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Adjusting your BFO offset using audio spectrum analyzer software

In adjusting the BFO  you have to be careful to not transmit both the signal AND the carrier.  If the filter has been shifted too far, it is possible to have the carrier transmitting unintentionally. Roman K7TXL found this out while trying to transmit FT8 signals.  He found two spikes on his SDR.

In his experience he has found that the only reliable approach, that doesn’t involve complex test instruments, is to download and use an audio spectrum analyzer program.

Roman uses SpectrumView by WD6CNF.

SpectrumView software

WD6CNF Audio Spectrum Analyzer runs on the Windows operating system and has the following features:

  • Analyzes audio from 10 Hz to 20 kHz
  • Takes an input from a microphone or wave file
  • Has variable displays
    • Fast Fourier Transform (FFT) display
      • Variable sample rates (8000 Hz, 11025 Hz, 22050 Hz, 44100 Hz)
      • Variable transform sizes (1k, 2k, 4k, 8k)
      • Upper and lower limits adjustable
      • Continuous, averaged, peak hold
      • Selectable foreground/backgrounds
      • Variable markers (2)
      • Save a reference plot, compare with the foreground plot
    • Time display (oscilloscope)
      • Triggered sweep
    • Waterfall display (color or B/W)

Alignment procedure from Roman K7TXL

  1.  Disconnect antenna from ubitx, or better, find an unused part of the band with just hiss, no other signals
  2. Run audio out from ubitx into PC/Mac/Linux audio in
  3. Run spectrum analyzer and configure it to listen to the audio in
  4. Turn up volume on ubitx until levels are reasonably high but not clipping in the spectrum analyzer
  5. For the ubitx 5 (not the earlier versions), you will want to set the BFO somewhere around 11.055-11.056 MHz as a starting point.
  6. I am going to presume that you started with the BFO frequency on the high side, say 11.057 and you are lowering it. As you do so, you should see on the spectrum analyzer that the noise’s high frequency roll off frequency increase. At some point it will no longer get any higher no matter what you do with the BFO value. The point you want is right as you touch that max value. You might have to back off and watch the max value decrease, then adjusting again until you find the maximum.
  7. Save the BFO frequency using the PTT where you get maximum high frequency response. Roman found that the BFO frequency was about 1.5-2 kHz lower than the frequency where he got the most “bass” response in his headphones, thus the final result had less bass response.
  8. If you have an SDR, you can double-check your results: Run ubitx into a dummy load and have an SDR with separate tiny (or no) antenna nearby, either run WSJT-X “Tune”, or a single frequency of audio into the ubitx mic. The SDR will likely be sensitive enough to pick up stray RF (even with the dummy load) and you should be able to see only one, not two spikes on the display (make sure you zoom in maximum to a narrow bandwidth view).

After this adjustment, Roman has managed to make a few longer distance FT8 contacts, i.e. Seattle to Alaska on 40m.

Reference