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
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.
