Sam's Interferometer Measurement Display System 1 (SG-MD1)
Installation and Operation Manual

Version 1.31 (30-May-16)

Copyright © 1994-2008
Samuel M. Goldwasser
--- All Rights Reserved ---

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Table of Contents


SG-MD1 is a 5.5" x 4" PCB that attaches to the REF signal from an HP/Agilent or similar two-frequency laser, and the MEAS signal from an optical reciever through an interferometer returned from a movable retro-reflector (mirror or cube-corner), and provides a visual readout of position change in increments of the wavelength of light at 632.8 nm. The intent of this system is to be used to demonstrate the principles of precision measurement based on two-frequency laser-based interferometry.

Photo of SG-MD1 PCB in Action

In a nutshell, the two-frequency laser sends out a pair of superimposed beams that differ in frequency by a relatively constant amount (called REF), one polarized horizontally and the other polarized vertically. Optics separate the two beams sending one to a (generally) fixed reflector, and the other to some tool or device whose position needs to be measured precisely. Both beams then return to an optical receiver where the difference frequency (called MEAS) is generated. With no movement, the phase relationship between the two beams is constant and MEAS equals REF. But if the tool or whatever moves, the frequency and thus phase relationship between REF and MEAS will change due to doppler shift. A phase change of 360 degrees represents a position change of 1/2 wavelength of the light from the laser, approximately 632.8 nm for the HeNe lasers most often used. The precise wavelength is nominally 632.991372 nm with a wavelength accuracy of 0.1 ppm for the typical HP/Agilent laser.

Using the two-frequency approach rather than a basic Michelson or similar interferometer, among other things makes these systems more immune to misalignment by eliminating issues of fringe counting and direction, and fringe contrast.

In the diagram below, SG-MD1 replaces the "Signal Processing" block and provides a means of demonstration the basic principles of precision measurement using two-frequency laser interferomtery. Aside from the low cost, this also allows for much more hands-on since the schematics are available and all signals are accessible for monitoring.

Principles of operation

SG-MD1 is conceptionally quite simple. It consists of dual differential line receivers for REF and MEAS, edge detection, synchronizer, 8 digit BCD up/down counter and display, UP and DOWN monitor circuit, and control PAL.

The logic inside U6 looks at the REF and MEAS edges and converts these to Up and Down pulses to the counter and monitor LEDs. Depending on mode, it may suppress strictly alternating Up and Down pulses to eliminate activity in the display when the position isn't changing. Simultaneous Up and Down pulses are always suppressed. It also provides the proper signals to implement signed-magnitude mode in the unsigned BCD counter including the Sign Overflow bits.

Required equipment to use SG-MD1

Depending on the objectives, an interferometric measurement system can be put together from commercial or home-built components. The latter would have an accuracy and precision virtually equal to that of the commercial version but would have a net cost that is a small fraction of it, and would have potentially much greater educational value.

Note that some basic soldering/assembly skills will be needed to wire up the cables to the power supplies, laser, optical receiver, and SG-MD1. If that capability is NOT available, then consider a complete system with SG-MD1 included instead.

In addition to the SG-MD1 PCB, the following must be provided:

SG-MD1 PCB jumpers, connectors, and user controls

Seven two-pin headers set up SG-MD1:

The connectors on the SG-MD1 PCB are:

The only user control is the RESET button. :-)

SG-MD1 display and indicators

The main readout is an 8 digit high brightness orange seven segment LED display.

The UP (green) and DOWN (red) monitor LEDs provide a rough indication of measurement activity. Each of these are pulsed on by a retriggerable monostable when an up or down pulse is generated. Note that the activity of these LEDs is independent of the setting of the NOSCOS jumper - they always only show activity that isn't strictly altnernating up and down. In other words, two successive MEAS pulses without a REF pulse is required for the UP LED to pulse, and two successive REF pulses without a MEAS pulse is required for the DOWN LED to pulse.

Note that when changing very slowly, both LEDs may sometimes come on simultanenously even if the counter seems to be strictly incrementing or decrementing due to even slight jitter in the REF or MEAS signals, and unequal high and low times, especially if using X2 resolution.

The default pulse width for the monitor monostables is about 1 ms, so counts in the range of 0 to 1,000 per second will result in a roughly linear variation in brightness. This can be altered by replacing C1 and C2 with different uF values, which may be desirable if motion is mostly very slow or very fast.

Selecting resolution

The resolution determines both the minimum detectable incrment in position as well as the maximum range before overflow or wraparound on the display.

Any combination of these may be used but of course, increasing resolution also reduces the maximum position change that cna be displayed (though I doubt this is of much concern!). Also, higher resolution tends to increase the effects of noise and jitter on display behavior at very slow count rates and will mostly show up in the monitor LEDs both flashing at low count rates. The recommended settings for SG-MD1 where resolution isn't critical is to bypass the edge FFs but to leave JB7 open.

Connecting REF and MEAS

The inputs on SG-MD1 are to a dual channel differential line receiver. Each input has an impedance of approximately 8K ohms (internal to the chip). Positions for 3 resistors to set levels and sensitivity are provided on the PCB. The more common configurations would be:

The HP/Agilent signals can be used single-ended (e.g., only using REF, not both REF and ~REF) but the noise immunity will be reduced.

Mini-coax or twisted pair is recommended for the REF and MEAS cables. Note that the REF and MEAS channels are processed identically on SG-MD1 so the only chnage resulting from swapping them will be to invert the direction of counting. As labeled, UP is when the frequency of MEAS is greater than REF.

Setting up the interferometer

Any of the interferometer configurations can be used with SG-MD1. These are shown in:

What optical components are available will influence which configuration is most appropriate and easiest to implement.

Some options for the "Tool" in a demonstration system include:

A sample diagram of a configuration using a plane mirror interferometer and voice coil poisitioner with SG-MD1 is shown in:

The function generator provides a means of moving a mirror by a very small and well controlled amount, with its signal monitored by an oscilloscope. This could be done in a variety of other ways, including simply a micrometer adjustmentable linear stage.

A version using a plane mirror interferometer is shown, because that makes more sense for attaching to a loudspeaker than a bulky cube-corner.

And the butcher block version (pre-SG-MD1 using a frequency counter):

Photo of Two-Frequency Interferometer Laser Tester

Aligning the interferometer

Power the laser and optical receiver and wait for the laser to come READY.

With genuine HP/Agilent optics, it's very easy to get all this working together. Even modest misalignment can be tolerated, though the signal quality may degrade somewhat. Once everything is aligned, make sure it's all locked down. Except for the remote reflector (cube-corner or mirror), nothing else should move!

Initial test of SG-MD1!

Once the laser, optics, and optical receiver are set up, it's time to connect SG-MD1.

Once proper operation has been confirmed, the entire system can be powered at the same time. Then, when the laser comes READY, just press the RESET button.

My present setup includes both SG-MD1 and an HP-5508A.

Photo of Sam's Interferometer Laser Tester with SG-MD1 and HP-5508A

From left to right are frequency counter on top of 5 VDC power supplies for SG-MD1 and the buffer amp for the loudspeaker, SG-MD1 on top of the HP-5508A, my power supply for 5517A lasers (presently unused), and function generator on top of oscilloscope. The readouts differ both because SG-MD1 displays half-wavelengths compared to the HP-5508A's microinches, and they weren't initialized identically.


A large variety of problems can be caused by issues with DC power. So, first check that the +5 VDC and GND connections are secure and that the supply voltage is within +4.75 to +5.25 VDC from no load to max load. Install the Lamp Test (LT) jumper to turn on all segments of the display to get close to max load.

Most other problems are related to either REF and/MEAS signal integrity, or interferometer alignment.

Some common problems and solutions:

If the solution to your problem isn't in the above list, performing the following static logic tests on SG-MD1 will go a long way toward confirming that it isn't at fault. In addition to the 5 VDC power supply, this will require a pair of pushbuttons with debounced TTL-level outputs:

  1. Configure the resistors on the REF and MEAS inputs for single-ended TTL.
  2. Configure the jumpers as desired.
  3. Attach your debounced pushbutton outputs to the REF and MEAS connectors. (Don't forget GND!)
  4. Apply power and press RESET. The display should read 0 or 00,000,000 depending on the settings of LZS and SGNMAG.

It may be desirable to install 10 uF capacitors across C1 and C2 so the monitor LEDs will pulse for a longer amount of time with each count. (For electrolytic caps, plus should be on pins 15 and 7.) However, the ~1 ms flashes using the default capacitors should be clearly visible under reduced lighting conditions.

SG-MD1 is now running just as it would with the real REF and MEAS inputs except that you are providing them manually!

Tests of other configurations should be self evident.

If none of this is able to identify the problem, it may be necessary to use an oscilloscope to probe the internal signals.

SG-MD1 Specifications

The following assumes that the RES jumper is set for X2 and the edge FFs are present. Different settings will change the maximum measurement range in an obvious manner. Also see the section on "Selecting resolution", above.

SG-MD1 Status

A prototype SG-MD1 has been built and tested. Some minor bugs were fixed and Rev. 1.2 artwork is complete. SG-MD1 has been in extensive use in my interferometer testing rig for several years. However, times change and the µMD1 Micro Measurement Display using a microcontroller instead of discrete TTL is infinitely better and easier to build. There are therefore no plans to do anything further with SG-MD1. The End. :-)

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    -- end V1.31 --