# Understanding Frequency Modulation - Beginner

Discussion in 'Wireless & RF Design' started by Nathan Hale, Aug 16, 2016.

1. ### Nathan Hale Thread Starter Active Member

Oct 28, 2011
125
2
Hi guys. i am trying to understand how frequency modulation works. Please look at the picture below.
I see the information signal which let us suppose is 4000Hz. ( Let us pretend Elvis is signing at 4000 Hz..)
I see a carrier signal which , we will pretend, is 97.5 MHz. ( Lets pretend it is a radio station in NY.)
Now look at the end result after modulation ( the FM Signal.)
The signal's frequency is almost constantly changing!
My question to you guys is, if the frequency of the FM signal is changing, how can i still hear Elvis at 97.5 MHz? Shouldn't the song be all over the radio dial? Shouldn't the song be at 96MHz for a moment and then at 103 MHz the next moment? I am asking this because looking at the FM signal, the only conclusion that i can make is that the frequency is constantly changing.
But we know that the song is always constantly playing at 97.5MHz. Why is the song not being transmitted all over the dial?

2. ### crutschow Expert

Mar 14, 2008
13,523
3,391
That picture shows an extreme level of modulation.
In the actual radio station the frequency is not changing near as much as shown.
The maximum frequency deviation for USA FM stations is ±75KHz around the roughly 100MHz center carrier frequency.
Thus if you looked at the carrier with an oscilloscope you would barely see this relatively small deviation.
So you can see that the signal is not spread all over the FM band.
Here's some further info on FM broadcasting.

Nathan Hale likes this.
3. ### Nathan Hale Thread Starter Active Member

Oct 28, 2011
125
2
So, are you saying that my radio somehow detects the deviation and still keeps the song playing at 97.5 MHz?

4. ### nsaspook AAC Fanatic!

Aug 27, 2009
3,020
2,386
Because your modulation (Information) signal is centered around zero and the possible frequency swing from the carrier frequency is limited to the designed (and assigned) bandwidth of the channel to a usually fairly narrow frequency range.

This (SDR radio) is the FM band with a three channels display showing the modulation, the display range is then switched for a single channel display.

5. ### crutschow Expert

Mar 14, 2008
13,523
3,391
Yes, it detects (demodulates) the modulation deviation and converts it back to the original audio.
I don't understand what you mean by "still keeps the song playing at 97.5 MHz"(?).

Aug 8, 2011
51
7
Just to keep thing's simple for "OLE STUPID" (ME) .
On the transmit side , your VCO is generating a "CARRIER" at 97.5 Mhz , the "MODULATOR" circuitry in your transmitter is superimposing the audio signal "ONTO" that "CARRIER" (that's the Quick & Dirty).
The other associated circuitry within your transmitter controls power level's and "DEVIATION" (the 75Khz width of the audio signal) .
Like I said , this is the "Quick & Dirty" explanation, thing's get much more "involved" from here especially when you start adding "SUB-CARRIER" signals & Modulation schemes.

7. ### AnalogKid Distinguished Member

Aug 1, 2013
4,709
1,301
Yes. Each station on the FM dial is not a single frequency. Each station is allocated a frequency band. In FM in the US, that band is 150 kHz wide. Inside it is the FM mono signal (left and right stereo signals added together) that has been filtered to remove all frequencies above 15 kHz, called the "sum" signal. Next is a constant "pilot" tone at 19 kHz. Next is the difference signal (the remainder, what is left over when the left and right signals are subtracted from each other). The difference signal is amplitude modulated on a 38 kHz subcarrier within the 150 kHz frequency envelope). Next are more subcarriers with SCA on them, extra audio and data signals. Bottom line, FM stereo broadcasting is not nearly as simple a concept as plain old AM mono.

More to your question, you're correct, the transmitter does not transmit a single frequency such as 97.5 MHz. But as Wally said, it also does not wander all over the place. It wanders all over a very narrowly defined place in a very specific manner. Here is a conceptual example; the real world numbers are not exactly like this.

For an audio signal that is defined as +/-1.0 V peak at "full volume", the instantaneous value of the audio signal always is somewhere between +1.0 V and -1.0 V. A constant value of 0 V equates to silence. For an FM transmitter at a nominal 100 MHz and a modulation bandwidth of 75 kHz, here are some frequencies:

Audio = 0 V --> Transmitter output is 100.000 MHz
Audio = 0.5 V --> Transmitter output is 100.0375 MHz (100,037,500 Hz)
Audio = 1.0 V --> Transmitter output is 100.075 MHz
Audio = -0.5 V --> Transmitter output is 99.9625 MHz
Audio = -1.0 V --> Transmitter output is 99.025 MHz

Note that there is no direct correlation between the frequency of the audio and the frequency deviation of the transmitter. It is the amplitude of the audio that changes (modulates) the frequency of the transmitter, hence the name. For FM, the instantaneous amplitude of the modulating signal produces an instantaneous value of the transmitted carrier. For a UHF walkie-talkie, the carrier is somewhere between 450 to 470 MHz, but the deviation is only +/-25 kHz for a single spoken audio channel. Because the deviation is so narrow, many more individual channels can fit within the allocated frequency band.

For completeness, the frequencies of the modulating signal do affect the transmitter output, but that gets in sideband spread, Bessel functions, and some seriously hairball mathematics.

ak

Last edited: Aug 17, 2016
Nathan Hale likes this.
8. ### crutschow Expert

Mar 14, 2008
13,523
3,391
To summarise, the audio amplitude determines the amount of FM carrier frequency deviation, and the audio frequency determines how rapidly this deviation occurs.

AnalogKid likes this.
9. ### BR-549 Well-Known Member

Sep 22, 2013
2,184
421
I would suggest a review and study of various modulations. Then I would suggest that you buy a sdr dongle and tune in and study these different modulations.

Most dongles will only go down in frequency to the fm band, maybe a little lower. You can see wideband fm signals here.
You wont be able to see broadcast am radio stations, but you can find narrow band am on the aircraft frequencies.
You will find narrow band fm on the police bands. There are hams bands too.

And many other curious looking signals.

It will cost you about 20 bucks.

10. ### Nathan Hale Thread Starter Active Member

Oct 28, 2011
125
2
Thank you all for your replies. After so long, I have finally grasped the concept!!

11. ### Nathan Hale Thread Starter Active Member

Oct 28, 2011
125
2
Now that's what i call a Great practical example! Keep up the good work. examples like these make concepts much more easier to understand.

12. ### ScottWang Moderator

Aug 23, 2012
4,934
777
If you want to know about more deep knowledge that is AFC, and it was also called AFT.
AFC -- Automatic frequency control.
AFT -- Automatic Fine Tuning.

13. ### atferrari AAC Fanatic!

Jan 6, 2004
2,673
785
Never heard AFT before!

14. ### ScottWang Moderator

Aug 23, 2012
4,934
777
Me too, I only learned the AFC.