In preparation for the arrival of a Rb GPSDO here, I have been
watching the 100 Hz sideband on 10 MHz WWV all day long, and
recording its reported variations -- presumably due to CODEC clocking
jitter and drift.
With a sample rate from the Flex3K of 48 kHz, and an effective FFT
resolution of around 11 mHz per cell, I have been watching this on
SpectrumLab, and find frequency deviations of around 0.2 mHz RMS, and
+/- 1 mHz p-p. The fine-scale resolution comes from SpectrumLabs
interpolation routines for finding peak frequencies.
Sounds pretty good -- but this implies an accuracy in the CODEC clock
of around 0.004 ppm !! (10^-9). Furthermore, I see essentially zero
long-term drift at this resolution.
Now, how can this be? The Flex3K, if it used its internal 0.1 ppm
clock as the clock for the CODEC, should give about 25x worse than
this. And if it isn't using the internal TCXO for the CODEC, then why
aren't we using the same superb CODEC clock for driving the whole rig?
Just doesn't make any sense to me...
- de Dave, N7AIG
Dr. David McClain
Chief Technical Officer
Refined Audiometrics Laboratory
4391 N. Camino Ferreo
Tucson, AZ 85750
email: d...@refined-audiometrics.com
phone: 1.520.390.3995
web: http://refined-audiometrics.com
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