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)
---------------------------------------------------------------------------= =AD-------------------------------------------- [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