Use a FM Notch Filter to Improve the Performance of your Software-defined Radio

Posted by GPIO Labs on

FM radio 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 overwhelm or saturate the receiver in a software-defined radio or RF receiver. This in turn impacts signal reception. You might for instance hear static instead of audio.

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. 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.

The next two images show the effect of a FM notch filter in practice.

The image below 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 FM notch filter was inserted between the antenna and the RTL-SDR.

With the use of this filter 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

  • The first stage of a SDR is an internal low noise amplifier
  • An external LNA is added to a SDR

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 a LNA can suppress the amplified FM signals before they impact the receiver. Sometimes it might make sense to add the notch filter before a LNA. 

In the case of a SDR, by connecting the FM Notch filter to the RF input port, you're essentially adding it before the first internal LNA.

FM Notch filters are a very important accessory in any software-defined radio or receiver kit as they help protect from saturation and improve signal reception.

 Different types of Notch Filters