Hi Sheldon,
Thank you very much again!!
I tried small signal ac analysis with the low pass filter available in the ahdl library and it does not seem to work with ac analysis; the filter however is a time domain model and does not have any laplace functions in it.
I have one other question -
I am currently reading up on the phase domain model of the PLL available at
http://www.designers-guide.org/Analysis/PLLnoise.pdfCan this model be used to run small signal ac analysis to get gain/phase plots?