I have a few questions about harmonic distortion as follows:

1. In a specific circuit, we assume that there is no phase delay to the circuit, what about the relative phase between the fundamental wave and its 2nd harmonic distortion for a single-tone input of an opa? Is the relative phase difference is fixed? If it is fixed, we assume that the value is a and there is additive phase delay of the circuit b, then what about the relative phase between the fundamental wave and its 2nd harmonic distortion?

2. How about the relative phase between the fundamental wave and its 2nd harmonic distortion for cascade circuit of two op amplifiers(of the same type)? For instance, if the input fundamental wave is 1Vpp of amplitude and frequency of 1MHz, and 2nd harmonic distortion is -60dBc for the specific input frequency and amplitude of an opa, when it goes through the opa(the gain is 1), then the fundamental signal will produce a 2MHz signal (2nd harmonic distortion) of 1mVpp(Vh1). If the fundamental signal and Vh1 go through another opa (of the same case with above), then then the fundamental will produce another 2MHz signal of 1mVpp(Vh2), then how to calculate the total 2nd harmonic distortion of the 1MHz fundamental signal?

3. Is there a universal mathematical mode for an opa? How to calculate the harmonic distortion accurately?

4. I find a figure(as shown in figure below) about harmonic distortion about AD829. For a general application, I abtain a 2nd harmonic distortion of -65 dBc( f=156.25kHz, 2Vpp,RL=100 ohm), but from the figure, I can only abtain about -55 dBc. I don't know why. Does the figure show the harmonic distortion of worst case to most extend?

Thank you very much!