Skip to main content

FM Modulation Explained by Army Signal Corps Video

 FM modulation is so elegant it almost feels like a magic trick. After finally watching a 1940s U.S. Army Signal Corps training film, I realized just how simple frequency modulation really is—both on transmit and receive. Seeing the LC tank, limiter, and discriminator explained visually made everything click. Even better, that understanding immediately paid off in a modern, practical way: fixing an overly aggressive CW sidetone in my video QSLs using nothing more than an audio limiter.


I finally understand FM modulation and it's so simple I'm amazed it wasn't developed first. I found an old US Army signal corp video that describes how both FM transmit and receive work.



Audio Modulation

I had not understood that modulating the transmit frequency was this simple. It all makes sense now that I see it, but I hadn't considered how simple this could be. The video demonstrates the use of an LC tank circuit to  generate the required RF. By placing a a condenser mic in parallel with the capacitor in the tank circuit, voice modulation changes the capacitance of the tank. That changes the resonant frequency of the tank, and therefore the frequency it produces. And that's it! So simple!



Receive Circuit

Limter

The video goes on to explain that on the receive side, since what we want to modulate are changes in frequency, there's not point in keeping hte edges of the incoming waves that just contain noise and no frequency modulaiton information. Consequetly, a limiter is employed to limit off the tops and bottoms of the incoming intermediate frequency radio waves. (Yes, that does  mean that there's still a mixer involved.)


Discriminator

Finally, the film explains how the frequency variations are turned into audio utilizing the approximately linear section of a tanks frequeqncy response curve to create DC output levels proportional to the distance the incoming frequency sits away from the resonant frequency of the tank.




Using What I Learned

I made a lateral applicaiton of my new found knowledge this morning. I wanted to calm down the really loud Rockmite sidetone in my video QSLs. It occurred to me that an audio limiter might be just the thing to clip the amplitude  tops off the sidetone while leaving the much quiter receive CW untouched. Sure enough, going 15 dB down with the limiter did the trick!

Notic that the limiter's cutting off the tops of the sidetone wave while leaving most of the other audio untouched.



Understanding FM modulation turned out to be less about math and more about learning style. Getting to see the ideas of FM laid out simply, and more importantly, calmly in terms of familiar LC circuits made all the difference. Once amplitude is stripped away with a limiter, frequency becomes the only thing that matters—and discriminators turn that motion back into sound with beautiful simplicity. 

On a front completely different from vacuum-tube receivers, once I had the idea of limiters in mind I was able to solve my sidetone issue as well!




Labels:

Comments

Post a Comment

Please leave your comments on this topic:

Popular posts from this blog

Cool Math Tricks: Deriving the Divergence, (Del or Nabla) into New (Cylindrical) Coordinate Systems

Now available as a Kindle ebook for 99 cents ! Get a spiffy ebook, and fund more physics The following is a pretty lengthy procedure, but converting the divergence, (nabla, del) operator between coordinate systems comes up pretty often. While there are tables for converting between common coordinate systems , there seem to be fewer explanations of the procedure for deriving the conversion, so here goes! What do we actually want? To convert the Cartesian nabla to the nabla for another coordinate system, say… cylindrical coordinates. What we’ll need: 1. The Cartesian Nabla: 2. A set of equations relating the Cartesian coordinates to cylindrical coordinates: 3. A set of equations relating the Cartesian basis vectors to the basis vectors of the new coordinate system: How to do it: Use the chain rule for differentiation to convert the derivatives with respect to the Cartesian variables to derivatives with respect to the cylindrical variables. The chain ...
10 comments
Read more

The Valentine's Day Magnetic Monopole

There's an assymetry to the form of the two Maxwell's equations shown in picture 1.  While the divergence of the electric field is proportional to the electric charge density at a given point, the divergence of the magnetic field is equal to zero.  This is typically explained in the following way.  While we know that electrons, the fundamental electric charge carriers exist, evidence seems to indicate that magnetic monopoles, the particles that would carry magnetic 'charge', either don't exist, or, the energies required to create them are so high that they are exceedingly rare.  That doesn't stop us from looking for them though! Keeping with the theme of Fairbank[1] and his academic progeny over the semester break, today's post is about the discovery of a magnetic monopole candidate event by one of the Fairbank's graduate students, Blas Cabrera[2].  Cabrera was utilizing a loop type of magnetic monopole detector.  Its operation is in...
1 comment
Read more

More Cowbell! Record Production using Google Forms and Charts

First, the what : This article shows how to embed a new Google Form into any web page. To demonstrate ths, a chart and form that allow blog readers to control the recording levels of each instrument in Blue Oyster Cult's "(Don't Fear) The Reaper" is used. HTML code from the Google version of the form included on this page is shown and the parts that need to be modified are highlighted. Next, the why : Google recently released an e-mail form feature that allows users of Google Documents to create an e-mail a form that automatically places each user's input into an associated spreadsheet. As it turns out, with a little bit of work, the forms that are created by Google Docs can be embedded into any web page. Now, The Goods: Click on the instrument you want turned up, click the submit button and then refresh the page. Through the magic of Google Forms as soon as you click on submit and refresh this web page, the data chart will update immediately. Turn up the:...
Post a Comment
Read more