[time-nuts] Question about SoundCard stability?

David McClain dbm at refined-audiometrics.com
Wed Oct 13 06:06:24 UTC 2010


Indeed... when I make the FFT bins substantially narrower than the  
expected drift cycle amplitude, I do in fact see the drift cycles in  
the reported "interpolated peak frequency".

However, now that the bins are so narrow, the window duration is  
significant in relation to the cycle periods of interest (20 minutes  
per window, cool down time is 15 minutes). And so the amplitude of  
the drift is very small -- much smaller than might have been expected.

Again, I think this is due to convolving with the window period --  
taking the average deviation over the period of the window, and that  
would be much smaller than the peak deviation experienced from TCXO  
effects.

What seems amazing here is that one can derive a valid soundcard  
frequency correction by using the shorter period windows and using  
reported interpolated peak frequencies. I guess that is still  
reasonable since, when you do make the deviation visible with long  
window periods, you want those deviations to be equal-ripple about  
the zero deviation line. And using an averaged deviation works just  
as well for all averaging periods. There is a trade-off between  
accuracy and one's human patience...

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 22:52, David McClain wrote:

> Yes, indeed. I demodulate in AM mode, specifically to remove any  
> sensitivity to the LO wandering with ambient temperature.
>
> And no I wasn't seeing any variation bigger than 4 ppb with a 0.1  
> ppm TCXO wander. That's what the quandary was all about.
>
> I think I have answered the question... You cannot get around the  
> uncertainty principle, which states that your precision in  
> resolving frequencies is limited by the inverse of your resolution  
> in time. Attempting some hair-brained "interpolation" across a peak  
> in the FFT is just a mathematical game without any meaning.
>
> A *proper* interpolation in frequency space is performed by zero- 
> padding the time record. When you do that, you introduce many inter- 
> bin sidelobes. But more to the point, when the FFT bin-size is the  
> same width as the expected drift amplitude, you get a broad,  
> convolved bin content from the duration of the window, and  
> attempting to say, on the basis of adjacent bin amplitudes, that  
> you know where the frequency of *the peak* is to any better than  
> the bin-width is just nonsense.
>
> So SpectrumLab, while offering a fancy interpolated peak frequency,  
> must be interpreted with caution. What it reports can be wildly off- 
> base.
>
> 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 19:47, Mark Spencer wrote:
>
>> Just curious, if you have set your radio to AM mode to remove any  
>> variations due
>> to wonder in the radio LO, how would any minor deviations in the  
>> TCXO of the
>> radio affect the measurement ?  If you are looking at the  
>> difference between a
>> 10.000000 mhz carrier and a 10.000100 mhz sub carrier wouldn't the  
>> difference
>> always appear to be 100 hz in AM mode even if your TCXO drifted  
>> slightly ?
>>
>> You might be seeing the affects of varrying doppler shift between  
>> WWV and WWVH
>> (although I'd expect the variation to be larger.)
>>
>> Sorry it's been a long day for me and I may be missing something  
>> obvious (:
>>
>> All the best Mark Spencer
>>
>>
>>
>>
>> ----- Original Message ----
>> From: David McClain <dbm at refined-audiometrics.com>
>> To: time-nuts at febo.com
>> Sent: Tue, October 12, 2010 4:48:56 PM
>> Subject: [time-nuts] Question about SoundCard stability?
>>
>> 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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