Re: #uBITX Firmware KD8CEC - IF-Shift etc. #ubitx

Jerry Gaffke

A straightforward implementation of an SSB transmitter would be to have a sharp crystal filter,
let's assume nice even numbers of 2khz wide between 11.999000 and 12.001000 mhz.
Also a BFO that is 500 hz beyond the edge of the crystal filter, the frequency of the BFO
is the frequency of the suppressed carrier of the transmitted (or received) SSB signal. 

To create an upper sideband signal, we might use the filter to allow through only the stuff that 
is between 500 hz and 2500 hz above the carrier.  So the BFO should be at 11.998500 mhz,
and the audio frequencies that get through are between 500 and 2500 hz.

To create a lower sideband signal, we use the filter to allow through only the stuff that
is between 500 hz and 2500 hz below the carrier.  So the BFO should be at 12.001500 mhz.

The critical issue for Gerald is likely the 3000 hz difference (in this case) between the
11.998500 mhz and the 12.001500 mhz BFO.  This needs to be adjustable somehow
to account for variations in filter width and different preferences regarding how far the BFO
should be from the filter passband.  (The further the BFO is from the filter passband,
the more high frequency content the audio will have.)  

On the uBitx, the LSB/USB selection is carried out by moving the second local oscillator
between 33 and 57 mhz, the BFO is always on the low side of the 12 mhz crystal filter.
So a little bit harder to think about than the "straightforward implementation" presented above.

A fix for the code as shipped from HFSignals would be to modify the code here:
such that this value in ubitx_20.ino  can be varied within the configuration menu 
    #define SECOND_OSC_USB (56995000l)

Procedure would be to adjust the BFO using the existing configuration routines for best audio
for LSB reception, then adjust SECOND_OSC_USB for best audio on USB reception.
When transmitting, everything just runs backwards and all those relationships still apply,
so audio allowed through the 12mhz filter should be identical.

Jerry, KE7ER

On Sat, Mar 10, 2018 at 09:15 am, ge_clipboard wrote:

The sound of SSB signals with my uBITX are very strong on the high side. In LSB I can adjust the received sound with the IF-Shift with a very pleasing effect, but this does not work the same with USB. And I have noticed, that using the IF-Shift, does also influence the TX signal where the output is becoming a DSB signal with lots of power in the other sideband. I have also tried to adjust the BFO, but without positive results.

Would it be possible to have the IF-Shift function just with the RX and leave the TX on its own? What am I doing wrong?

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