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Design >> RF Design >> Flicker noise of active device in LC VCO
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Message started by jlee on Mar 23^rd, 2008, 7:27pm

Title: Flicker noise of active device in LC VCO
Post by jlee on Mar 23^rd, 2008, 7:27pm

Dear guys,

I have two questions about flicker noise upconversion of active device (negative transconductance) in LC VCO.

1. For close-in phase noise (1k ~ 100k), it is observed that the noise contribution is dominated by upconverted flicker noise in active device. My question is how can we explain this observation in physical insight? Is it pure AM-PM action by nonlinear cap or harmonic mixing (say 2nd harmonic) ? How to prove that by simulation?

2. In "A 1.5V, 1.7mA 700 MHz CMOS LC Oscillator with no upconverted flicker noise" by K. Hoshino, it is proposed that an inductor inserted at active device source can help to improve close-in phase noise. Why such an inductor can achieve that?

Regards

Jason

Title: Re: Flicker noise of active device in LC VCO
Post by emad on Mar 25^th, 2008, 2:02am

Flicker noise up-conversion mechanisms are fairly complex. They are mostly based on AM-FM conversion through the varactor and other non-linear caps. However, the capacitance at the common mode point also plays a role that is not AM-FM based. It works more on shifting the switching edge back and forth so it is PM not FM.

The premise in Hoshino's paper is different. The noise filter tunes out the capacitance at the common mode point at a single frequency. It tries to widen this tune-out range by capacitance switching. The noise of the noise filter allows you to inflate the size of the current source (hence reducing its flicker noise) without fear of loading the common mode point.

Emad

Title: Re: Flicker noise of active device in LC VCO
Post by jlee on Mar 26^th, 2008, 1:46am

Thank you, emad.

Do you think it is a robust way to get excellent close-in phase noise by noise fliter (inducotr and cap at active device source) over process and temperature?

Regards

Jason

Title: Re: Flicker noise of active device in LC VCO
Post by emad on Apr 1^st, 2008, 12:58am

It is good for bias flicker noise but it does not dramatically improve flicker of the switches. It will improve pushing though.

Emad

Title: Re: Flicker noise of active device in LC VCO
Post by jlee on Apr 9^th, 2008, 7:00pm

Thanks for your input.

Considering only flicker noise from differential pairs exists in VCO, which structrure is more attractive to get flicker-free phase noise, NMOS only or complementary (PMOS only is not selected for its low effective Gm for a given current)?

Regards

Jason