On 1/4/2012 11:13 AM, Said Jackson wrote:
Hi Ed,
Since the oscillator is typically free-running in an NCO, and periodically corrected by
phase drops to stay "on frequency" the error you are seeing may be caused be
the offset in your crystal, combined with the limited digital resolution of the NCO
trying to correct for this offset.
If this is the case, your measured error would change with temperature.
Does it do that?
Yes, it does. Cooling the unit by using a heatsink causes a significant
increase in the output frequency. The change is large enough to be
obvious on an oscilloscope where one channel is the 10 MHz from the
Z3801A and the other channel is the 10 MHz from the CW12.
If yes, the NCO resolution is not fractionally corrected all the way. Replacing
the on board Tcxo with an ocxo hand-tuned to the correct frequency would help,
but that sort of defeats the purpose of the product..
Yes, it appears that the CW12 is just not appropriate for what I was
thinking of doing. However, to be fair, the 1 PPS output is much better
than the older receivers. I'll have to think about where I go from here.
Ed
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