[time-nuts] Navsync CW-12 GPS Board - I don't believe it!!
azelio.boriani at screen.it
Wed Jan 4 20:31:23 UTC 2012
Interesting... this explains why they use a 20MHz OCXO (without EFC) in the
FTS125 to clock the receiver that has an NCO steered that clocks a PLL that
synchronize another OCXO (with EFC). Yes, I have opened up an FTS125.
On Wed, Jan 4, 2012 at 8:32 PM, Ed Palmer <ed_palmer at sasktel.net> wrote:
> 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.
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