Decreasing intermod on the Kenwood TM-733 receivers

Posted on Feb 6, 2014

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
Click here to download the full size of the above Circuit.

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...

Leave Comment

characters left:

Related Circuits

  • New Circuits



    Popular Circuits

    Car battery tester with LM3914
    2x300W or 600W Power audio Amplifier
    Drive Two-Six White LEDs (WLEDs) in Parallel
    mc2833 fm transmitter circuit design electronic project
    Led Running Lights
    applied robotics
    police siren pcb
    Light Sensor Switch Circuit
    control of remote domestic system
    Signal Generators Circuits
    Regency ATC-1 Converter
    circuit to test servo motor (pole rotor) of parabolic antenna
    Unity gain buffer circuit diagram of the MAX4102 4103