To begin with: Let me clarify that I'm using icfb 5.1.0 and spectre 7.1.1 for my simulations.
My circuit has an opamp which supplies the tuning voltage to the succeeding VCO stage. It has to be mentioned that a feedback path exists from the oscillator to the opamp and thus I set out to ascertain the stability of the feedback loop present. I'm making use of the pstb analysis with the iprobe component placed between the output of opamp and tuning input of VCO.
What puzzles me is that the loop gain magnitude from the pstb always starts from 0 dB! But the entire circuit works fine displaying a good tuning curve. One more point to add is that the opamp is a behavioural model where I'm free to change its Gain and GBW.
I'm attaching the plot here and its unclear to me if I'm missing some basic understanding of the pss and pstb analyses.
Could anyone throw some light on this?
Thanks in advance!