PSS PNoise setup for dynamic comparator

Hello everyone,

I am trying to measure the noise of a dynamic comparator.

The comparator operates with frequency FCLK = 40MHz. Besides, there is an auto-zero clock, which is equal to the sampling frequency of the ADC ( maybe FS = 40KHz or 2MHz depends on the ADC)

The comparator will measure the input in some first periods of FCLK, then come into auto-zero periode ( some periods of FCLK) and then take a rest in the rest time of sampling phase.

In the setup of PSS, I use shooting option, auto calculate for the beat frequency --> Beat frequency = Fs = 40K (or 2M)

In the PNoise, in output sweep range, i put from 1 Hz to FCLK/2 = 20MHz. I use sample(jitter)/edged crossing with sampling ratio = FCLK/FS

The noise will be measured at the rising edge when the output = VTH

The problem is when I change the FS, the measured point is the same ( because the FCLK is the same, auto-zero haven't turn on yet at this point so the FS should not effect to the performance), but the noise is so much different. 

So, my question is:

Can I use the sample ratio like this to measure the FCLK/2 range instead of pss/2 ? 

Is there any problem with my setup that leads to the different noise following the FS ?

Thank you