ADVANCED LESSON 65logomidi2.gif

             
LEARNING  OBJECTIVES and NOTES
Operating Practices and Procedures - Intermodulation
8c.1 Understand how to identify whether the distant transmitter or the local receiver is producing intermodulation products.

Intermodulation occurs when two strong signals enter the early stages of a receiver, usually the receive RF amplifier or the receive mixer. One should be there, but the other is a spurious or harmonic signal. The unwanted signal mixes with the wanted signal modulating it to produce new frequencies called intermodulation products.There are two sources of the unwanted signal:

1. The distant transmitter is been overdriven, producing spurious or harmonics which are not been adequately filtered by the transmitters low pass filters. This can be corrected at the transmitter end by checking the output spectrum with a spectrum analyser and then adjusting the transmitter and low pass filters to reduce or remove the unwanted signal. The unwanted signal may also come from a different transmitter. This enters the RF stage and again modulates the wanted signal. One solution is to filter out the unwanted signal. These problems are common at repeater sites where a number of high power commercial transmitters may also be operating.

2. The receiver itself could be producing spurious or harmonic frequencies in the VFO or synthesiser. These would mix with the wanted signal to produce unwanted intermodulation products. The cure for this is to check the output from the VFO or synthesiser with a spectrum analyser for signals other than the wanted signal. Then to set about removing them by, for example, adding band pass or low pass filters between the VFO and the mixer.
Receivers can also produce intermodulation signals from the reception of two strong signals. For example you are listening on 21.000MHz. The  Rf stage receives strong signals at 100MHZ and 79MHz which mix to produce an intermodulation signal on 21MHz.

Also remember the rusty bolt effect. A rusty bolt on your gutter acts as a rectifier. Two strong signals can combine to produce intermodulation signals that are picked up by your receiver.

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