MRW said:
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?
Thanks again!
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).
Cheers
PeteS