Tone testing of two-way / amateur radio passband

less than 1 minute read

Audio spectrum sweep in 30 second steps consisting of: 400Hz, 1200Hz, 2100Hz, 2300Hz, 2500Hz at -3dB.

A typical test would consist of using an inexpensive digital VOM to test audio bandwidth of a transceiver.

  1. calibrate for non-ideal sound card and VOM using 0 dB reference. Play back the tones INTO THE DESIRED LOAD, and note the voltages at each tone.
  2. Measure loss at discrete frequencies added by the audio circuitry.
dBm at 600 Ω Volts RMS
-20 0.08
-15 0.14
-12 0.19
-10 0.24
-8 0.31
-6 0.39
-4 0.49
-2 0.65
0 0.77

Of course, you might calculate things yourself by realizing that:

dB = 20*log10(V/V0)

W = 10((dBm-30)/10)

V = √(P*R)

The important thing is amount of level change vs. frequency, rather than absolute level (unless you can’t get proper output on any frequency).

Audio passband graph NBFM 12.5kHz
Real audio bandpass of 12.5kHz RF bandwidth FM transceiver.

Audio response significantly worse than this graph might cause trouble with digital modes, especially digital SSTV, which uses 2.3..2.5 kHz bandwidth. You also would experience power dropoff on PSK31, which should be compensatable with audio level changes.

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