[time-nuts] Question about SoundCard stability?

David McClain dbm at refined-audiometrics.com
Tue Oct 12 23:48:56 UTC 2010


Hi,

I have a Flex-3000 receiver, running freely on its internal TCXO (0.1  
ppm). I have been recording the reported deviations in the  
measurements of the 100 Hz sizeband of 10 MHz WWV all day long. I do  
this in AM detection mode, to remove any variations due to the wander  
in the radio LO. Recording from the 48 kHz audio stream and into  
SpectrumLab for analysis.

After taking out the measured frequency error in the  
"soundcard" (which I believe is the Flex Radio internal CODEC), of  
17.5 mHz, I'm seeing frequency deviations of 0.2 mHz RMS, and +/- 1  
mHz p-p, with no measurable long term drift. The FFT uses a bin size  
of 11.44 mHz. SpectrumLab interpolates to the peak assuming high SNR  
and a Hann window, with no nearby interference.

This implies that my "soundcard" is giving me a stability of 0.004  
ppm, or around 10^-9. How can this be? I already know that the TCXO  
wanders about by as much as +/- 1 Hz at 10 MHz due to temperature  
variations in the room. (10^-7). If that TCXO were used to derive the  
soundcard (CODEC) clock then I should be seeing variations of 25x  
larger. And I can assure you the CODEC is *not* being driven by an  
OCXO or GPSDO of any kind.

Anyone?

Dr. David McClain, N7AIG
Chief Technical Officer
Refined Audiometrics Laboratory
4391 N. Camino Ferreo
Tucson, AZ  85750

email: dbm at refined-audiometrics.com
phone: 1.520.390.3995
web: http://refined-audiometrics.com





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