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Design >> RF Design >> AM-to-PM conversion
https://designers-guide.org/forum/YaBB.pl?num=1171668335

Message started by chenyan on Feb 16^th, 2007, 3:25pm

Title: AM-to-PM conversion
Post by chenyan on Feb 16^th, 2007, 3:25pm

Amplitude noise to phase noise conversion carried out by the varactor in VCO is one of the phase noise sources. What I can not understand is:
the am noise modulating the varactor has to be low frequency to be converted to PM around the carrier, however the low frequncy am noise is attenuated by the LC tank and thus should be small, while the am noise at frequency around the carrier can only be converted to pn noise far away from the carrier. Then there should be very tiny amount of am noise converted to PM around the carrier, which is opposite to my simulation and experience.
What I have misunderstood? Please help me out.

Title: Re: AM-to-PM conversion
Post by RFICDUDE on Feb 16^th, 2007, 8:45pm

The varactor has only one terminal connected to the tank, the other terminal is connected to the control voltage input. Any noise present on the control input will modulate the capacitance of the varactor and thus modulate the frequency (or phase) of the VCO which shows up as PM noise around the carrier.