graphic below shows the general appearance of the PCB with most of the parts installed for the three levels of gain:


SG-OR3 PCB Version 1.02: 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-OR3 V1.02 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. Also note that if OR3 is used near maximum gain to be able to detect a very low level optical input (<2 µW), it may need to be installed in a shielded box (not included).

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

    Inspect the parts closely, especially the (yellow) ceramic capacitors as they may all appear 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, brown-black-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.

  12. ( ) Install C7, C8, C10, C11, C17, C18 (0.1 µF).
  13. ( ) Install C12, C13, C14, C19 (10 nF).
  14. ( ) Install C20, C21 (1 nF).

  15. ( ) Install R1 (2.7K ohms, red-violet-red).
  16. ( ) Install R2, R5, R10 (100K ohms, brown-black-yellow).
  17. ( ) Install R3 (36K ohms, orange-black-orange)
  18. ( ) Install R4, R18 (10K ohms, brown-black-orange).
  19. ( ) Install R6, R11 (15K ohms, brown-green-orange).
  20. ( ) Install R7, R12 (2.2K ohms, red-red-red).
  21. ( ) Install R8, R13 (220 ohms, red-red-brown).
  22. ( ) Install R20 (1K ohms, brown-black-red).
  23. ( ) Install R9 (10K ohm trim-pot, labeled 103).

  24. ( ) Install D1, D2 (1N4148 diode). Pay attention to polarity.
  25. ( ) Install SIG (green LED. The longer lead goes to the left and the flat goes to the right. See note above with respect to soldering LEDs.

  26. ( ) Install JP1 (3-pin jumper block). The jumper shold be placed on the top position for high gain and bottom position for low gain. JP1 may be omitted and replaced with a jumper wire if the desired position is known.

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

  28. ( ) Install SKT2 (2 pin male-female socket strip). Although the PD can be soldered, the use of the socket strip is recommended as the photodiode is 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.

  29. ( ) Prepare the photodiode. Cut a ~4x5 mm piece of the circular polarizer (CP) sheet to attach to the front of the photodiode. The CP sheet has protective film on both sides. Carefully peel it off. One side of the sheet has adhesive to stick to the PD. Press it firmly in place. The polarization axis is at 45 degrees so it can be attached with the edges aligned with those of the PD. For installation in the solder strip, the leads of the PD can be cut to a length of ~1/8 inch.

    In the trivial triviality department, the front face of the PDs in the kits will have an "X" shaped rainbow appearance due to the behavior of the Quarter WavePlate on the back of the CP. Why might this be?

  30. ( ) Install J2 (3-pin header or screw terminal block). For the header, the plastic tab faces toward the center of the PCB. For the terminal block, the access holes face out.

  31. ( ) Install SKT1 (8 pin socket). Pay attention to orientation.

  32. ( ) Plug U3 (SN75179B) into the 8 pin socket. Pay attention to notch or dot. Confirm that all pins are seated properly in the socket. The SN75179B can be damaged if installed incorrectly.

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

  34. ( ) Smoke test #2: Apply power. Nothing should smoke. ;-)

  35. ( ) Operational test: A varying source of light or a two frequency laser with a split frequency of 3 MHz or less will be required. If a laser isn't immediately available, an LED flashlight with adjustable brightness may suffice as they usually use PWM to control brightness. (with a flashlight, the SIG LED may not be that bright due to the low PWM frequency.) An oscilloscope is highly desirable as well since the SIG LED can only show that something is there but not what it looks like. The signal could be from a local AM radio station! ;( :)

    With no optical input, the Signal LED should be off if the Gain trim-pot is not above around 1/2 max (with both gain stages populated). But a human hand near the input introduce enough signal to set it off, so move out of the way. Or consider the response to your AC personality to be a sufficient confirmation that at least most of the circuit works. ;-) With a laser or other signal source, the LED should be on. Attaching the output to µMD1 or µMD2 would be a confirming test.

Gain Options

OR3 as configured above has two gain options selectable by the jumper on JP1 which differ by approximately a factor of 10. The high gain setting with the jumper in the upper position results in a usable response down to below 2 µ at 2 MHz but may be more prone to pickup of RFI. The low gain setting with the jumper in the bottom position should be usable with beam power above around 25 µW and should be quiet at any setting of the gain trim-pot. One or both of the jumper settings should be satisfactory for most applications. And OR3 has been tested from below 2 µW to above 1 mW without moving the jumper or adjusting the trim-pot.

However, where it is known that the beam power will be high or in an electrically noisey environment, eliminating the second gain stage may be useful. See OR3 Gain Options which describes these in more detail. BUT AGAIN, THERE IS USUALLY NO NEED TO USE ANYTHING BUT THE DEFAULT, ABOVE.

Theory of Operation

OR3 consists of an impedance matching stage, 1 or 2 stages of amplification, a differential-to-TTL converter, TTL-to-RS422 converter, and SIG 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 SN75179B 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. ;-)