ubitx V3 with panadapter #ubitx

Impulsive.force@...
 

I have a V3 ubitx. I have added the I.F (45Mhz) 50Mhz buffer board (https://ubitx.net/2018/07/21/full-sdr-receive-with-g4hup-buffer-amp/) and taken this to an SDRplay RSP1A. I have added a USB hub and soundcard (soundblaster) in with the ubitx so a single USB lead runs the radiono and the soundblaster.
This arrangement means I can switch the audio into FLDIGI between the soundblaster or the RSP1A and I have full CAT control of the ubitx and RSP1A. The RSP1A has its LO set to 45Mhz and USB for digital modes.
The ubitx is as supplied and I have not changed the calibration etc

I have noticed that the 'audio frequency' of a known BPSK31 signal appears at a slightly different frequency between the soundblaster and the RSP1A. I think it is about 200Hz but might be 700Hz as its difficult to measure it without an actual fixed reference. Also for instance, if I tune the ubitx to a morse signal, the signal is not in the same place via the RSP1A.

Does anyone know why there is (appears to be) this difference? I cant see a reason from the ubitx documentation

Thank you for any explanation

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