On 6/20/16 7:51 AM, Nick Sayer via time-nuts wrote:
I'm considering taking a shot at the next ARRL frequency measurement
contest.
The assumption going in is that the signal is CW, with at least a
half minute or so of just solid "on" at one point or another and that
reception is reasonably good.
I've got a good TIA and excellent references, but that's the easy
part, it seems to me. It seems to me that what I really need to do is
make a synthesized heterodyne receiver that can present an accurately
tuned RF band pass - say, 10 kHz wide with the synthesizer set for 5
kHz steps - to the TIA, with some manually tunable high-pass and
low-pass filtering to isolate the signal of interest. If the mixer
got its LO from a synthesizer with a GPSDO reference, it seems to me
that you could then measure the frequency of the signal of interest
(now an audio frequency, so you can listen to it too) with the TIA
(also getting the GPSDO reference) and then do simple math to arrive
at the actual RF frequency.
Anybody have any thoughts?
Most of the folks doing the FMT these days use some sort of audio
spectrum analyzer program and estimate the frequency using that.
The signal isn't very high SNR (unless you're in Newington and they are
radiating from W1AW<grin>) - I'm not sure a narrow band filter followed
by a counter would be the best way to go.
You need a good reference to calibrate the frequency of your receiver -
depending on its design, you might just be able to measure your 10MHz
reference to figure out the offset for your receiver, and if it's stable
over the time required, you're good to go.
The ionospheric Doppler will spread the signal a few tenths of a Hz, so
getting millihertz is more random luck of the draw.
I note also that the last ARRL FMT ran at 10PM EDT with a transmitter in
California (where it was 7PM).. this is a particularly BAD time of day
to do the test, because the ionosphere is changing effective height and
attenuation so it greatly penalizes folks who are relying on skywave
propagation.
You can practice using WWV/WWVH, by the way.. nice AM signal with a good
carrier.
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