Alice,
   What type of circuit follows your switched-capacitor filter? Is it a sampled-data circuit, such as a sample and hold or ADC, or is it a continuous-time circuit, such as a continuous-time anti-alias filter?  A sampled-data circuit is only sensitive to the noise your SC filter produces at the instant when it samples, while a continous-time circuit is sensitive to noise produced at all time during the clock cycle.

If your SC filter is followed by a continuous-time circuit, then you are interested in the continuous-time behavior. SpectreRF computes this directly. If your SC filter is followed by a sampled-data circuit, then you are interested in the discrete-time behavior of your circuit. In this case, you must either use SpectreRF's strobed-noise feature or add an ideal sample-and-hold to the output of you circuit so that only the noise at the sample point is measured.

-Ken

You don't say how it is failing to converge. Is it failing to converge in DC, in transient or the tstab interval of PSS, or is the outer PSS iteration failing to converge.

-Ken