# VCO pulling effect and S-parameter - RF Cafe Forums

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trashbox

Post subject: VCO pulling effect and S-parameter Posted: Sun Jul 17, 2005 5:53 am

Captain

Joined: Mon May 24, 2004 11:30 pm

Posts: 10

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.

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Guest

Post subject: S-parametersPosted: Mon Jul 18, 2005 7:02 pm

S11 and S22 are the input and output impedances, expressed as the reflection coefficients. So solve the formula for reflection coefficient for the desired impedance, given your characteristic impedance:

p=(Z-Z0)/(Z+Z0) -> Z = Z0(1+p)/(1-p).

"VCO pull" is a characterization test, not an operating condition. The more "pull" you have, the poorer the performance as a signal generator will be. The load presented by a nonlinear device such as a mixer or divider will provide a varying impedance to the VCO, thus degrading its spectral purity if the VCO pull is large.

Hope this helps!

Good Luck!

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trashbox

Post subject: Posted: Mon Jul 18, 2005 11:33 pm

Captain

Joined: Mon May 24, 2004 11:30 pm

Posts: 10

thank you,GUEST!

Posted  11/12/2012