abnormality in SP analysis in Spectre

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gunturikishore

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Hi all,

I am doing the Sparameter analysis for a circuit and consisting of Three Transisotrs at a node typical sizes being 120/.18u,60/.18u and 60/.18u and I connect them at that node to Gate, Source and Drain Resp.

Now I connected at tht node the port instance form analogLib and when I do sp analysis it comes out to be 0.230 - j 0.34 on Z-smith. Now when I connect a 1p F series capacitance in b/n the port and node it turns out to be .670 - j .93.
Now my doubt is how a series capacitance varying the real part of the Z-smith. I expect only the imaginary part to change. My frequency of operation is 2.1G.

Thank you in advance.
 

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