Bob Camp wrote:
HiIf you dig into the circuit, the resolution of the DDS is indeed 7x10^-13 per LSB. Bob
Thanks for the confirmation.
On Dec 9, 2011, at 9:51 PM, Bob Smither wrote:Bob Smither wrote: <snip>My documentation states that the full range of adjustment is: 7f ff ff ff => +383 Hz 80 00 00 00 => -383 HzThis scale factor is clearly wrong - using it and applying an appropriate correction changes the frequency way too much. From: http://article.gmane.org/gmane.comp.time.nuts/9473/match=fe+5680+pin+outs (thanks Jose!) a better estimate is 140000 / Hz. Using data after 20 hours: http://www.c-c-i.com/node/155 the estimated frequency offset is 1.36E-3 Hz. Using the above scale factor gives a correction of -190 counts. Here is the command I used to (temporarily) program the new offset: 2e 09 00 27 ff ff ff 42 bd (the bd is the exclusive-or checksum of the offset) Now I wait ...
After 13 hours with the above correction the offset appeared to be ~ +3E-4 Hz or 3E-11. This is close to the expected aging of the unit (2E-11 / day).
A plot showing the reduced drift from the offset correction of -190 is here: http://c-c-i.com/node/156Adding a correction for the +3E-4 Hz to the previous correction gives a new offset of -229 counts. Shortly after I got this set up, a power line glitch reset things so I took the opportunity to adjust the time interval to ~ 100 usec. I then applied the -229 counts as:
2e 09 00 27 ff ff ff 1b e4 (the e4 is the checksum) and starting measuring again. It looks like this is chasing the aging of the unit. More later. -- Bob Smither [email protected] ========================================================================== A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed. -- Second amendment to the Constitution of the united States ========================================================================== Circuit Concepts, Inc. 281-331-2744
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