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# FM are the same PM at certain conditions?

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#### electronics_kumar

FM PM relations

by some book, i found that both are same at "certain conditions".. what are the conditions that they are same...

Re: FM PM relations

This is my intuitive approach:
An instantaneous change in Phase produces a change in frequency and viceversa.
If the modulating signal is varying the PM or FM signals are the same. The only diferences are probably with DC or DC level. In FM a DC level will shift the carrier to another frequency. In PM a DC level will not change the freq. of the carrier.
You can use an FM radio to speak to a PM radio (Marine VHF used to be PM).

Re: FM PM relations

albert22 wrote
The only diferences are probably with DC or DC level. In FM a DC level will shift the carrier to another frequency. In PM a DC level will not change the freq. of the carrier

what do you want to share ...i couldnot get it ...may be unabale to decode your valuable info... could you tell it in a simple jargon so that i will understand it

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

Re: FM PM relations

In both cases we have to cnosider the phase of the signal for demodulation.
as long as you look for phase variation, it does not matter wether the phase variation is done by frequency change or the phase change itself.
Regards

FM PM relations

As I understand it so far, Phase modulation is basically just a 'return to zero' version of FM modulation?

Re: FM PM relations

PM chnage in a 1 period..FM in a haft period!!!

Re: FM PM relations

When the signal is 0 the carrier is at its center frequency.

So PM would be inmune to a DC offset and FM not.
// could you throw more expalanation on this words ..i could understand it properly...

Re: FM PM relations

They are almost the same. PM modulations can be received on the FM receiver and vice-versa. This is because in both modulations there will be frequency components around the carrier frequencies. But in PM these components are created by change in phase of carrier. But in FM its due to intentional change of frequency of carrier.

Actually in PM I think some parts of the audio spectrum (may be LF) is more preserved than the FM. So on receiving PM in an FM receiver there will be some change in audio quality.

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