PSS determines the period of the small-signal analyses

Here is a description about PSS analysis in Cadence:

PSS is a large-signal analysis and determines the period of the small-signal analyses. PSS requires that multiple periodic stimuli be coperiodic.


Can anyone explain the bold part?

I think you used more than one stimuli ( sources,ports etc. ) that have no minimum common frequency.
When you use PSS, the stimulis should have at least common beat freqeuency which is at the same time biggest searching period.

Thanks. Sorry for the confusion. Those statements are from Cadence tutorial. Here is the page: http://wendang.baidu.com/view/4ec559...fc77db2de.html
I have read some articles about PSS analysis and here is my understanding relating to fundamental frequency.

Take an example of an circuit including two sources, source1 900MHz and source2 10MHz; the fundamental frequency is the largest common one of the two, therefore, f_fund = 10MHz.
And could you explain how PSS determines the period of the small-signal analyses. I am new to the software and I am trying to use it.