questions on simulating THD' of s/h?

I've finished a fully differential bps s/h block and am working on the
charaterization of the specs. now I've got a bunch of questions
concerning the usage of cadence spectre, ADE and calculator. Could
anyone please help me out?

1. what is the best/reliable method of simulating/measuring thd of s/h
circuit? Seems to me there are 3 ways: 1) using calculator's thd
function; 2) using dft function and calculate the harmonics from the
plot; 3)using fourier analysis...

2. If I use thd function (or dft func), how many samples do you
recommend? The default is 64.

For this method, what I did is to click the gernerated s/h output plot
and apply thd func. Is this correct?

3. How many cycles I need to run in the transient analysis? I run 30.1
cycles (of the input signal period). According to my memory, the longer
the better accuracy. Is this correct?

4. I read the fourier analysis help (spectre -h fourier), but got
confused by the first sentence right away: "The ratiometric Fourier
analyzer measures the Fourier coefficients of two different signals at
a specified fundamental frequency without loading the circuit..."? Why
"two different signals"?

5. What is the typical/acceptable thd?

Thank you for your kind help!

 

Anyone?