'Rx BFO' setting and frequency calibration

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Joined: 2009/12/07 - 18:03
'Rx BFO' setting and frequency calibration

When I began WSPR I used to calibrate the frequency of my FT-897D using an audio waterfall program (SpectrumLab) and adjust the TCXO until I got the correct audio tone from GB3VHF on 144.430MHz. I could usually get to within 10Hz at 144MHz, equivalent to 2Hz at 28MHz where I mainly operate. But the TCXO frequency does shift when the radio gets moved and as the process was fiddly I wanted a way to maintain good accuracy without opening up the radio.

I read the linear regression method described in the user guide. However as the FT-897D derives all its SSB/CW frequencies from a single TCXO all I needed was a simple correction proportional to the operating frequency. Eventually as I only operate only one band I decided just to scale the 144MHz error to the appropriate band and offset the VFO by the required amount. The VFO operates in 10Hz steps so this way I need never have an error of more than 5Hz without ever taking the lid off. However I wondered whether I could reduce that error further with the 'Rx BFO' setting under the 'Advanced' tab?

The meaning of the 'Rx BFO' value is not fully explained in the User Guide but looking at the nominal value of 1500Hz I hoped it might be a way of adjusting the centre of the computer audio band. If this could be adjusted in 1Hz steps (and if it would also shift the Tx audio band too) then it might provide the fine tuning I was looking for.

To check the idea I tried different Rx BFO values and watched what happened to the spectrogram traces and the spot frequencies appearing in the list below it. The spectrogram traces behaved just as I imagined. Using a lower Rx BFO value (e.g. 1450Hz) shifted the traces up by 50Hz, presumably because the audio frequencies from the receiver are now 50Hz higher than what the software is now expecting. But the spot frequencies shown for incoming signals did not change.

As nobody has mentioned it before I guess this is not a bug. But can someone tell me what is happening? Is this 'Rx BFO' value what I thought it was, and if so why does changing it not move the incoming spot frequencies? If the 'Rx BFO' is something else then what is it?

- Nick