PSS and number of harmonics in crystal oscillators

Hi,

I'm simulating a test bench consisting of a crystal oscillator, followed by several inverter stages that act as buffers.

I want to simulate the phase noise at the output of the last buffer, and eventually see how the added buffers contribute to the total phase noise.

I'm using PSS+PNOISE(sources option). The PSS uses the Twotier Method and the oinnode+ is on the crystal pin (not at the output of the last buffer). In the PNOISE instead, since I want to calculate the PN at the output of the last buffer, I used as output (Positive Output Node) the output from the last buffer

Is this setting correct?

Moreover....

Since the output is a squared waveform(due to the buffers) I'm selecting an oversample factor of 8 and an high number of harmonics. However, PN results increases when increases the number of harmonics.

Why this? Is an higher number of harmonics more accurate?

Note that from 40 harmonics on, the PSS fails to converge.

Thanks

Giuseppe