That's exactly what it is. Perhaps the OP doesn't realise that FM converts (at the modulation stage) an amplitude to a frequency deviation around the carrier. This leads to the inescapable conclusion that the demodulation process must convert a frequency difference [from a carrier, which at this is probably not the same one] to an amplitude :)
Dear OP: Some basics of FM might help (google is your friend).
Thanks all! Happy Holidays! I am familiarizing myself with FM.
If I gathered it right, the demodulated signal (in amplitude) would "sketch" out the audio waveform, no? Does this indicate that at the zero deviation the modulator would put at 0 V and the value would go up as high as 75 mV at the higher limit of the audio range?
No, the "audio range" is an entirely different animal. If I read Dan Hollands' interpretation correctly, it means that when the carrier is just at the carrier frequency, the output of the discriminator is at
0 volts. But when the carrier frequency deviates from center, the discriminator puts out a voltage that is proportinal to the instantaneous difference between the FM modulated carrier _right_ _now_ and the center freq.
This can happen at such a fast rate that the resulting instantaneous changes can be processed as audio, recovering the information from the station. Another side benefit is "automatic frequency control," which uses this 75 mv/22.5khz or whatever it was, after filtering, to feed back to the local oscillator and kind of "lock in" to the signal's frequency.
As Rich said, it depends on your point of view. Instaneously, you will have a DC voltage proportional to the difference between the carrier and the *current* frequency* (this is somewhat simplified, as there will usually be a low pass filter involved, so what you will have is the integral of past history DC levels)
If the original carrier was modulated with audio, then the output of the demodulator should be that audio, although not necessarily at the same amplitude.
In pure infornation terms, the 'information' in the signal is S(fc - fi) where fc is the carrier and fi is the instaneous frequency and S is the integral over some specified time. Depending on the point of view, one may take the instaneous difference [not taking the integral], or the integral (as in the case of filtered audio).
At zero deviation, there is no 'information' signal, so the output should be 0 offset from some arbitrary level, but not necessarily 0V.
The 75mV is simply a spec showing the transfer curve of the demodulator. The actual output may be higher or lower depending on the actual deviation of the carrier (there are a number of different specs, each with their own deviation limits).
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