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The relation between phase noise and varactor capacitance

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phase noise dependence

Hello
in lc vco, can anyone explain why intuitively the phase noise is inversely proportional to the varactor capacitance?, also, why increasing length of the mosfet increases its thermal noise??

thanks
 

Re: capacitor array for wide tuning range vco

It depends on the frequency vs control voltage curve, the top and bottom boundary of the linearer part of two curves should meet and overlap some.
Generally two curves should overlap 20%~50%.

Hope it helps.
 

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    ravet

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Re: phase noise dependence

If ω is not a constant,
Phase noise L(Δω) ∝ ω^2
ω^2 = 1/LC
So L(Δω) ∝ 1/LC
If ω is a constant, LC must be a constant, because the Q of inductor is less than that of varactor, larger varactor means larger Q, then the phase noise is inversely proportional to the varactor capacitance.

For more details, you can read this book: Thomas Lee, The Design of CMOS Radio-Frequency Integrated Circuit.

About the mosfet, if it is parallel in the LC tank, it can be seen as a current noise source, inoise^2=4KTgm∝W/L the thermal noise is inversely proportional to the length.
If it is serial, it can be seen as a voltage noise source, vnoise^2=4kTR∝L/W, then the thermal noise is proportional to the length.
 

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    ravet

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Re: phase noise dependence

Hi,
Thermal noise increases with length.

We know thermal noise is 4kTRf

Now R is the resistance of MOS, which is directly proportional to Length of the MOS gate.

So as length increases thermal noise increases.

thanks
sarfraz
 

    R

    ravet

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Re: phase noise dependence

lijianheng said:
If ω is not a constant,
Phase noise L(Δω) ∝ ω^2
ω^2 = 1/LC
So L(Δω) ∝ 1/LC
If ω is a constant, LC must be a constant, because the Q of inductor is less than that of varactor, larger varactor means larger Q, then the phase noise is inversely proportional to the varactor capacitance.

For more details, you can read this book: Thomas Lee, The Design of CMOS Radio-Frequency Integrated Circuit.

About the mosfet, if it is parallel in the LC tank, it can be seen as a current noise source, inoise^2=4KTgm∝W/L the thermal noise is inversely proportional to the length.
If it is serial, it can be seen as a voltage noise source, vnoise^2=4kTR∝L/W, then the thermal noise is proportional to the length.
Click to expand...

thanks for replying
about the mos noise, i think the actual noise is the current noise , which proportional to the gm, aint it?, i mean that decreasing the actual noise will decrease total noise, am i right?
 

Re: phase noise dependence

In lc vco, mosfet noise is current noise. It convert to voltage noise by the lc load. Decreasing mosfet noise will decrease the output phase noise.
But gm can't be too small, gmRp must be larger than 1 to maintain vco oscillate.
 

Re: phase noise dependence

The length of Mos increase leads to area increase, its noise will decrease.
 

Re: phase noise dependence

adwxl said:
The length of Mos increase leads to area increase, its noise will decrease.
Click to expand...

you must distinguish flicker from thermal noise,
 

Re: phase noise dependence

AA

The phase noise proportional to the oscillation frequency , and so , inverse proportional to capacitance, also, for tail current source, you concern with the currnet noise and so , increasing its gm will increase its thermal noise,, also, for the cross-coupled pairs , you concerned with input refered noise voltage ,a as it will convert to phase noise ; AM-to-FM conversion, so increasing their gm will reduce their thermal noise contribution in phase noise,,,,,,,,


regards
 

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