Digital BFO Mod: Terrible Audio! #bitx40help

Ryan Flowers

Hello Everyone,

I've been working on my own digital VFO/BFO for my BITX40. The goal: Switch sidebands without inverting the VFO to 19mhz for 40M and 26mhz for 20M. I've got it working, mostly, but I'm having one heck of a time with audio quality. The 12mhz crystals not 12mhz but rather 11,984,800 and currently I have my sidebands at 11994420 for USB and 11996450 for LSB. These are 2030 and 4060hz away from the crystal frequency, and I am having a hard time visualizing how that works. But at any rate, these are the what I came up with mostly through trial and error. 

I'm currently using a modified version of the uBITX sketch. I've added the offset frequency (11984800, named vfoOffset) and lsbCarrier and am using the following to do my tuning:

  if (isUSB){
    si5351bx_setfreq(0, vfoOffset + frequency);
    si5351bx_setfreq(1, usbCarrier);
    si5351bx_setfreq(0, vfoOffset + frequency);
    si5351bx_setfreq(1, lsbCarrier);

I've also added menu items for adjusting the USB/LSB and Offset easily, and using these I adjusted for best inbound audio and ended up with the aforementioned values. When I tune the radio to 7074mhz and select USB, I'm not getting *any* decodes on FT8- but it works on LSB... What on earth? The other thing is that audio output (as tested by transmitting into a dummy load and listening on my Kenwood) is just terribly choppy and unintelligible. I've tried varying the signal output strength of the si5351 clocks from 2ma to 8ma, no difference. It sounds terribly over-modulated.

I have gotten in over my head, but I'm determined. Can somebody help me understand this so I can not just complete this project but get a better understanding? Thanks so much.

Ryan Flowers - W7RLF
Multi Band BITX40

Join to automatically receive all group messages.