Your questions are very open ended, so my response will necessarily be somewhat vague.
Quote:1) can split noise in AM-PM be considered a general way to treat noise in any circuit?
The noise is the noise, and while you can always choose to interpret it as either AM or PM noise, or some combination of the two, the circuit itself will not care. Thus, you tend to only decompose noise into its components when displaying the noise. The circuit and the simulator itself will use the undecomposed noise.
Quote:2) In case of an ideal amplifier is it possible to consider only amplitude additive noise just because PM component is neglected?(noise is about white)
In the case of a time-invariant non-distorting amplifier the noise added by the amplifier will be stationary, which means it will have equal amounts of AM and PM noise. As such, when considering the noise produced by an such an amplifier, we would never decompose it into its components. However, the input of the amplifier may contain noise that has unequal AM and PM components, in which case that will also be true at the output.
Quote:3) AM noise in resonators is often neglected and all noise is supposed to be PM. Is it true because we suppose to have a complete conversion AM to PM due to the feedback?
Resonators are generally linear circuits, so the noise they produce is stationary, meaning that it contains equal amounts of AM and PM noise. So again, if we were considering the resonator alone, we would never decompose the noise into its components.
But I suspect that is not the question you are asking. Presumably by resonator, you actually mean resonant oscillator? Except in rare cases, the noise at the output of oscillators is almost purely in the phase. The natural amplitude control mechanisms of the oscillator act to suppress the amplitude variations. (It is wrong to say the feedback converts the amplitude noise to phase noise, the feedback suppresses the amplitude noise.)
Quote:4) Additive PM noise spectral density depends on the input signal level, instead multiplicative noise is independent on the input signal level. Is this also true for FM white noise generated from white phase noise?
I really do not like the term 'Additive PM noise'. I usually associate 'additive' noise as stationary noise. AM and PM noise only springs from modulation processes, which are more aptly described as 'multiplicative'.
PM and FM noise are the basically the same thing. It is just the FM noise is scaled by the frequency. Generally we talk about FM noise at the output of an frequency modulator. And a frequency modulator is the same as a phase modulator where the input signal has been passed through an integrator.
-Ken