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I am trying to make an FM receiver. I have a radio transmitter and it's able to pick up and demodulate the signal from that when the transmitter is beside it, but the problem is when I try and pick up any other radio stations. The variable capacitor is between 10 pF - 60 pF maybe is this too narrow of a range? or would there be another reason for this?

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  • \$\begingroup\$ There could be many other reasons. This is way too broad at the moment; we don't have any idea what debugging you have done or what test equipment you have available or what frequency your transmitter is tuned to. Also I see no LO or IF filter or FM demodulator in your circuit, so how does it work? Is it intended to be a TRF set? \$\endgroup\$ – Brian Drummond Nov 1 '20 at 15:30
  • \$\begingroup\$ Go and search for "FM receiver circuit" and not how they all are significantly different from your circuit! Also tell us where the circuit comes from and since this is "RF" also show how you build it. \$\endgroup\$ – Bimpelrekkie Nov 1 '20 at 16:18
  • \$\begingroup\$ This isn't a good circuit. What's your goal here? If you want to achieve a broadcast FM receiver look at a heavily integrated modern architecture such as using an Si4702 and an MCU if you want to build from scratch study the typical superhet FM receiver archiecture, if you want to do remote control or other data get nRF24L01 clones, RFM69's etc. \$\endgroup\$ – Chris Stratton Nov 1 '20 at 16:24
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A simple home-made AM receiver, covering the FM broadcast band (88 -108 MHz), can be used to tune in to and receive FM radio stations, through a technique called 'slope detection'.

When a station is off-tuned on such a receiver, either slope of the frequency response of the tuned circuit would convert the FM signal to AM and thus enable demodulation.

Nearby strong stations would make the job easier.

Stations, with rated power ranging from 1 to 10 kW and located 6 km away (as the crow flies), are received fairly well, using this simple receiver.

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Stations with higher power ratings, located nearby, have been received with similar receivers even without using the 1.5 V cell.

High 'Q' tuned circuits are a must for such receivers.

Should you be lucky to have high power stations nearby, here's a good resource to build a number of simple FM receivers.

https://www.mikroe.com/ebooks/radio-receivers-from-crystal-set-to-stereo/the-simplest-fm-receiver

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I am trying to make an FM receiver. FM receiver not picking up any stations

The circuit you have posted is an AM receiver and it ain't a very good AM receiver either. You need to load the diode detector (an op-amp input is nowhere near good enough) so that it properly half wave rectifies but, of course, it'll be a bit rubbish on FM.

Delving deeper, it's impossible to say that it is even tuning in the range 88 to 108 MHz because component values are not shown and, critically, construction details are neither.

Finally, you need to DC bias the op-amp input at about half rail for it to stand a chance of working on AM. Plenty of problems to fix.

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  • \$\begingroup\$ While this one is probably too simple to work, it's worth noting that with a degree of channel selectivity (from eg, a superhet design with an IF filter), then an AM architecture tuned to the correct frequency can intelligibly receive wideband FM broadcasts as the deviation of the modulated signal up and down the filter skirt produces an amplitude variation picked up by the AM detector. \$\endgroup\$ – Chris Stratton Nov 1 '20 at 16:19
  • \$\begingroup\$ @ChrisStratton isn't it more appropriate to put this comment under the question? Also, by correct frequency you mean offset to one side of the FM centre frequency, right? \$\endgroup\$ – Andy aka Nov 1 '20 at 16:21
  • \$\begingroup\$ It's specifically a response to your point of this being an AM detector. By correct frequency, I mean tune a band where FM rather than AM is traditionally used. \$\endgroup\$ – Chris Stratton Nov 1 '20 at 17:05

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