FM or PM?

Take a normal analogue FM radio system (like the ones in a car).

Now FM is defined as

cos(Wct + k integral( f (t) dt ) )

and PM as

cos(Wct + kf(t) )

Where f(t) is the signal.

where k is a constant depending on the depth of modulation etc.

Now it is known that broadcast 'FM' uses pre-emphasis at the
transmitter (a 6dB/octave slope with break-frequencies of say 1kHz to
15kHz  - not sure exactly on the frequencies here).

A 6dB/Octave slope is differentiation over a certain frequency range
(the pre-emphasis range).
Surely this cancels out the integration in FM and it becomes PM.

At the receiver a PLL or demodulator will ideally give d/dt [integral
f(t) dt ] = f(t).
ie rate of change of phase.

However, at the receiver is a de-emphasis circuit which has a slope of
-6dB/Octave which will cancel out the effect of the previous
differentiation. Hence FM with pre and de-emphasis is surely PM over
the necessary freq range and outside of this freq range it is true FM.

F.

sheepshaggerx@yahoo.co.uk wrote:
> Take a normal analogue FM radio system (like the ones in a car).
> 
> Now FM is defined as
> 
> cos(Wct + k integral( f (t) dt ) )
> 
> and PM as
> 
> cos(Wct + kf(t) )
> 
> Where f(t) is the signal.
> 
> where k is a constant depending on the depth of modulation etc.
> 
> Now it is known that broadcast 'FM' uses pre-emphasis at the
> transmitter (a 6dB/octave slope with break-frequencies of say 1kHz to
> 15kHz  - not sure exactly on the frequencies here).
> 
> A 6dB/Octave slope is differentiation over a certain frequency range
> (the pre-emphasis range).
> Surely this cancels out the integration in FM and it becomes PM.
> 
> At the receiver a PLL or demodulator will ideally give d/dt [integral
> f(t) dt ] = f(t).
> ie rate of change of phase.
> 
> However, at the receiver is a de-emphasis circuit which has a slope of
> -6dB/Octave which will cancel out the effect of the previous
> differentiation. Hence FM with pre and de-emphasis is surely PM over
> the necessary freq range and outside of this freq range it is true FM.

Go to the head of the class. Is there a question ?

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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