Re: Intermodulation Performance

Warren Allgyer <allgyer@...>


I may have needed to define my terms better. It was not -30 dBc but -24 dB referenced to one of the two tones used for testing. That is a figure of merit that I normally use to set drive levels and I consider it the maximum acceptable although, as you point out, there is no spec for most modern equipment and no FCC level other than "must not cause interference outside the signal bandwidth". Modern equipment like the Icom 7300 uses a general specification of -50 dBc for all unwanted emissions and this presumably includes IMD products outside the filter bandpass.

Maybe another way of looking at it: Testing on 80 meters, through a very sharp bandpass filter that limits harmonics and spurs to -60 dBc or better, I set the Drive level to produce +37 dBm on CW. I then put a single tone at -43 dBV into the audio input to get +30 dBm output on SSB. I then increased the tone input in 1 dB increments until I achieved 1 dB of compression. That happened at an audio level of -34 dBV, 9 dB higher than the level to make one watt, and the RF output with that input was +38 dBm. An increase of 9 dB of audio produced an increase of 8 dB in RF output; 1 dB of compression.

I then added a second tone at the same level and measured the difference between one of the tones and the first adjacent product: -15 dB. Opposite sideband products were less than 30 dB down from the primary tone level.

Operation of my radio at the 1 dB compression point results in unacceptable (to me) IMD as well as unacceptable (to everyone) splatter in the opposite sideband and outside the filter sideband on the desired sideband. I could not achieve my desire for -24 dB for the first IMD product with any more than -41 dBV of audio drive which produced an RF output of 1.6 watts.


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