RFICDUDE
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Ah ok, I think I can help here.
We are talking about "nonlinear responses" when a particular signal is applied to a nonlinear circuit.
AM-AM and AM-PM is the first harmonic (fundamental) response when the amplitude of a single sinusoidal input signal is swept (large signal gain response). AM-AM AM-PM contains information about odd order nonlinearity (3rd order and higher) as the amplitude is swept such that the amplifier enters gain compression.
IIP3 is related to the 3rd order nonlinear response when a two-tone input signal is input to a nonlinear circuit. The intercept point is a practical abstraction making it easy to compare the linearity of circuits, but it is the measurement of a response to a specific stimulus signal. IM3 is particularly useful in circuits/systems where the amplitudes are such that the 3rd order distortion is the most significant even order contributor around the desired signal (i.e. receivers, LNA, downconverters, etc.).
Spectral regrowth is the nonlinear response to an arbitrary stimulus signal exhibiting amplitude modulation. It is accurate to say that IM3 is the spectral regrowth when a two-tone signal is the stimulus and the amplitude is low enough that higher order nonlinearity (5th, 7th, etc) is not contributing significantly to the response.
Now AM-AM AM-PM and IM3 are characterizations of the nonlinearity because the stimulus signals are known and analytic, so the underlying nonlinearity can be extracted as power series coefficients by finding the values of coefficients that match the AM-AM AM-PM response or IM3.
Ok this is great info, but it doesn't answer your question about what to use during design.
For receivers, IM3 is good enough, but it can pay to do system level simulation with IM3 and then maybe envelope simulation with a modulated signal to determine any difference between the two. If everything stays 3rd order then the offset shouldn't change and IM3 can be used with good confidence. You can do something similar for IM2 situations too.
For transmitters, what I have seen work well is some designers correlate spectral regrowth against IM3 to find a crude offset between the two for modulated signals of interest. Then IM3 is use through the design phase since it is more straight forward to simulate and interpret the impact of design changes, but ultimately spectral regrowth needs to be checked using a AM-AM AM-PM model to pass signals through just to make sure to correlation is ok (the correlation is different for different signals). Also AM-AM AM-PM is only accurate for circuits that do not exhibit significant memory effects (IM3 dependent on tone-separation or tone-frequency). In this case AM-AM AM-PM will work for narrow tone-spacings but not so good for wider tone separation.
In the end you will likely need to use both IM3 and AM-AM AM-PM to gain confidence in the predictive spectral regrowth performance.
There are special cases such as OFDM where the signal can be modeled as a Gaussian or complex Gaussian amplitude modulation where the properties of the signal are known well enough to permit nonlinear calculations using the statistical properties of the assumed Gaussian signal. But this discussion gets a bit involved ...
The references below don't exactly answer your question, but they do contain some discussion about AM-AM AM-PM contributions to spectral regrowth.
J. S. Kenney and A. Leke, "Power amplifier spectral regrowth for digital cellular and PCS applications," Microwave J., vol. 38, no. 10, Oct., 1995, pp. 74.
Aikio, J.P.; Rahkonen, T.; , "A Comprehensive Analysis of AM–AM and AM–PM Conversion in an LDMOS RF Power Amplifier," Microwave Theory and Techniques, IEEE Transactions on , vol.57, no.2, pp.262-270, Feb. 2009
Sundstrom, L.; , "Fast and accurate ACLR estimation method," Radio and Wireless Conference, 2004 IEEE , vol., no., pp. 183- 186, 19-22 Sept. 2004
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