FM Notch Filters – why you need one with most SDRs

FM radio was invented in 1933 and is one of the most popular broadcast technologies out there. FM broadcast signals in most places in the world occupy the frequency range from 87.5 MHz to 108 MHz. These signals also happen to be very strong and can often overwhelm or saturate the receiver in a software-defined radio. This in turn prevents SDR from receiving all other signals.

If a user does not want to listen in to the FM broadcast band, then one way to mitigate the effect of this strong band of signals is to suppress or ‘notch’ them out. The product that can be used for this purpose is called a notch filter and in this case specifically a FM notch filter.

A good FM notch filter will significantly suppress signals in the FM band and cause little to no disruption to other signals outside the band. In the next two pictures you can see the effect of a FM notch filter as it suppresses all FM signals in the displayed frequency range.

The first picture shows a few strong FM signals as seen in SDR# using the RTL-SDR with an antenna connected to it.

Using an RTL-SDR to display FM band signals

The next picture below was generated with the same setup. However this time, a GPIO Labs FM Notch Filter was inserted between the antenna and the RTL-SDR. You can see that almost all FM signals have been eliminated.

The effect of an FM Notch Filter

A FM notch filter is especially useful when a low noise amplifier (LNA) is added prior to the SDR to amplify and detect weak signals. Most LNAs have higher gain values at lower frequencies. The picture below shows a typical gain versus frequency curve.

This results in the strong FM band signals becoming even stronger after amplification relative to other signals (for instance cellular) that the user might be interested in.

FM notch filters suppress amplified signals in the FM band while preserving everything else

Inserting a FM notch filter after the LNA can suppress the amplified FM signals before they impact the receiver.

FM Notch filters are a very important accessory in any SDR kit as they help protect the receiver from saturation and enable the user to make signal measurements that would otherwise be impossible.

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