I decided to try to work out the peak deviations in FM when transmitting from an IC-9700 and an IC-705 into a dummy load. The receiving transceiver is a SunSDR2 Pro without antenna connected. I used a software sig gen to feed both Icom radios with an audio tone via their USB audio interface.
This tone is adjusted whilst observing the FM transmission on another SDR receiver, or listening to the fundamental frequency with narrow CW filter. The injected tone’s frequency is adjusted to bring the main carrier frequency to the zero point (null) in the bessel function, the carrier at the fundamental will drop to zero on the sdr, and it will go silent on the receiver that is listening in CW mode.
- 1st fundamental crossing = 2.4048 modulation index
- Deviation = 2.4048 * AF (where AF is the audio frequency applied that results in a null carrier at fundamental RF frequency)
- Bandwidth = 2 * (max audio freq + peak deviation)
- Assume 3k peak audio frequency for voice used in bandwidth calculations
1830 Hz FIL1 = ±4.4 kHz dev
913 Hz FIL2 = ±2.195 kHz dev
FIL1 BW : 14.8kHz
FIL2 BW : 10.39kHz
1762 Hz FIL1 = ±4.24 kHz dev
889 Hz FIL2 = ±2.13 kHz dev
FIL1 BW : 14.48kHz
FIL2 BW: 10.26kHz