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Design >> RF Design >> noise folding in VCO
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Message started by chenyan on Dec 16^th, 2006, 2:59pm

Title: noise folding in VCO
Post by chenyan on Dec 16^th, 2006, 2:59pm

According to my calculation and also other publications. The noise contribution of differential pair to VCO phase noise doesnot change with W/L if Gm of this pair considered a square wave-like.

But in spectreRF simulation, the noise V^2/Hz decreases when W/L is inreased. My simulation was always in current-limted region a square-wave like Iout. Also the frequency changes from parasitics of the transistors are compensated with ideal caps so all the simulations oscillate at same freq. Tail current is also kept same therefore a almost same vout amplitude.

Does anyone know why? Thanks

Title: Re: noise folding in VCO
Post by sheldon on Dec 16^th, 2006, 3:13pm

Chen,

Could you provide the reference you are referring to by the statement, "The noise contribution of a
differential pair to VCO phase noise does not change with W/L if the Gm of this pair ...". It makes it
easier to respond to your question.

Best Regards,

Sheldon

Title: Re: noise folding in VCO
Post by chenyan on Dec 17^th, 2006, 7:20am

Sheldon,

You can find this statement in "The designer's guide to high-purity oscillators" by Hegazi on page 57.

Although the deduction is somehow wrong and unclear, but this statement however fits my calculation too.

Best regards
Chen

Title: Re: noise folding in VCO
Post by neoflash on Jan 1^st, 2007, 1:10am

I guess Emad's book contains something to reply your question.

1. larger W/L increase harmonics, the more harmonics, the more phase noise we have.
2. larger W/L increase Ctail, thus more upconversion of low frequency tail noise.

MIT published a new paper on it 2005 JSSCC Feb. You can go and have a look.

rfmems wrote on Dec 17^th, 2006, 7:20am: