Re: FM PM relations
Sorry if I was not clear.
In FM as the modulating signal varies, the frequency of the carrier is changed. When the signal is at its max. the deviation of the carrier is max.
When the signal is 0 the carrier is at its center frequency.
If you apply a DC (that is a constant) as the modulating signal, the carrier will change its frequency and will stay there until the DC goes to 0 again. This also occurs if the mod. signal has a mean value different than 0. (Or in another words, has a DC level or offset). One example of this is V(t) = Vdc + sin(wt). In this case the entire band will be shifted.
In PM the phase changes with the mod. signal. A change in phase produces a change in the freq. If the phase is constant, there is no change in the freq. Only when you are varying the phase, the freq varies.
If you apply a DC, the carrier wil shift its phase. There is going to be a freq change for an instant but then the carrier will return to its center freq. and stay there. There is no shift on the band.
So PM would be inmune to a DC offset and FM not.
A DC offset could be present when you are working with digital pulses. There are ways to code the digital pulses to obtain 0 DC offset. To minimize the freq. shift in FM transmition.
More mathematically, you can think that a FM results from the derivative of the signal applied to a PM modulator. The derivative of a constant is 0 so no FM will result. The derivative of a sine is a cosine so pure FM will result.
Hope that this is more clear. However this is my point of view, I dont think it as valuable but just trying to help.
Regards