Rather late response, sorry. Been a bit tied up the last few months... (for a change!)

The primary reason is that QPSS is a "mixed frequency-time" analysis, and so is partly in the frequency domain (the "moderate" signals) and partly in the time domain (the "large" signals). Since tdnoise relies on the time domain approach, that's why it isn't available.

Conceptually I would have thought it could be technically possible to strobe the noise output during the shooting interval used in QPSS (i.e. if the clock were the large signal), but this has not been implemented. I don't know if it really is possible (Ken would have a better idea than me), but it's worth contacting Cadence directly (via customer support) and suggesting it.

As an alternative though, if you can simulate over the common frequency of your large signal input and the clock, then it's possible with PSS. It just boils down to whether your common frequency is reasonable - and by adjusting the frequencies you may be able to make sure it runs in a reasonable time with a reasonable amount of memory.

Regards,

Andrew.