supply noise rejection for LC VCO - RF Cafe Forums
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Post subject: supply noise rejection for LC VCO Posted: Mon Feb 20, 2006 8:50 pm
Joined: Mon Feb 20, 2006 8:49 pm
If the supply is the much noiser than the ground, which biasing scheme is better for a cross-coupled LC VCO? Biasing from bottom with NFET or from the top with PFET? Thank you.
Post subject: VCOPosted: Wed Feb 22, 2006 4:04 pm
Joined: Wed Feb 22, 2006 3:51 pm
I suspect it depends on what junction capacitance is being used to control the frequency, and which voltage it references.
That said, if you really can maintain a good constant current from the supply (the PFET option), the current source will isolate the noise. But, if you simply use a current mirror, all bets are off - you really do need to keep constant current.
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