hi, i have done pnoise analysis  using spectre on a sample and hold circuit( nmos used as the  switch) .  I have used a sampling frequency of 625KHz and  signal frequency  of  62.5KHz. Hold  capacitor value is 14.5pF, which shuold give a total integrated noise  power  of 300p (kT/c). i used a maximum sideband value of  200.
The thing Iam confused with is that if  we  choose  stop frequency equal to  312.5KHz(fs/2), the total integrated noise is much less than kT/c . At the same time  if we choose stop frequency equal to 1GHz  the total integrated noise at the output equals the kT/c noise.
Even though we used the maximum sideband options, we still need to integrate till very high frequencies. Can anyone please tell me why it is like this ?