How to measure the S21 and S12 plot of a buffer stage in Spectre-RF?

Hi Guys:

I had integrated a buffer stage at the output of the designed VCO, for isolation. How do I measure the S21 and S12 plot of this buffer stage in Spectre-RF to observe the isolation characteristic. Thanks in advance

Rgds

If you are just interested in the S12 and S21 of the buffer, connect input and output ports on the buffer, do a S parameter analysis. All the S parameters are stored. You can use direct plot to see the S parameters.

Hi Guys:

The input of the buffer is from a autonomous circuit (VCO), How much input impedance shall I set in the input port of the buffer, I'm using a 50 Ohms open drain, source follower buffer. Thanks in advance

Rgds

How did you do that? if you have any answer please tell me.

You can not define the output impedance of an oscillator.Instead, you may check "Pulling" characteristic of your VCO.
It looks like load pull technique. You can change your load impedance automatically that is seen by VCO only on entire Smith Chart and observe pulling effect for each point.Namely, you'll find a optimum region which gives you minimum frequency shift, the impedances that are in your optimum region will be input impedance of yur buffer.