Hi Bill,
On 1/9/2017 5:28 PM, Bill Somerville wrote:
> I have just been looking at a problem with FeqCal when FTol>DF, I have a
> fix for that but I also noticed that, when I connect another WSJT-X
> instance as a tone generator back to back (digital loopback) with an
> instance running FreqCal, I see slightly broader peaks from the DFT for
> all but a few frequencies. These cause the peakup routine be less
> accurate. For the following tone frequencies, and probably others:
>
> 500Hz 750Hz 1000Hz 1250Hz 1500Hz 1750Hz 2000Hz 2250Hz ... 4500Hz
>
> I see accuracy>3DP but most other frequencies I see accuracy to only
> ~1.5DP.
This is one of the ways in which my initial implementation of FreqCal
mode uses quick-and-dirty code. Steve's explanation is surely correct;
a windowed FFT would not show these (potentially undesirable) effects.
But you should not be fooled by seeing a frequency "measurement"
displayed to 3 decimal places. The bin width for a 55296-point DFT (or
FFT) with 12000 Hz sample rate is 12000/55296 = 0.217 Hz, so there's no
way we're going to get accuracies much better than 0.2 (or maybe 0.1)
Hz. This is by design; since we're using HF-propagated signals, which
can have Doppler shifts or spreads greater than 0.2 Hz, there is little
point in doing longer FFTs. Much better to do as FreqCal does it now:
do many FFTs each of length several seconds, and see how consistent the
results may be.
Still, it will probably be best to put a window on the FFT.
-- Joe
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