I am a physician with a hobby interest in communication electronics.

I am having a difficult time understanding the concept of frequency modulation (FM). If a voltage-controlled oscillator is used to alter the carrier frequency, i.e. the frequency of the carrier will vary according to the AMPLITUDE of the modulating sgnal, how can the resultant signal accurately differentiate a low volume frequency (low voltage amplitude), from a modulating frequency of the exact same frequency but with a higher voltage amplitude (i. e. higher volume)? It seems that the resulting carrier frequency change will be of higher frequency in the latter case.

Could anyone help with the concept I am missing?

Thank you very much!

D. Peter Van

 

This is just to clarify my question. If, for example, I am playing the "C" note on the piano, if I play it softly, the carrier frequency will change to a certain frequency according to the amplitude of the signal. If I then play the same note loudly, the carrier frequency will then be transmitted at a higher frequency, changing the pitch that the receiving circuit sees. Am I correct, or am I missing a concept here?

Thank you again!

D. Peter Van

 

The VCO's instantaneous output frequency will follow the input voltage.
Imagine a plot of some arbitrary modulating signal vs time. Apply this signal to a VCO. Now imagine a plot of the VCO output frequency vs time. With a perfect VCO, the waveforms on the two graphs will be identical.
To demodulate the FM, you need a frequency discriminator. The output of this will be a voltage which is proportional to the input frequency.

 

This is just to clarify my question. If, for example, I am playing the "C" note on the piano, if I play it softly, the carrier frequency will change to a certain frequency according to the amplitude of the signal. If I then play the same note loudly, the carrier frequency will then be transmitted at a higher frequency, changing the pitch that the receiving circuit sees. Am I correct, or am I missing a concept here?

Thank you again!

D. Peter Van

No, it won't change the pitch, it will produce the same pitch at a higher amplitude (imagine that!).

 

Ahhh....thank you. It is starting to gel in my mind....but what about loudness and softness of music for example. I think I can now understand how the frequencies modulate the signal, but how would the dynamics of music or voice, or whatever be reproduced and then discriminated on the other side if only frequencies of the voltage changes in the signal are changing the carrier instantaneous frequency? Am I making any sense?

Sorry for an additional question; this has been "bugging" me for a while. I would love to move on to building something like a transmitter, but I am trying to make myself understand each part as I proceed!

D. Peter Van

 

OK, suppose your VCO is centered at 100MHz, and has a "gain" of 10kHz per volt. Now, drive your VCO with a 1kHz square wave that goes from -1V to +1V. The input starts out at -1V, and the VCO outputs 99.99MHz. After 0.5ms (half the period of 1kHz), the input switches to +1V, causing the VCO output to suddenly change to 100.01MHz.
OK, now double the square wave amplitude, and change the frequency to 2kHz. The input starts out at -2V, and the VCO outputs 99.98MHz. After 0.25ms (half the period of 2kHz), the input switches to +2V, causing the VCO output to suddenly change to 100.02MHz.
The change in modulating voltage (2V peak-to-peak to 4V peak-to-peak) is a change from soft to loud. The change in modulating frequency (1kHz to 2kHz) is a change in pitch. The VCO followed both (it deviated farther, and more rapidly).
You could substitute sine waves (or any waveform) for the square wave, and the VCO frequency will track the voltage of the modulating signal.
In summary, the frequency follows the instantaneous voltage of the modulating signal.

 

YES...GOT IT! Change in pitch changes the frequency more rapidly; change in loudness (amplitude) changes the frequency farther! Now that makes better sense to me.....

Thank you very much...I cannot beleive I stumbled upon this forum...what a great resource for people from all different backgrounds who are interested in electronic things. You guys should be commended for this excellent website!

D. Peter Van