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Design >> RF Design >> LC VCO phase noise versus Q factor
https://designers-guide.org/forum/YaBB.pl?num=1423734449

Message started by JV on Feb 12^th, 2015, 1:47am

Title: LC VCO phase noise versus Q factor
Post by JV on Feb 12^th, 2015, 1:47am

Hi guys,

Could someone please enlighten me why the phase noise of an LC VCO drops with 6dB when doubling the loaded Q factor (while keeping all other parameters constant)? Looking at Leeson's formula, it has the Rp losses in the numerator (Ps=V²/Rp) and Q squared in the denominator.

I would expect that doubling the Q factor of the LC tank while keeping L and C constant, implies doubling the Rp as well. This would result in only 3dB improvement of the phase noise, is this correct?

What am I missing here?

Thanks!

Title: Re: LC VCO phase noise versus Q factor
Post by Jeffrey987 on Mar 12^th, 2015, 11:00am

It depends if you assume V constant? Is your VCO current limited or voltage limited?
If it is current limited, Ps = V^2 / Rp ~ i^2 Rp, if it is voltage limited, it is as you assume in leesons formula.

Title: Re: LC VCO phase noise versus Q factor
Post by aaron_do on Mar 13^th, 2015, 11:34pm

Quote:

I would expect that doubling the Q factor of the LC tank while keeping L and C constant, implies doubling the Rp as well.

Hi,

why do you think that doubling Rp would make the phase noise worse? If I made Rp infinite (which is equivalent to removing it), what do you think would happen to the noise performance?

Aaron