Need Information about FM and AM modulators


Joined Apr 2, 2007
in a previos thread abt demodulation this was one helpful reply i got.
"Google the terms "ratio detector" and "foster-seeley discriminator"


Joined Feb 24, 2006
Do you have enough background knowledge to dive right in or would it be better to start with a refresher on AC circuits. Tell ya what - here's a question.

If I take two sine waves at frequencies f1 and f2, and I multiply them together, what frequencies will come out of the multiplier. I'll even give you a hint, there are four of them. That's an AM modulator.


Joined Jan 22, 2004
here's a general overview:

the FM way of transmission begins with an r-f carrier wave and an audio freq. which we call modulating signal. when the r-f carrier is modulated with an audio signal, the freq. of the r-f carriers varies with the amplitude of the modulating signal.

at the receiver end, the antenna receives FM signals within its band and the tuner stage selects a specific band by way of the tuning cap. then the r-f amp strenghtens the FM signal. the local oscillator generates a constant amplitude r-f signal which is mixed with the FM signal producing an intermediate freq. (i-f). these i-f stages are usually tuned to 455kHz. the i-f stages likewise passes and at the same time amplifies the i-f carrier.

the FM detector stage is different from the AM detector stage. the FM detector must convert the freq. variations into its audio representation. then the detected audio signal is fed thru a de-emphasis network. this networks restores the relative amplitude of the signal freq. components. at the transmitter, the high freqs. are further amplified which we call pre-emphasis which is to improve the signal-to-noise ratio for transmission. so in the receiver end the reverse is done.

after the de-emphasis process, the audio signal is finally amplified to be able it to drive the speaker. you will also notice that in the receiver system there is an automatic freq. control (AFC) whichs keeps the receiver oscillator properly tuned.

some of the AM circuits are also used in the FM function.

well there's also the FM multiplex fundamentals but its a different goat to deal with.:)