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Design >> RF Design >> VCO basic question
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Message started by raja.cedt on Aug 27^th, 2010, 6:43am

Title: VCO basic question
Post by raja.cedt on Aug 27^th, 2010, 6:43am

hi,
through math i came to know that series resistance with inductor in lc tank reduce the resonance frequency where as with series resistance with varactor will increase. But i want to know from theory..can any one explain it...i am sure it is because of series resistance inductor Q will go down and varactor Q will increase and i am guessing this is the main reason.

Thanks,
Rajasekhar.

Title: Re: VCO basic question
Post by vp1953 on Aug 27^th, 2010, 3:59pm

HI Raja,

Q for a inductor/capacitor is always defined as magnitude of reactive part/resistive part, so it always comes down when the series resistance increases regardless of whether it is capacitive or inductive.

when you have a series resistance with the inductor or capacitor, it has to be converted to its parallel equivalent and the parallel equivalent is used to calculate the resonant frequency via w=1/sqrt(LC).

For an inductor, with a series resistance, the parallel inductance value increases and is given by (w^2.L^2+R^2)/(w^2.L) where R and L are the series resistance and inductance. Hence the resonant frequency comes down.

For the capacitor, the parallel capacitance value decreases and hence the resonant frequency increases.

Title: Re: VCO basic question
Post by raja.cedt on Aug 28^th, 2010, 11:47am

hi,
tx for your reply, so what you are saying is correct so during conversion from parallel to series you will get large inductor and small cap you would see the difference. But i am trying to get this feel from Phase diagram..any how tx for your reply.

Thanks,
rajasekhar.