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GPS-Based Frequency Reference Circuit Mods & Additions

The short term frequency stability of the GPS-Based Frequency Reference described in the March and April issues can be improved with some circuit changes, as described here. It's also quite easy to add an RS-232C serial output, so the NMEA data stream from the GPS receiver module can be fed out to a PC for other purposes.

By Jim Rowe

FEW DAYS after the March issue of SILICON CHIP had been published with the first article describing the GPS-Based Frequency Reference, an email arrived from New Zealand reader Dr Bruce Griffiths advising that the method

Click for larger image
Fig.1: adding a serial data output port to the GPS-Based Frequency Reference is easy. Here's how to do it.
used for cascading the synchronous frequency dividers IC4, IC5 and IC6 was not the best way. Correcting this would improve the divider’s reliability and hence the short-term frequency stability.

After studying the data, it turned out that Dr Griffiths was correct. As a result, I performed some "surgery" on the prototype reference, changing the divider configuration over to one which should give more reliable synchronous division. Then I set it up again and let it run for a few days, while I monitored its operation.

Sure enough the short-term stability did seem to be better, although not dramatically so. The benefits were mainly in terms of reduced jitter in the "raw" PLL error correction voltage, before filtering.

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