That's not precisely true. You can get a frequency estimate that is substantially more precise than 1/T if the snr is high. Consider super-resolution in an interferometer which is mathematically similar. What you give up is ambiguity. Probably one of the oldest techniques is that of Prony, but there are lots of others
On Oct 12, 2010, at 10:52 PM, David McClain <[email protected]> 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-binq 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: [email protected] > 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 <[email protected]> >> To: [email protected] >> 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: [email protected] >> phone: 1.520.390.3995 >> web: http://refined-audiometrics.com >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
