Strange Interference

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Active Member
Feb 10, 2019
Baltimore County, MD
band pass filter for that particular frequency
You do not want a bandpass filter. You want a notch filter to trap out that particular frequency and pass everything else. A bandpass filter passes a range of frequencies and attenuates all other frequencies outside of that range..
Last edited:


Dec 19, 2002
Vista, CA
Ok guys, I think I found the offending freq using a spectrum sweep. The scanner stopped on 152.6000 and it was a constant broadcast of that data noise. That’s the only one I could find. On the screen it was showing FM while receiving this pager noise. What does that mean?

Would the next step be to email PAR and see if they can do a band pass filter for that particular frequency?

I really appreciate all the help on this guys.
Yes, that would be the "next step".

You now have the center frequency that the notch filter (not "band pass" - the other poster is right, you need a "notch" filter or "band reject" filter) needs to be designed for. PAR can go from there.

You need to be aware that all notch filters will have a stop band" of frequenciesthat will be attenuated around the center - it won't just attenuate the one center frequency. Usually, the better the quality of the filter the smaller the stop band around the center (assuming you only want that one frequency atenuated). And of course, the more expensive.

So, if you have desired frequencies close to that undesired one you may find those frequencies greatly are somewhat reduced in strength themselves. Usually, with those PAR filters, that's within about 1 MHz of the center - but that's a question you need to ask them.

As to the "FM" reading - that's normal. All US land based and sea based above 30MHz LMR two-way (and paging one-way) are narrow band FM either +/- 5 kHz deviation ("old" narrowband) or +/- 2.5 kHz deviation ("new" narrowband) unless they are some form of digital modulation like P25, NXDN, or DMR. Even those digital schemes can use frequency modulation to deviate around a center as part of the component of the modulation scheme (phase, frequency, amplitude). Civilian and military aircraft, however, still use AM or amplitude modulation analog.

Ham radio FM, paging, GMRS, NOAA weather boradcasts, remote broadcast, and Marine (water craft) are still using the wider form of narrowband at +/- 5 kHz deviation in the US while most other LMR traffic from 138 MHz to 512 MHz has shifted or soon will shift to the smaller lower bandwidth 2.5 kHz deviation. That is for analog FM voice. The newer digital modes inherently use the narrow 12.5 kHz bandwidth limits and some, like NXDN, will fit within a 6.25 kHz bandwidth.

Anyway, more info than you need, sorry. Point is, unless you are listening to aircraft transmissions, your scanner will usually default to FM or NFM (or FMN which is the same as FMN but can shift with brand) with the former designed for the wider form of narrow band or +/- 5 kHz deviation while the latter is designed for the more narrow newer form using +/- 2.5 kHz deviation.

Consumer scanners usually use an internally programmed "lookup table" to determine which form of FM to use based on the FCC assigned bandwidth limits for a given frequency range. But they are not always accurate. Just because your scanner shows "FM" in the display does not neccesarily mean the detected signal is actually a +/- 5 kHz deviated FM signal. It could be the narrower form. You just usually have to try and experiment for which seems to work best for you. Paging signals, as far as I know, of the type you are having trouble with, are exempt from the newer narrow band requirements so they are usually the wider version ("FM") but it depends on exactly what kind of paging it is and which part in the FCC rules it falls under.

Also, if your scanner has a "WFM" mode that usually means the really wide deviation setting for US FM broadcast stations like your normal car radio recieves. Those have a really wide deviation out to +/- 75 kHz or so with an allowed bandwidth of 200 kHz or so. Old analog TV also used wide band FM for the audio (in the US) but with slightly less deviation than the regular FM broadcasts used. Back before digital TV took over you could use that "WFM" setting to listen to the audio from TV stations as well if the scanner could recieve those frquencies. New digital TV uses a digital scheme that no current scanner can demodulate, unfortunately.

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