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.
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.
Cheers,
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
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-boun...@febo.com [mailto:time-nuts-
boun...@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: d...@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: d...@refined-audiometrics.com
phone: 1.520.390.3995
web: http://refined-audiometrics.com
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