Decreasing intermod on the Kenwood TM-733 receivers

The Kenwood TM-733A is an older dual-band mobile radio. This radio is capable of 50 watts transmit on VHF, 35 watts on UHF, full-duplex crossband repeat, and simultaneous receive on two VHF or UHF frequencies. It does have a problem, however: It is terribly sensitive to being overloaded by other signals. The effects of overload (or `intermod` - co
Decreasing intermod on the Kenwood TM-733 receivers - schematic

mmonly referred to as IMD meaning "InterModulation Distortion") manifests itself as the reception of signals that aren`t really on frequency. During a `bout of intermod, one will often hear QSOs of other repeaters (amateur and not) as well as odd bleeps and buzzes from paging and telemetry systems. Because intermod is a mix of multiple signals, you cannot experience it from just one signal - it takes at least two signals! Even before I got my own `733, I was aware of the problem: Back in early 90`s a friend of mine bought a TM-732 - the predecessor to the TM-733. It had some pretty nice features and a reasonably intelligent (I thought) operator interface. But it had terrible intermod problems: It would seem to squeal and squawk at almost any time, anywhere due to front-end overload. He asked me to look into it and see if there was something that I could do about it. For an FM receiver, the stronger the signal, the less noisy it is. When quantifying sensitivity measurements, a common standard is to have a signal with a 1 KHz tone at a deviation of 3 KHz. The "SINAD" is the ratio of that 1 KHz tone to the background noise. The "12 dB SINAD" measurement is how much signal is required to get that tone 12dB above the "popcorn" noise one hears on a weak FM signal. How noisy is a signal with 12dB of SINAD Pretty noisy - but if the other person is talking in a normal voice (i. e. not quietly or far from the microphone) 12dB of...

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