Construction Guidelines for Basic Quadrature-Sin-Cos Decoder and Quad-A-B Interface Kits

Version 1.00 (27-Mar-2021)

Copyright © 1994-2021
Sam Goldwasser
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This manual provides instructions for the construction of a quadrature-sin-cos decoder and simple interface to quad-A-B for µMD0, µMD1, or µMD2. While the optical parts (including the photodiodes) are capable of many MHz, the electronics at this point is very rudimentary. Someday this may change but for higher performance, a user-supplied (or designed) dual pre-amp and interface will be required.

There are presently two available kits:

  1. Quadrature-Sin-Cos decoder: This takes the output of an interferometer and converts it to electrical signals phase-shifted by 90 degrees.

    Quad-Sin-Cos Decoder using Variable Attenuator Plate and Circular Polarizers. Optical Layout (Left) and Typical Parts (Right)

    Quad-Sin-Cos Decoder Parts List

      Prt#  Qty Description 
      AP1    1  Variable attenuator plate used as adjustable NPBS
      CP1    2  Pieces of Circular Polarizer (CP) sheet (or cut)
      CS2    4  Microscope cover slips to attach CP pieces
      PD1    1  Photodiode for Sin/A channel
      PD2    1  Photodiode for Cos/B channel

    The photo shows the typical optical parts and photodiodes.

    R2 and R3 can serve as load resistors for the photodiodes if bandwidth is not critical, or as the feedback resistors for proper trans-impedance amplifiers. R1 is simply to protect the PDs should they be installed backwards, and the power supply should a PD fail shorted.

    Quad-Sin-Cos Interface to Atmega 328P Nano 3.0 / µMD0

    Here are the very long detailed parts lists. :) The numbering refers to schematic above:

     Prt#  Description                 Comments
      U1   LM393P, 8 pin DIP           Dual voltage comparator
      U2   UA9638, 8 pin DIP           Dual RS422 line driver
     LED1  LED, red                    Thresholded Sin, A
     LED2  LED, green                  Thresholded Sin, B
      R0   Resistor, 1K, 1/4W          Photodiode protection
      R1   Resistor, 100K, 1/4W        Sin (Channel A) load resistor*
      R2   Trim-pot, 10K               Sin (Channel A) threshold
      R3   Resistor, 100K, 1/4W        Sin (Channel A) trim-pot isolation
      R4   Resistor, 470M, 1/4W        Sin (Channel A) hysteresis
      R5   Resistor, 2K, 1/4W          Red LED current limiting
      R6   Resistor, 1K, 1/4W          Channel A pullup
      R7   Resistor, 100K, 1/4W        Cos (Channel B) load resistor*
      R8   Trim-pot, 10K               Cos (Channel B) threshold
      R9   Resistor, 100K, 1/4W        Cos (Channel B) trim-pot isolation
      R10   Resistor, 470M, 1/4W       Cos (Channel B) hysteresis
      R11  Resistor, 47K, 1/4W         Green LED current limiting
      R12   Resistor, 1K, 1/4W         Channel B pullup