Hi Jim,
Can you give any more precise hints, e.g., links to articles? You
mentioned Prony, Music, and Espirit. You caught my attention with
your analogy to interferometry...
Cheers,
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 13, 2010, at 05:27, Jim Lux wrote:
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 <d...@refined-
audiometrics.com> 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
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