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I am trying to build a FM receiver circuit with simplest elements, transistors, capacitors (an op-amp such as LM 741 would be fine too) for analysis purposes (not real-time radio). The circuit should be able to tune into FM station, and output frequecies close to that station. I don't need demodulation, I will do that seperately (the plan is I interface with the analog output through an Arduino, I want to capture the data, and code digital demodulation). What is the simplest circuit for this task?

I guess this request would fall under software-defined-radio, so I am trying to build a hobbyist's, simplest dongle.

Thanks,

BBSysDyn
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    You will need some kind of demodulation. The Arduino cannot capture and process analog signals at anywhere near the speed needed to do demodulation in software. Also, you seem very much a beginner. Crystal radios can't be used with FM. If a crystal radio would be of any use to you, you would be using AM, not FM. So, what are you working with? AM or FM, and what frequency range? – JRE Jun 01 '17 at 10:07
  • Thanks for the comment - yes I am interested in FM. I'd like to be able to tune in to any FM station, so a variable component is necessary. – BBSysDyn Jun 01 '17 at 10:24
  • I see, Arduino analog input is read at 10khz max, so I guess this is not enough for FM input. Then I'd need to digitize it before offloading it to Arduino / external computer like @analogsystemsrf suggested..? – BBSysDyn Jun 01 '17 at 10:31
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    Reasons not to use a 741: https://electronics.stackexchange.com/questions/304521/reasons-not-to-use-a-741-op-amp – Andy aka Jun 01 '17 at 10:44
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    Your whole question makes no sense. You talk about "FM Radio", but then say you don't want to demodulate. However, FM is modulation. In fact, it stands for *frequency modulation*. You want to make a frequency modulation radio receiver without demodulation. So how is the modulation method relevant if you're not going to demodulate the signal anyway. You just want a RF narrow band filter and amplifier? But then what does that have to do with FM? Closing this mess. – Olin Lathrop Jun 01 '17 at 10:58
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    SDR using an Arduino? Bad idea. – Enric Blanco Jun 01 '17 at 11:22
  • I was going to use the Arduino to capture the data. I am not trying to implement real-time radio on Arduino. Mainly I want to capture a slice of the FM radio spectrum, and analyze if offline, demodulate it, play it, etc. I am looking for the simplest circuit to implement that. – BBSysDyn Jun 01 '17 at 11:23
  • A lot of justified resistance to this question. OP has a way to go before the complexity of his endeavour becomes clear. SDR technology only became available relatively recently because it is SO COMPLEX and the reason it is available to the general public now and not just the military is because it became cheaper for consumer applications through incremental performance increases in DSP processing. However it does not work without massive computational overhead operating at high speeds. I still think questions like this should be left open as the collected wisdom will be of value to others. – KalleMP Sep 08 '18 at 09:11

1 Answers1

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schematic

simulate this circuit – Schematic created using CircuitLab

So you want to SELECT one station in the FM band?

How about digitizing ALL the stations, then use digital filtering. IN that case, Fsample of 80MHz will downconvert the entire FM band of 88-108 to 8MHz --- 28MHz.

If you want just one station, use the NE602 with LO 40MHz higher 128--148MHz, creating low-side IF into some LC filtering and 40MHz (OPamp?) amplification, then down convert again with another NE602 using fixedLO for the 2nd IF to get amplified and digitized by Ardunio [phasenoise aka jitter of Sample clock may set your SNR]

I'm assuming you want a quality musical output. If you just want low-rate data (rate << Ardunio 10KHz), implement a lowIF frontend. That is, for 88MHz data at 2Kbit/second, use 88.005MHz LO to produce 5KHz IF. Do not mix down to DC (that is, do not use Zero-IF because you'll then need I+Q IF strips to preserve the FM data)

analogsystemsrf
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