S-parameter and VCO pulling effect

[trashbox]

Hi all,
Would you give me any hints about the following questions:

1) If I know the s-parameter of a block(such as a buffer amplifier), how to calculate the input impedance and output imdedance according to S11,S12,S21 and S22?

2) Many papers mentioned that VCO's pulling effect is due to the varying load.I am puzzled why the load impedance will vary if the VCO is fixed to a mixer or a divider in PLL?

Thank you very much.

 

[armsgroup]

this the relation between z matrix and s matrix

 

[trashbox]

To armsgroup:

Thank you very much. Would you give me more information(or the original source or URL) about this relation?

 

[toonafishy]

The impedance looking into the base (BJT) of the switching core of a Gilbert Cell mixer (or a divider) changes as the core switches. The effect is much less looking into gates (MOS) but it still changes. This changing load pulls the VCO unless there is a buffer between them.