FM vs PM

Martin Czech czech at Micronas.Com
Mon Aug 7 14:17:14 CEST 2000


I think it must be integrators and differentiators,
otherwise the operations will not perfrom correctly.
Of course , lp and hp filters are approximations to that,
but in the flat band they won't work



:::The difference between PM[phase mod] & FM is that PM is not only 
:::proportional to amplitude, but frequency of the modulating source as well.. 
:::to create FM from a PM modulator you need a 6db/oct LPF at the modulator 
:::i/p. Conversely, to create a PM from FM modulator, a 6db/oct HPF is used. It 
:::should be possible to use a simple RC network so long as the modulating i/p 
:::is high-z. Of course, the DC offset will create differing effects between a 
:::'synthetic PM' and a real PM.
:::Cheers,
:::Dan


The difference comes very clear, if a rectangular modulator is used.
In the FM case, the carrier phase pointer moves faster and slower, according
to the sqa pattern. In PM the phase pointer will move generally always 
with the same speed, except for those moments where the input sqa skips,
there it will skip too.

Result: The ideal FM chain produces a smooth waveform, the ideal
PM chain gives abrupt jumping waveforms.

Therefore we can predict a certain "brightness" for PM,
which really observed.

m.c.




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