questions about S(2,1) for the tank of LC VCO

[mingpangzi]

Hi, I am designing a 30 Ghz LC VCO, and I used a typical equivalent LRC model for the tank--L,C,R in parallel.

How I find S(2,1) is quite different from what we get in the tutorial. Usually we get a hat shape for S(2,1), but now I get a sharp V-shape for it.

Could anybody give more comments on it to help us understand it?

Thanks!

 

[BigBoss]

It is normal....
You have connected your circuit serially and at resonace frequency it behaves as a blocking circuit at its resonace, therefore S21 is lower than 0 around that frquency.
The hat which you have seen before is input impedance of the circuit.To see this, you connect only one port to the circuit in parallel and make your simulation again.

 

[mingpangzi]

It is normal....
You have connected your circuit serially and at resonace frequency it behaves as a blocking circuit at its resonace, therefore S21 is lower than 0 around that frquency.
The hat which you have seen before is input impedance of the circuit.To see this, you connect only one port to the circuit in parallel and make your simulation again.

Thanks for your comment.
I check the manual, the hat shape is also S(2,1). I upload the picture.
And in my case, the phase shift is less than 2 degrees, I am not sure this is the problem, I find S(1,1) can not have a larger than 1 gain when the phase shift is 0, when I use osctest.

 

[kspalla]

Check for the wider freq range.
These filters have null and possibly you are verifying at the same freq.