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This report describes how to build the electronics for a resolver-based absolute angular position encoder for use on a telescope. This system is now in use on the VLA radio telescope azimuth and elevation positioning. The circuits are ideal for upgrading an existing encoder system to one with higher resolution or higher reliability. The report includes schematics, board designs, program source code, and excruciating detail about the design.

Be sure to check Appendix E - Errata. All the errors here have been corrected in the Production Version.

This upgrade to the VLA encoders provides 0.08 arcsecond (0.00002 degree) resolution at low power and high reliability while minimizing RFI emission.

Printable version, but no schematics or board designs (download these seperately): encoder.ps 385KB.

VLA Technical Report No. 76

Absolute Position Encoder Electronics Upgrade

Bob Broilo

February 18th, 2000

Contents

1. Introduction
2. Design Goals
- 2.1 "Bit 13" Errors
- 2.2 "PN3DB" Errors
- 2.3 "Flipper Bit" Problems
- 2.4 Serviceability
- 2.5 Other Improvements
3. Basic Design
- 3.1 Design Problems
4. Final Design
- 4.1 Encoder - Coarse System
- 4.2 Encoder - Fine System
- 4.3 Encoder - Fiber Transmitter
- 4.4 Data Receiver/Buffer - DRB
5. Performance
6. Retrofit Process
7. Adjustments
8. Further Improvements
9. Electrical Specifications
10. References
Appendix A - Schematics
Appendix B - Program Listings
Appendix C - Printed Circuit Boards
Appendix D - Parts List
Appendix E - Errata
Production Version