As per this building simple radio it is the simplest AM radio receiver.


obviously this is very poor AM receiver.(but it was working for someone in comment)

I'm wondering what will happen when this receiver is very near to FM radios transmitter? will it work?

if no, How will the simplest FM radio receiver look like?

  • \$\begingroup\$ Don't edit follow up questions into the actual one. If you have a new question, ask a new question. \$\endgroup\$
    – PlasmaHH
    Commented Feb 14, 2016 at 0:04
  • \$\begingroup\$ need to wait 4 days for next question :( \$\endgroup\$ Commented Feb 14, 2016 at 0:13
  • \$\begingroup\$ Sorry, but I just had to chuckle (in a goodly way) when I saw that schematic ! Just how strong would the field need to be to make that work, I wonder? \$\endgroup\$
    – N.G. near
    Commented Sep 24, 2016 at 6:39
  • \$\begingroup\$ This is the simplest unit with digital tuning. Low cost and small. Code included: electronics.stackexchange.com/questions/306497/… \$\endgroup\$
    – SDsolar
    Commented Nov 1, 2017 at 2:41

3 Answers 3


For a totally passive FM set you can use something like a resonant circuit that is tuned somewhat off the carrier frequency so variations in carrier frequency result in changes in output voltage from the detector at the baseband frequency. This is called 'slope detection'.

For example, from this website:

This should also work to pick up AM signals.

enter image description here

An even simpler circuit is presented here.

enter image description here

  • 1
    \$\begingroup\$ I was just about to post the second one. \$\endgroup\$
    – Passerby
    Commented Feb 13, 2016 at 22:42

I demonstrated this to a ham club years ago. Thereby hangs a tale... At the end, I'll explain my possibly simplest FM receiver.

I got called by the FCC to help investigate interference at the local airport. So I loaded up my gear, and went out to talk to the folks. They said the interference was intermittent, but it was people talking and some beeps.

"Got a tape" I ask. "Sure". So they play the tape. Imagine my horror when I hear the repeater that belongs to my ham club! I pulled out my radio, sent the appropriate codes, and shut the repeater down. I told them that I was the president of the ham club, and that the repeater would not go back on the air till the problem was resolved.

So I pulled the repeater and put it on the bench. The PLL, which determines the transmit frequency was hopping between the airport frequency and ours, thousands of times a second! Component drift and some other design difficulties prevented this from being "seen" by the onboard computer and having the transmitter shut down. (Kenwood VHF repeater controller, won't buy again..) The repeater was actually transmitting on BOTH frequencies!

I fixed the component drift issues, retuned the repeater, added a notch filter for the airport frequency on the transmitter output as insurance, and put it back in service.

Back at the club I explained the problem. I got told in no uncertain terms by one of the members that it was IMPOSSIBLE for an AM reciever (aircraft) to get audio from an FM transmitter. I played the tape.. The voice of the person they had recorded was the guy claiming this was IMPOSSIBLE.. (VBG)

So how does this happen? Nothing is ever perfect. The antenna system for our repeater (and any other repeater) has a passband, and the transmitter itself isn't perfectly "flat". So as the carrier is driven to different frequencies by the FM modulation, there is a little bit of AM modulation put on the carrier by the passband of the system. This can also happen on the receiver side.

So I set up a demo. The filters for repeaters are called "cans" because they are large hollow metal cans, internally silver plated, and adjusted by mechanical tuning. These "Cans" are very narrow, High Q filters. I set up a can for our repeater transmit frequency, and added a simple diode and earphone. With careful tuning I was able to demonstrate what was essentially a crystal radio, with only the antenna, can, diode and headphone.

The "simpler" circuit above has seven components counting headphone. I did it in four. Now it certainly isn't the least expensive circuit, these cans are quite expensive.

All that it takes is to tune the filter such that the FM transmitter carrier frequency is not centered in the filter but is off to the side just a bit, but still within the passband of the filter.

It's called "Slope detection".



The absolutely simplest concept must not just include a schematic. It needs to consider ease of construction and all that.

Take a look at the construction and dteails for this one: Single-station FM radio

It was simplicity itself to build and a pleasure to use.


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