If two signals fall within the passband of a receiver they will both compete to be heard. They will also heterodyne together in the detector stage, producing an audio tone equal to their carrier frequency difference.

For example, suppose we have an AM receiver with a 5 kHz bandwidth and a 455 kHz IF. If two signals appear on the band such that one appears at an IF of 456 kHz and the other is at 454 kHz, then both are within the receiver passband and both will be heard in the output.

However, the 2 kHz difference in their carrier frequency will produce a 2 kHz heterodyne audio tone difference signal in the output of the AM detector.

In some receivers, a tunable high-Q (narrow and deep) notch filter is in the IF amplifier circuit. This tunable filter can be turned on and then adjusted to attenuate the unwanted interfering signal, reducing the irritating heterodyne.

Attenuation figures for good receivers vary from –35 to –65 dB or so (the more negative the better). There are some trade-offs in notch filter design. First, the notch filter Q is more easily achieved at low IF frequencies (such as 50 kHz to 500 kHz) than at high IF frequencies (e.g. 9 MHz and up).

Also, the higher the Q the better the attenuation of the undesired squeal, but the touchier it is to tune. Some happy middle ground between the irritating squeal and the touchy tune is mandated here.

Some receivers use audio filters rather than IF filters to help reduce the heterodyne squeal. In the AM broadcast band, channel spacing is 9 or 10 kHz (depending on the part of the world), and the transmitted audio bandwidth is 5 kHz. Designers of AM broadcast receivers may insert an R–C low-pass filter with a –3 dB point just above 4 or 5 kHz right after the detector in order to suppress the audio heterodyne.

This R–C filter is called a ‘tweet filter’ in the slang of the electronic service/repair trade. Another audio approach is to sharply limit the bandpass of the audio amplifiers. Although the shortwave bands typically only need 3 kHz bandwidth for communications, and 5 kHz for broadcast, the tweet filter and audio roll-off might not be sufficient. In receivers that lack an effective IF notch filter, an audio notch filter can be provided.

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