[time-nuts] Question about SoundCard stability?
dbm at refined-audiometrics.com
Wed Oct 13 07:44:43 UTC 2010
OTOH, I do see the virtue in the phase examination approach... A
longer FFT would average over any short-term variations in a
cyclostationary process. If you take shorter FFT's you can catch the
signal in the act of drifting, and perhaps use the phase examination
to augment your frequency estimate. However, if the signal is
drifting, you'd have to account for that in the phase advancement too...
Dr. David McClain
Chief Technical Officer
Refined Audiometrics Laboratory
4391 N. Camino Ferreo
Tucson, AZ 85750
email: dbm at refined-audiometrics.com
On Oct 12, 2010, at 23:25, John Miles wrote:
>> 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.
> Well, not entirely -- it's common enough to see FFT applications that
> compute frequency readings at sub-bin precision by tracking atan
> (Q,I) across
> multiple time records. That is a well-defined thing to do, since the
> relationship between the time-record length and the period of the
> signal in a given bin is what's ultimately being measured. But
> this sounds
> like a case where the readings reported by the software are based on
> assumptions that aren't valid.
> What is the connection between the Flex 3000 and the PC like?
> Where does
> the "48 kHz" rate you mentioned come from, exactly? If, for
> instance, the
> 48 kHz is some fraction of the same TCXO that's driving the baseband
> conversion in the receiver, then it could make sense if the frequency
> readings appear mysteriously constant. The "drift" would be in the
> wall-clock duration of the time record in this case, influencing
> the true
> frequency of the FFT bin in ways the software doesn't know about.
> In other words, as far as SpectrumLab is concerned, the frequency
> with bin 123 of a 1024-bin record at 48 kHz is exactly
> 2882.8125000... Hz,
> because it's assuming that the 48 kHz sample rate is also exact.
> If the
> latter isn't true, and it won't be, then the former won't be true
>> 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.
> It doesn't work that way (or shouldn't, at least, if they are
> claiming to
> report true peak-frequency readings).
> -- john, KE5FX
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