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Design >> RF Design >> how to make the ring vco start?
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Message started by neil on Apr 20^th, 2008, 8:59am

Title: how to make the ring vco start?
Post by neil on Apr 20^th, 2008, 8:59am

hi,everyone,
i am design a 3-order ring vco with the CSL(CSA) delay cell( single input),
it doesn't start up.and i don't know what level the CSL input should be biased to get the gain A0>=2.
it looks like A0>=2 can't be obtained. i guess the input MOS is in linear region,and it 's gain is very small.
...i use the .ic inatiation ,and the vco can start.but it can start without giving a inatiation.
..i don't know why...anyone can help me? thanks.

Title: Re: how to make the ring vco start?
Post by Stefan on Apr 20^th, 2008, 9:05am

Maybe it's not the A0, but missing transient noise sources or even better missing initial conditions that will make the whole chain even _start_ oscillating. An insufficient A0 should show a damped oscillation that would finish soon, is that what you're seeing at your output ?

Title: Re: how to make the ring vco start?
Post by Berti on Apr 22^nd, 2008, 12:11am

Neil,

I agree with Stefan. The simulator will even find a stable operating point for a ring-oscillator constisting
of CMOS inverters. However, in reality, this operating point won't be stable because of noise sources, and it will
start to oscillate. Only initial condition can help to happen this in simulation. Nevertheless, you should take a
carefull look at your circuit to make sure that this is really the case.

Regards

Title: Re: how to make the ring vco start?
Post by loose-electron on May 14^th, 2008, 3:35pm

Agree with the rest, the op point will get found where everything sits at half scale, because your models are ideal and matched, with no offsets or noise. Generally, an IC on one node of the ring oscillator (tug something off the center line) will get the thing to perform as expected.