Also known as: FM notch filter, FM band-stop filter, FM trap, broadcast FM filter
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An FM broadcast notch filter is a band-stop filter that rejects the 88–108 MHz broadcast-FM band before it reaches a receiver’s front end. On software-defined radios — especially cheap direct-sampling dongles and wideband DDC receivers — the local broadcast-FM transmitters are often the strongest signals at the antenna by tens of dB, and left unfiltered they push the front end into overload.1
Key takeaways Every DDC / direct-sampling SDR wants one. Broadcast FM (88–108 MHz) is usually the loudest thing at your antenna; unfiltered it overloads the front end, raising the noise floor and generating intermodulation products across the band you actually want. A ~$25 band-stop filter drops broadcast FM by >50 dB with <0.5 dB loss elsewhere — often the single cheapest way to lower your noise floor.
Overview
A wideband receiver shares one ADC and one front-end gain stage across everything in its capture. Its dynamic range is finite, so the strongest signal present sets the gain the whole capture must live within. In most populated areas that strongest signal is broadcast FM. When it drives the front end near clipping, two things happen: the noise floor rises (burying weak signals like a distant control channel), and non-linear mixing produces intermodulation products — phantom carriers — elsewhere in the spectrum.
Because broadcast FM sits in a fixed, well-defined band, a band-stop filter tuned to 88–108 MHz removes the offender while leaving the VHF/UHF public-safety and business bands essentially untouched. This is distinct from a broadcast-FM band-pass (which keeps only FM) — a notch keeps everything except FM.
How it works
The filter is a passive LC (or SAW) network with a deep rejection null across ~88–108 MHz and low insertion loss on either side. Typical numbers for the common SDR-oriented parts: >50 dB of stop-band attenuation, <0.5 dB insertion loss outside the notch, on SMA connectors. Good designs keep the adjacent VHF airband (108–137 MHz) minimally affected. Install it inline between the antenna (or the LNA) and the SDR; being passive and bidirectional, it needs no power.
Relevance to SDR
GopherTrunk decodes weak P25, DMR and
TETRA control channels that a raised
noise floor can bury, and the wideband role: wideband path is especially sensitive
because one overloaded capture starves every tap on it — the same front-end-overload
failure the Airspy wideband notes describe (clip_ratio non-zero,
weak sites at the noise floor). If a wideband dongle shows clipping or a stubbornly high
noise floor in a metro area, an FM notch filter is usually the first and cheapest fix,
ahead of adding an attenuator or lowering gain.
Where to buy
Common, inexpensive options on Amazon: the RTL-SDR Blog Broadcast FM Block Filter (~$25, >50 dB rejection) and the Nooelec Flamingo+ FM (higher rejection, low airband impact). Equivalent notch filters are also widely sold on eBay and AliExpress.
Sources
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RTL-SDR.com — background on broadcast-FM overload of SDR front ends and the use of 88–108 MHz band-stop filters to lower the noise floor and suppress intermodulation on direct-sampling and DDC receivers. ↩