In an ongoing testing effort to get the highest speeds from VARA-FM, I have been examining the characteristics of the "9600 baud" 6-pin mini-din input on my Icom-7100. Although there is only one input pin, the treatment of the incoming signal can be switched to wideband via a menu setting.
My first experiment has been to try the Bessel zero method of determining deviation. My first experiment was to generate a sine wave at 1250 Hz to find the amplitude of the signal that would result in 3kHz deviation. The result was that the radio went into TX fault (flashing red) and stopped transmitting, due to overdriving the input signal. The carrier amplitude had not reach a null.
The next experiment was to set the amplitude of the signal to 500mVpp as specified in the manual and increase the frequency from 200Hz until I saw the Bessel Zero (minimum carrier amplitude). To my surprise, this occurred at only 227Hz, indicating a deviation of only 548 Hz (227 x 2.41).
The same experiment at 300mVpp saw the Bessel zero at about 250Hz.
I'm reasonably confident that my measurements are within 10%, although I've seen 60Hz sidebands on the carrier and audio sidebands about 30dB down. I have tested 5800Hz at the same amplitude and see the carrier, the J1 sidebands at +/- 5800Hz from the carrier, and very weak J2 sidebands at +/- 11,600 Hz, as I would have expected.
My question is that is this a reasonable result? Is the deviation really that low or should I be looking further for faults in the setup or method?
Setup: Leader AF generator. Audio voltage measured on Tek 'scope. Transmitting into dummy load next to Elecraft K3 with 2m transverter and no antenna. IF-out to SoftRock-IF plugged into Steinberg UR22C audio interface. Viewing signal with HDSDR.
73, Chris VE3NRT