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Design >> RF Design >> equivalant impedance of loaded LC tank in VCO
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Message started by chenyan on Sep 20^th, 2006, 10:11am

Title: equivalant impedance of loaded LC tank in VCO
Post by chenyan on Sep 20^th, 2006, 10:11am

As known,in a diff. LC VCO, the oscillation voltage amplitude V=Z(f)*i, Z(f)is the loaded tank impedance.
With PSS simulation, we have the output voltage, with the current i known, it is easy to calcultate the loaded impedance at different harmonics.
Question is how to get this current, where should I probe or how should I calculator? I have tried the current on the transistor drain, but the result seems strange.
It seems that it gives only the impedance of the LC tank, and the transistor loading and changing of Idc or Vdc doesnot effect Req at all.

Somebody help me out, thanks.

Title: Re: equivalant impedance of loaded LC tank in VCO
Post by Nick on Sep 27^th, 2006, 7:58am

Hi,

Have you tried the xf analysis?
It allows you to plot the real and imaginary part of the impedance seen between two nodes.

Nick

Title: Re: equivalant impedance of loaded LC tank in VCO
Post by chenyan on Sep 28^th, 2006, 2:01am

Thanks Nick.

But what I tried to do is stead state analysis of VCO, so PSS should be just fine.

Hegazi in his book said a voltage biased oscillator is inferior to a current biased oscillator, because for voltage-biased, when one transistor is in triode and the other off, it will load the LC tank and results in a lower loaded Q. So I did PSS sim, took the output voltage and the current of transistor drain. get the Z(w) for the fundmental.But I could not find this effect.

Also, I found out Z(w) I calculated at certain harmonics is inductive, that is for sure wrong. So my question is if my way is wrong, which curent shoule be taken to calculate the Z(w) and loaded Q.