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LC VCO phase noise versus Q factor (Read 2849 times)

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LC VCO phase noise versus Q factor
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=V�/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!

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Jeffrey987 Junior Member Offline Posts: 26	Re: LC VCO phase noise versus Q factor Reply #1 - 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.
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aaron_do

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Re: LC VCO phase noise versus Q factor
Reply #2 - 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

there is no energy in matter other than that received from the environment - Nikola Tesla

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