How to get Ceq and Req of a node?

[teem]

Hi,

I am newbie on circuits. I am trying to get the equivalent R and C of a designated node with respect to frequency. First attached image is my example, it’s a simple CS amplifer, and I use below ocean script to calculate equivalent Req and Ceq:

…
analysis(‘sp ?start 10M ?stop 50G ?ports list(“/PORT0”))
…
y11=YP(1 1)
Ceq=imag(y11)/2/acos(-1)/xval(y11)
Req=1/real(y11)
plot(Ceq Req)

The results are shown in 2nd attached image.

I am quite baffled by the results. Here are my questions:

1. Is my C2 + PORT0 connection correct?

2. Is my script to calculate Ceq and Req correct?

3. If yes, I am expecting the Ceq be close to C0(1pF) and Req be close to R0(1K ohms) and should be constant over a wide range until inductance effects kicks in at quite high frequency, am I right?

4. But the results showed the Ceq and Req are not close to constant and showing roll-off behavior. How to explain that?

5. Why the Ceq be as high as ~1.4pF at freq=10MHz. Shouldn't it be close to 1pF?

Thanks a lot in advance for your help to correct my poor knowledge.