The AM/PM characteristic of a RF power amplifier graphs the phase difference of the input and the output signal in dependance of the amplitude of the input signal. 

I have found a formula, which is stated to give a measure of the phase distortion: 

Now, I wonder: how is this magnitude related to the AM/PM ? Unfortunately, it is not mentioned what exactly Phi denotes. But I am pretty sure it denotes the phase difference of the input and the output signal. So in that case, kp should yields the slope of the AM/PM, doesn't it?

Yes.

great, thanks !

yes  sounds reasonable.

Ques  Why isn't IIR filtering used in sdr radio, as it is a short algorythm and can be procedes fasterthan FIR filtering ?

Did you mean to ask me that question? I don't see the link to my original question. And I have no clue about it. I mean, I'm grateful if you are trying to give me some more input.