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Design >> RF Design >> Phase noise by switch transistor in OFF state
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Message started by Visjnoe on Jan 4^th, 2008, 9:04am

Title: Phase noise by switch transistor in OFF state
Post by Visjnoe on Jan 4^th, 2008, 9:04am

Dear all,

I'm examining a 1.8GHz CMOS VCO and its phase noise performance. The VCO has discrete frequency tuning by means of a capacitor bank.
The capacitor bank is a classical implementation, meaning binary scaled capacitances which are switched by an NMOS transistor in series.

When I examine the phase noise contributions (phase @100kHz) for a certain capacitor bank setting, I notice that the largest contributions
come from the NMOS switches that are in the OFF state (thermal noise).

This is the first time I see switch transistors (in the OFF state) dominating VCO phase noise performance and I'm doubting the simulator.
For one, I don't think thermal noise is well modeled for transistors in the OFF state.

Has anyone encountered this before?

Regards

Peter

Title: Re: Phase noise by switch transistor in OFF state
Post by Ken Kundert on Jan 4^th, 2008, 8:47pm

Yes, you are not the first. Check out the "Oscillator Puzzle" on the Theory page.

-Ken

Title: Re: Phase noise by switch transistor in OFF state
Post by Visjnoe on Jan 7^th, 2008, 11:30am

Ken,

thank you for mentioning the interesting paper which analyzes this peculiar situation.

Kind Regards

Peter