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Measurements >> Phase Noise and Jitter Measurements >> reconciling ISF/LTV model and Mixer model for LC VCO phase noise
https://designers-guide.org/forum/YaBB.pl?num=1392318046

Message started by love_analog on Feb 13^th, 2014, 11:00am

Title: reconciling ISF/LTV model and Mixer model for LC VCO phase noise
Post by love_analog on Feb 13^th, 2014, 11:00am

Sorry if this is a dump question
a) According to LTV theory, you just take the fourier transform of ISF and multiply with the various noise sources (gm stages and bias current) to get the final phase noise.
The noise at wosc gets multiplied by c1
There is no talk about AM and PM from noise values at different frequencies
b) According to mixer theory,
Low-frequency noise (thermal) mixes up to create two correlated sidebands resulting in only amplitude modulation (AM) noise. High-frequency bias noise (which is only thermal) downconverts into a single noise sideband in the passband of the LC tank, containing both AM and PM noise.

Questions...
a) So if there is no AM-PM conversion than mixer theory predicts lower phase noise than ISF
b) In addition mixer theory seems to indicate that noise at wosc gets mixed to dc and 2*wosc so it won't impact final phase noise either.
c) Mixer theory claims Low freq flicker noise gets upconverted to 3wosc, 5wosc etc. which flows through the cap and causes phase modulation. It also gets converted via AM-PM since any variation in current changes the swing of the VCO.

Title: Re: reconciling ISF/LTV model and Mixer model for LC VCO phase noise
Post by aaron_do on Feb 13^th, 2014, 5:30pm

Hi,

I'm not an expert on this by any stretch, so maybe somebody can answer this properly, but here is a link that might be helpful. Basically what I got from it is that LTV theory cannot be used to find phase noise in oscillators.

http://www.kevinaylward.co.uk/ee/phasenoise/LTV.xht

regards,
Aaron