Post subject: VCO pulling effect and S-parameter Posted:
Sun Jul 17, 2005 5:53 am
Joined: Mon May 24,
2004 11:30 pm
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?
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.
Post subject: S-parametersPosted: Mon Jul 18, 2005
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!
Post subject: Posted: Mon Jul 18, 2005 11:33 pm
Joined: Mon May 24, 2004 11:30 pm