What is Intermodulation?

Intermodulation is the "Multiplication" of one frequency by other frequencies. This occurs due to non-linearities in the input stages of receivers prior to the main I.F. filtering. A trigonemetrical identity shows that when one Sine or Cosine function is multiplied by another then the resulting function contains new frequencies consisting of the sum and difference of the frequencies of the two original functions. This process is used in all superheterodyne receivers to convert the input frequency to the I.F. frequency in a misnamed stage known as a Mixer.  Mixing is commonly thought of as "Addition" whereas it is "Multiplication" which is happening here. 

This effect can happen in any stage which shows non-linearity, and will always happen to some extent in the "Mixer".  Good design attempts to minimise this effect, however for Wireless Microphones it is best to always avoid frequencies with 3rd order hits within the receiver pass band and essential to avoid 3rd order hits below 25 kHz. Direct hits result in "Birdies", whistling sounds as the direct hits "Beat" together. Although in theory a direct hit is on exactly the same frequency, in practice there will always be some difference and this difference will be in the low audio range resulting in audible whistles. 

Hits outside the audible band may affect any audio processing (e.g. companding) which is being used as, although the human ear cannot hear them, the audio processing circuitry may do. In the case of Wireless Microphones, intermodulation can also occur in the output stages of transmitters, since they are often operated in close proximity.

Winmod is a simple piece of software we and our distributors use to evaluate and generate frequencies free of intermodulation.