So a few of my buddies and I (all undergraduate Electrical Engineers) were discussing FM signals. We found we all understood the modulation and signal profile itself, but none of us could think of a good circuit example for how the carrier wave and modulating wave are combined.

I could instant think of how to code an MCU and then use a filter to generate the FM signal so we all accepted that possibility.

But knowing how long FM has been used and that the rise of modern, cheap MCUs is relatively recent in electronics, there must be a host of way to accomplish this modulation scheme.

I'm just wondering if any of you know of any easy ways to do FM? Simple circuits would be great! But I'm also interested in any and all more complex methods (explanations would be appreciated!)

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    \$\begingroup\$ Are you saying that all FM signal generation circuits you've found via Google or other references all use a micro? \$\endgroup\$ – Roger Rowland Mar 5 '17 at 6:32
  • \$\begingroup\$ @RogerRowland I agree, that's hard to believe. I tried to address that with my answer, so that OP can match what they'll find online with their own understanding (which is probably not bad at all) and a description of what a minimal FM transmitter needs to have. \$\endgroup\$ – Marcus Müller Mar 5 '17 at 6:38
  • \$\begingroup\$ @MarcusMüller yes it smells to me like little or no research has been done before asking here, but considering your generous answer, I've up voted it. \$\endgroup\$ – Roger Rowland Mar 5 '17 at 6:44
  • \$\begingroup\$ No I am saying that that is the one that I could easily (on the spot) explain and conceptualize.... I clearly know that there are other methods and I just wanted to see if anyone here could give a better explanation... If you google FM techniques the results (for the most part) is an endless list of articles explaining what FM is with limited detail on Hardware implementation. \$\endgroup\$ – Wolfgang_Horton Mar 5 '17 at 8:45
  • \$\begingroup\$ I apologize for treating this spread like a discussion forum. I realize now that it is strictly for Q/A (I got messaged at for trying to start a discussion on a different question.. whoops!) So in summary, I have a decent understanding of some FM circuitry from my searching but I wanted to discuss some more detailed/alternate methods... I am aware it's not only software defined \$\endgroup\$ – Wolfgang_Horton Mar 5 '17 at 8:50

So yes, defining an RF signal's shape in software is a relatively new thing (it's called Software Defined Radio, and a lot of things do it under the hood these days).

I can't believe you didn't find anything via Google using "FM transmitter circuit", but assuming you found something but didn't understand it, I'll post a rough explanation here. I'll explicitly not post a schematic; it's really easy to find hundreds of those, and since you're undergrads, it's going to be relatively much fun for you finding the things described below.

What you'll need is something that is a Voltage-Controlled Oscillator (VCO). By applying a varying voltage (in the case of audio transmissions, the amplified voltage from the microphone) to the control input of that oscillator, you change the frequency of that oscillator. Done! Sometimes, you use second, non-changing oscillator and a mixer to convert that varying frequency up the channel frequency you need it at.

So, in a minimal FM transmitter circuit, you'd be looking for:

  • input amplifier & filtering, if any
  • a VCO, typically consisting of one or two transistors in conjunction with caps and/or inductances that oscillate,
  • and output amplifying stage (typically, a couple transistors and filters)
  • an antenna
  • \$\begingroup\$ Okay cool. I've seen circuits with 555 timers and variable caps/controlling capacitance through transistor... and also some more complex circuits (relatively...). I suppose maybe not the best question. I am new to this forum (tonight haha) and was originally treating it like a discussion board but now I realize it is not. \$\endgroup\$ – Wolfgang_Horton Mar 5 '17 at 8:48
  • \$\begingroup\$ NE555 is not what you want to look at when generating RF frequencies. It doesn't work for frequencies that high. Really, Google "FM transmitter circuit minimal" or so. \$\endgroup\$ – Marcus Müller Mar 5 '17 at 9:15
  • \$\begingroup\$ I agree.. But 555 could work just fine for say an RFID card operating at 125-135kHz that requires FM for it's com protocol \$\endgroup\$ – Wolfgang_Horton Mar 5 '17 at 9:25
  • \$\begingroup\$ Granted most 125-135kHz cards don't have much of a command response protocol and simply spam their data using fsk or ask/ook via load modulation of the carrier wave without caring who is listening.... and 555 wouldn't be able to get up to the 13.56Mhz that most sophisticated RFID protocols (where you would actually need FM on the reader side) are based off.. \$\endgroup\$ – Wolfgang_Horton Mar 5 '17 at 9:28
  • \$\begingroup\$ I read about this circuit "circuitstoday.com/simple-fm-transmitter-circuit" and it's operation totally makes sense. Thanks for the advice \$\endgroup\$ – Wolfgang_Horton Mar 5 '17 at 9:39

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The colpitts oscillator with varactor tuning is pretty good for audio and data. See above. I would recommend this method because I've used it several times for very low power data coupling. I've also used it from 80 MHz to 400 MHz and with data rates up to 10 Mbps. Audio will be fine of course but don't overmodulate.

Don't build it and break the law by transmitting too much power. There are regulations for these things. If you want more circuit ideas google "colpitts oscillator with varactor tuning" and select images.


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