[time-nuts] Question about SoundCard stability?

David McClain dbm at refined-audiometrics.com
Wed Oct 13 00:03:26 UTC 2010


> Best guess is that the codec in the Flex runs on the same TCXO as  
> the rest of the radio.

That's my best guess too... so how is it that I'm seeing 25x less  
variation in the audio signal than the TCXO is capable of yielding?

I have the central air conditioner cycling on and off all day (Tucson  
desert), and I can see the radio wander by as much as +/-2-4 Hz at  
times, at 10 MHz. It shows an irregular sawtooth deviation in sync  
with the cycling of the air conditioner, with a period of about 45  
minutes, 15 of which are cool down times. This is mid-day, so the  
ionosphere is not producing much in the way of Doppler shifting. And  
even if it were, that shouldn't be measurable to me at the audio  
frequencies.

Dr. David McClain
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



On Oct 12, 2010, at 16:58, Bob Camp wrote:

> Hi
>
> Best guess is that the codec in the Flex runs on the same TCXO as  
> the rest of the radio.
>
> Bob
>
>
> On Oct 12, 2010, at 7:48 PM, David McClain wrote:
>
>> 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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