VCO(ring oscillator) phase noise/flicker noise analysis question!

HI, I am doing some analysis of ring oscillator,expecially the phase
noise analysis.

I am using st120nm(0.13um) digital process to do the simulation.


As lots of papers[1][2] have pointed out that the flicker noise in
the
tail current are one of the main contribution to the total phase
noise. Therefore, I want to find a some way to build a practical low-
pass filter within the tail current part.


However, I can not find any effect when I build a low-pass filter
together with the ring oscillator.


As a simple examle, I build almost same circuit which has discussed
in
the paper [1][2], and add a very big capacitor between the bias point
and ground. and then run the PSS --PNOISE analysis, however , there
is
almost no difference between the phase noise results by adding/
excluding the capacitors. The capacitor may big as 100uf.


I can find the phase noise performance decrease at 30dB/Decade in the
normal PNOISE analysis, and by remove the sideband (-1), the phase
noise performance decrease at 20dB/Decade which assumes that the
flicker noise has been removed within the simulation environment. In
theory, I assume that add the filters, the phase noise performance
should be located between these two lines.


However, now, the difficulty for me is that I can not find any effect
in the simulation. Therefore, I think maybe the problems happens at
simulation environment setup of the device model.


can anyone give me some suggestion or ideas?


The oscillator working almost at 4-5GHz,


the relative harmonic: 1
start stop range at 1K ----10M
sweep type for nNO.steps 300-500
sideband: 7/10 (exclude -1 for the purpose of removing flicker noise
analysis as mentioned above)


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[1] Analysis and design of low-phase-noise ring oscillators
Dai, L.; Harjani, R.;
Low Power Electronics and Design, 2000. ISLPED '00. Proceedings of
the
2000 International Symposium on


[2] Phase Noise and Jitter in CMOS Ring Oscillators
Abidi, A.A.;
Solid-State Circuits, IEEE Journal of
Volume 41, Issue 8, Aug. 2006 Page(s):1803 - 1816