The Basics Of Frequency Modulation


[brmarcum] takes us back to analog building block basics with his Frequency Modulation and Demodulation tutorial. Frequency Modulation (FM) sounds simple at first, but understanding the electronics behind modulation and  demodulation of an FM signal can be confusing. We’ve covered the basics before, but FM is so tightly associated with broadcast radio that searches often become muddled with references to RF, stereo, antennas, and transmitters.

[brmarcum] hopes to fill that gap with a simple circuit that modulates an audio signal to FM, then demodulates and amplifies it to be played on a small speaker. He used a Digilent Analog Discovery kit in his experiments, but an oscilloscope (an older analog scope would be perfect here) would work for output. Signal generation duties could easily be handled by a 555 circuit at the low end, and a computer sound card at the higher end.

[brmarcum] obviously put some time into his tutorial, but it’s not a tome of FM modulation. He’s broken down the modulation and demodulation circuits into their basic op-amp stages with examples of what the signal should look like on a scope after each stage. That’s the beauty here. By building and testing each section, anyone new to analog can learn how things work. In places where the theory behind what’s going on gets too in-depth for an Instructable, [brmarcum] gives links to Wikipedia.

7 thoughts on “The Basics Of Frequency Modulation

  1. We have lost all of contemporary music on our local NPR FM station. I now listen to FM almost as little as AM now. A local pirate has so few listeners so he just lets the playlist run out and dead air ensues all day. Makes it easier to track down!

    Digital radio and multiplexing go back to the 30’s. The inventor got ripped off, it helped win WW2 but he had to jump out a window. His wife won the legal mess.
    Edwin Armstrong.
    When a FM signal goes thru full limiting in your radio, it’s a square wave. I’d call that digital. Of course I don’t mean Morse, as that is digital also.

    1. ‘I’d call that digital.’
      You have to admit that your’e reaching quite a bit on that one right? Since only the frequency is relevent at the earliest stages, it can be amplified to clipping. At no point is the reciever required to store or manipulate a digital representation of anything. There are no 1’s and 0’s representing discreet frequency steps. Thats not to say that modern equipment can’t do these things, but just becuase the incoming signal gets clipped does not make it digital.

        1. That isn’t what digital means. The signal is encoded in the time between zero crossings not whether the signal is positive or negative. The time between zero crossings is a continuous value, thus it is an analogue signal. You clearly have no idea how FM modulation encodes the message nor the definitions of analogue and digital signals. All you have done is go, “Herp derp its a pulse train, must be digital.” without having a deeper understanding.

    2. Empire Of The Air: The Men Who Made Radio, by Tom Lewis. Fascinating geekbook, and the Ken Burns documentary. Sarnoff was a robber baron, DeForest was an inveterate tinkerer who seemed to know no real theory, just tried to capitalize on what he could, and Armstrong was the giant of the age, I think on a level with Tesla for sheer intellectual brilliance. Yes, too bad he chose defenestration in 1954. But his wife continued his patent fights, winning the last in 1967.

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