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Measurements >> Phase Noise and Jitter Measurements >> why the phase noise of ring osc is generally much bigger than driven circuit?
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Message started by luantan on Aug 20^th, 2008, 8:20am

Title: why the phase noise of ring osc is generally much bigger than driven circuit?
Post by luantan on Aug 20^th, 2008, 8:20am

Dear All:

In the phase noise simulatin, I find that the phase noise of ring oscillator is
generally much bigger than driven circuits (for example: cml clock buffer).

Why? The only difference I can think is that osc will "integrate the phase", so
the phase noise of osc will have 1/f^3 and 1/f^2 region. The driven circuit does
not "integrate the phase, so the phase noise only have 1/f and a noise
floor.
But this can not explain that osclillator's phase noise will be much bigger than
driven circuit, I think. The "integration property" will only make phase noise
decrease more quickly at higher frequency.

So, what is the real reason behind this phenomenon?

Any reply is highly appreciated. :)

---johnson

Title: Re: why the phase noise of ring osc is generally much bigger than driven circuit?
Post by Ken Kundert on Aug 20^th, 2008, 9:29am

Integrators have infinite gain at DC, and very high gain at low frequencies, so the integration can and does amplify noise at low frequencies. Another way of thinking of this is in terms of phase drift. In oscillators the phase can drift without bound. This is a manifestation of low frequency phase noise.

-Ken

Title: Re: why the phase noise of ring osc is generally much bigger than driven circuit?
Post by luantan on Aug 21^st, 2008, 7:28am

Dear Ken:

Yes, the noise is integrated in ring osc. This means that
for frequecny < 1Hz, the noise is amplified, for frequency > 1Hz,
the noise is decreased.
However I see that ring osc's phase noise is much bigger than
driven circuit even when frequecny >> 1 Hz.

---johnson :)

Title: Re: why the phase noise of ring osc is generally much bigger than driven circuit?
Post by Ken Kundert on Aug 21^st, 2008, 10:17am

There is nothing special about 1Hz. You are assuming a gain factor of one, but there is no reason to believe the gain factor on the integrator is unity. In fact, for a ring oscillator, I would expect the unity gain frequency on the integrator to be near the oscillation frequency.

-Ken

Title: Re: why the phase noise of ring osc is generally much bigger than driven circuit?
Post by luantan on Aug 24^th, 2008, 7:36am

Dear Ken:
Thanks for your immediate response, and that's was indeed helpful for me.
So, the real reason is that ring osc not only "integrates noise, but also
amplifies it.

However as far as Hajimiri's theory is concerned, I still have one question.
This well-known figure is quoted from his paper

In this figure, I can see integration, I can see periodical modulation.
But I can not see "amplification". May be 1/qmax is the "amplification"
factor. If it is, what about driven circuit? I think that driven circuit
also have this 1/qmax. So, this confuses me.

(By the way, here qmax is the maximum charge displacement across the capacitor
on osc's node. Hajimiri thinks that if deltaQ is injected into the node capacitor
The instantaneous voltage change deltaV will be
deltaV=deltaQ/C
So, hajimiri uses qmax to normalize his equations.)