Can a filter circuit be built for the FM broadcast range to only pass the frequencies not bearing a broadcast stream?

Question

I want to build a receiver that constantly sweeps the United States FM broadcast frequencies between 87.8 MHz to 108.0 MHz.

However, I want to filter out any frequency that has a radio station broadcasting on it.

Is there a way to dynamically detect a frequency with a broadcast on it and not pass it to the output circuit that has a speaker?

In operation: 1) the circuit receives the first frequency in the standard U.S. radio range. 2) if the frequency has a broadcast on it, ignore it, otherwise send it to the output speaker. 3) If no more frequencies in range, go back to 1), otherwise receive the next frequency in range and go back to 2).

I want to receive only those channels without a broadcast on them, sort of like the opposite of a squelch circuit.

I am a beginning electronics enthusiast and this is an experimental application to prove a concept. I'm asking for help because I haven't the slightest idea whether it would work and if it could, how to go about building such a thing.

Thanks all.

Answer 1

Of course it can. What you need to do is to perform a band scan and look at signal strengths. If you plot it, you would get something like this:

If the signal is above a certain threshold, some station is broadcasting on it and you can skip that frequency. If the signal is low, output it to the speakers.

Now on how to implement it. My approach would be to use one of the off-the-shelf FM receiver chips (e.g. ​Si4702) and a small micro to drive it. That's about all you need these days!

Watch out for any processing (the Si4702 has "Adaptive noise suppression"), which may interfere with what you are doing. Do your own research and pick a chip that suits your needs.

Answer 2

I want to filter out any frequency that has a radio station broadcasting on it.

The legal definition of Broadcasting is:-

“the dissemination of radio communications intended to be received by the public, directly or by the intermediary of relay stations.”

In the US it is illegal to operate an FM broadcast station without a license from the FCC, so all you should need to do is obtain a list of licensed stations and avoid those frequencies in your scan. This won't eliminate unlicensed stations of course, but as the FFC is taking aggressive action against illegal operators they don't tend to stay up for long.

Is there a way to dynamically detect a frequency with a broadcast on it

Not easily. You can certainly detect frequencies which have an FM transmission on them, but determining if they are broadcast transmissions is a bit harder. Since most (all?) FM broadcast stations transmit in stereo, you could look for the 19kHz pilot tone and squelch any station which has one, but if the signal is weak you may not be able to detect the tone.

Answer 3

Your specification would result in broadband thermal noise, because by definition if the Rx cannot lock onto carrier AND squelch is disabled if no carrier, then all you would hear is thermal noise.