how to set the FM TX deviation with an SDR
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Recently a question was raised about the optimum TX audio for digital modes on 2m FM. Great question.
The old guideline was to listen to your audio with a second FM radio (HT will work fine)? ?? "the sound from any of the sound card digi modes should be about the same loudness of a human voice with a microphone" That is a pretty rough guideline, and not a very accurate nor precise guideline.
First, some modes sound louder than others: the Olivia, THOR, MFSK modes are simple single-pitch sine waves but always played one tone at a time at a specific duration of each tone (the baud is tone per second).? Usually the pitch is 500 Hz to 2500 Hz for the multiple tones. So the tone duration (seconds) is simply? 1/baud. Longer duration tones are lower baud values and are easy on the ears; THOR, Olivia, MFSK Thus? each THOR 11 tone is twice as long as a
THOR22 tone, as well as a smaller bandwidth for the mode.
The psk type modes (PSK31, PSK63, 8PSK and VARA)? are changes in both phase and amplitude, so the "loudness" of phase-shift modes can be deceiving based on human ears. Humans hearing is more sensitive to sounds from
2000 - 3000 Hz, than sounds at 500 Hz, so higher pitch sounds
of any mode will appear louder than lower pitch sounds.
The optimum TX audio is based on the? deviation on an FM transmitter. Can we measure the deviation? (directly, or indirectly) and not the loudness based on human hearing and speaker quality.
Yes. If you have access to an SDR that can look at the RF spectrum (I will pick 2m FM) then adjust the amplitude of the TX audio until you get the following picture on your SDR app.? I am using an RSP (hardware) and usb cable and
RSP Spectrum Analyzer (software) with Windows 10.
For this test I? set the vfo on the radio (Icom 2820) to 145.690 MHz on an Icom dual-band FM rig. My sound "card" is a Masters Communications DRA
SR? feeding TX audio into the radio's? 6 pin MINI DIN data
jack. I set the Fldigi TUNE with the cursor to? 1500 Hz? in the waterfall. Be sure to turn OFF any TxID signal sounds, and turn off any PL tones - these will mess up the image. Don't try this on a analog voice repeater (that
needs PL tones).
Look at the RF Spectrum of the? 1500 Hz TUNE ? - note the carrier (@145.690 MHz) and the multiple sidebands. We get this picture (multiple sidebands) on FM, but not on AM or SSB. Louder TX audio results in more sidebands (and a drop in the carrier amplitude). However, on FM, you get the same POWER? (watts
to antenna) independent of the TX audio level.
When the amplitude of the ? pair of 1st sidebands?? matches the amplitude (height) of the original carrier --- STOP. Mark the TX audio levels (sound card TX setting, FLDIGI TX attenuator, Windows Speaker output).
The deviation is the product of the ? "modulation index"?? and the audio pitch in kHz. So at this level of TX audio, the modulation
index is about? 1.5? and the audio pitch is? 1.5 kHz? (1500
Hz) so the?? deviation is about? 2.25 kHz.?? That is about ideal.? You won't over modulate
and distort the sound card digi modes.
Questions?? contact me at
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What I've never understood is why the SDR receivers that I've used do not offer a window that looks like an oscilloscope display of what would be the equivalent of discriminator audio, like the scope on an old Cushman service monitor.? Then you could read the deviation right off the scope graticule, and observe any clipping, distortion, hum, etc.? SDR's have all this super-duper processing going on, so why use some of that computational oomph to show an oscilloscope view of the discriminator audio?
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Paul, ad7i
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K3EUI Barry
ad7i RADIO