[time-nuts] Question about SoundCard stability?

David McClain dbm at refined-audiometrics.com
Wed Oct 13 03:20:01 EDT 2010


Sorry, we must be talking past one another... I'm measuring the 100  
Hz sideband from 10 MHz WWV in AM demodulation. I should see about 5  
mHz cyclic drift, and I wasn't seeing it. I now know that is because  
the FFT bin size was about 11 mHz wide and the "interpolated" peak  
frequency assumes zero drift, infinite SNR, and no nearby  
interference. Hence the reported values are totally bogus in this case.

Once I resized the FFT bins to around 1 mHz, I could see the 5 mHz  
cyclic drift, reduced in amplitude by averaging effects.

Quandary solved... point of note regarding interpretation of  
SpectrumLab reported frequencies taken...

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 23:08, mike cook wrote:

>
>
> Le 13/10/2010 03:41, David McClain a écrit :
>>
>> Hi,
>>
>> No you aren't missing anything... except that I *DON'T* see the 5  
>> mHz drift that one should expect. Hence my contention that relying  
>> on interpolated frequencies from FFT peaks and their adjacent bins  
>> is bogus when the FFT bin size is as large or larger than the  
>> expected drift.
>>
> Am I being naive? I thought the point that Bob made in the original  
> reply, though unstated explicitly, was that if you are measuring  
> the signal against the same reference being used to generate it,  
> then you are only measuring the noise.  The drift will be masked.
>> Somehow that seems like it violated the uncertainty principle in  
>> the first place. And now I'm seeing that the principle rules supreme.
>>
>> I'm going to do another run, with a bin size about 1/4 the  
>> expected drift, to see if the reported "interpolated" peak  
>> frequencies really do show the drift one should expect.
>>
> I therefor don't expect that you will see any significant  
> difference and will not detect drift.
>> Cheers,
>>
>> 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 18:25, Randall Prentice wrote:
>>
>>> 1 Hz in 10Mhz is about the same ratio as 5mHz in 64Khz.
>>>
>>> This would make sense if the Clock for the A/D is divided off the  
>>> same
>>> TXCO.
>>>
>>> Or am I missing something.
>>>
>>> The reason I jumped in,  in a recent frequency measuring contest the
>>> winner was using that interpolation for his results.
>>>
>>> Regards
>>> Randall ZL2RJP
>>> -----Original Message-----
>>> From: time-nuts-bounces at febo.com [mailto:time-nuts- 
>>> bounces at febo.com] On
>>> Behalf Of David McClain
>>> Sent: Wednesday, 13 October 2010 2:18 p.m.
>>> To: Discussion of precise time and frequency measurement
>>> Subject: Re: [time-nuts] Question about SoundCard stability?
>>>
>>> Ahh.. so... Now since my TCXO is drifting to and fro by 1-2 Hz over
>>> the period of 45 minutes, why don't I see similar drift in the  
>>> 100 Hz
>>> audio signal, down around 5 mHz amplitude?
>>>
>>> This happens to be about the same size at the FFT bins. So I am led
>>> to conclude that "interpolated" peak frequencies are a bogus
>>> technique, and you can only truly count on variations on the  
>>> order of
>>> the FFT cell size as being measurable...
>>>
>>> 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 17:57, Bob Camp wrote:
>>>
>>>> Hi
>>>>
>>>> Yes it is a reasonable expectation as long as you don't have a lot
>>>> of drafts. A good TCXO can get down to sub 0.1 ppb over that  
>>>> period.
>>>>
>>>> Bob
>>>>
>>>>
>>>> On Oct 12, 2010, at 8:21 PM, David McClain wrote:
>>>>
>>>>> Okay, perhaps I should rephrase the question... Is is reasonable
>>>>> to expect a TCXO to perform at 4e-9 over the FFT window period of
>>>>> about 87 sec? I can only imagine that the enormous (87 s)
>>>>> averaging period is making my measurements look so good.
>>>>>
>>>>> 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
>>>>>
>>>>>
>>>>>
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