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.

  • 5
    \$\begingroup\$ What do you expect to hear on the unused channels? \$\endgroup\$
    – JRE
    Sep 29, 2016 at 21:20
  • \$\begingroup\$ I'm confused. You want to pass "it" through when there is no "it"? If there's nothing being broadcast on a particular channel, then there is nothing there to pass anywhere ... \$\endgroup\$
    – brhans
    Sep 29, 2016 at 21:51
  • \$\begingroup\$ @JRE This may be another EVP (aka. "ghosts talking on the radio") project. \$\endgroup\$
    – user39382
    Sep 29, 2016 at 23:03
  • 2
    \$\begingroup\$ Some radios have a squelch or "mute" circuit. Get ahold of that, stick an inverter in the gate logic (so "on" is "off") and you're done! All Noise, All The Time! \$\endgroup\$
    – gbarry
    Sep 29, 2016 at 23:59
  • \$\begingroup\$ Related question \$\endgroup\$ Sep 30, 2016 at 0:00

3 Answers 3


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:

enter image description here

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.


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.


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.

  • \$\begingroup\$ Not only thermal noise, there'll be plenty of background unintentional interfering signals (harmonics of digital clock and data lines, SMPS, brushed motors arcing, faraway FM stations that are too weak to lock) that won't be locked onto by an FM detector PLL but will contribute to the measured signal. \$\endgroup\$ Sep 29, 2016 at 23:11
  • \$\begingroup\$ yes amplitude noise with an FM discriminator will likely be unintelligble unless on the edge of the IF filter \$\endgroup\$ Sep 29, 2016 at 23:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.