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Design >> RF Design >> LC QVCO with shared bias node
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Message started by casual on Jun 29^th, 2010, 1:48am

Title: LC QVCO with shared bias node
Post by casual on Jun 29^th, 2010, 1:48am

From the CICC paper
"2-V 900MHz Quadrature Coupled LC Oscillators with Improved Amplitude and Phase Matchings" (C Lo, Howard Luong)

It stated that sharing the nmos bias node (drain nodes of the two LCVCO current source), it can improve amplitude mismatch and improve phase noise Pnoise.

From my simulation, it is not the case. I found that it is case by case.
Anyone know the real reason?

Title: Re: LC QVCO with shared bias node
Post by Mayank on Jun 29^th, 2010, 2:42am

I havent read the paper. BUT

Quote:

It stated that sharing the nmos bias node (drain nodes of the two LCVCO current source), it can improve amplitude mismatch and improve phase noise Pnoise.

Quadrature LC VCO comprises of two LC VCOs operating at 90 degrees phase diff.
The Drain of the TCS nmos is pulled up twice in one Period of Oscillation in a Single LC VCO. i.e. TCS Node has a major harmonic at 2*fosc.
If you connect the TCS nodes of two LC-VCOs, then if your QVCO is exactly symmetric, & ignoring higherr-order non-linearities,
the 2f fluctuation due to 1st VCO is out of phase to the 2f fluctuation due to the 2nd.
Hence, they should cancel & result in an improvement in Phase Noise.

--
Mayank.