[time-nuts] HF frequency counting receiver

[Graham / KE9H]

Most of the folks doing the FMT these days use some sort of audio
spectrum analyzer program and estimate the frequency using that.

Or use the audio spectrum analyzer to measure the difference between
the frequency being measured and the precision reference. You are
correct, it is usually not a classic counter.


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.

Well, even if the synthesizer is GPSDO referenced and locked, a digital
synthesizer
will have a minimum step size or resolution that it can generate.
You might not be able to hear it with your ear, but it is there.
For instance a DDS based synthesizer with a 32 bit tuning word and a 200 MHz
sampling clock will have a step size of 46 milliHertz.
So when you enter a decimal frequency into the synthesizer, you get the
closest frequency it can generate, which can have an error of up to +/- 23
milliHertz. And the actual error versus frequency entered breaks into sort
of
a Moire pattern if you plot it.  The errors are deterministic, but a user
normally does not have the
information to figure them out.

The ionospheric Doppler will spread the signal a few tenths of a Hz,  YES.

so getting millihertz is more random luck of the draw. NO,
it is all about how good your averaging method/strategy is, over the period
of the test measurement.
A lot of the Doppler error can be averaged out.
Some of it is an actual net vertical movement of the reflection point, as
you said, worst at sunrise/sunset.

Look at the scores/accuracy for some of the recent frequency measurement
tests.

--- Graham

==

On Mon, Jun 20, 2016 at 12:16 PM, jimlux <jimlux at earthlink.net> wrote:

> 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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