This is pretty self explanatory from the image above. Top section of the waterfall is the NooElec “Distill:FM” FM notch filter and the bottom section is a 75dB rejection FM notch that preserves Airband. I bought the bottom filter from GPIO Labs on eBay. I thought at first that the NooElec filter was doing an OK job for what it was, but it’s just not that great. If you’re not near any close FM stations or have a station that’s on the edge of being annoying, feel free free to chance on the NooElec, but the GPIO labs is at least 40dBfs (SDR# measurements may not be scientifically accurate) better at almost entirely removing FM broadcast. The station at 94.9Mhz above is KUOW which is a 100KW flamethrower not 1/4 mile from me. It images my RTLSDR and wrecks havoc everywhere. Even with the NooElec in line, I get imaging everywhere and can’t turn the gain up much on the RTLSDR. With the GPIO labs I can easily max out my RTL gain on FM and have KUOW do absolutely no imaging. At full gain I can’t even get a pilot tone out of KUOW to grab stereo information with the GPIO labs filter.
GPIO Labs also sells an even better 85dB rejection FM notch filter if you don’t mind losing Airband. Pass on the NooElec if you’re serious about needing proper FM rejection. I’ve bought many of their products and they’re all disappointing except for their new TCXO 0.5ppm RTLSDR’s. NooElec needs to up their game, because most of what they offer isn’t exactly good and isn’t exactly hard to build. If you don’t mind spending the extra $10, I’d go for the GPIO labs. I’d also recommend the official RTL-SDR.com FM notch. There are plenty of videos of it being tested with proper lab equipment and what it claims is exactly what it does.
*These tests were done with the same RTLSDR dongle, with the same SDR# settings, same gain settings, and same antenna. Nothing was moved, nothing was re-positioned and I was sitting in the same spot.