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Design >> Analog Design >> The influence of Kvco in PLL
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Message started by Monkeybad on Feb 17^th, 2008, 6:50pm

Title: The influence of Kvco in PLL
Post by Monkeybad on Feb 17^th, 2008, 6:50pm

Hi! Everybody!
In designing a VCO in PLL, the Kvco is an important parameter.
What's the influence of Kvco?
What's the relationship between the Kvco and jitters?
Is it good or bad when I have a very high Kvco?

Title: Re: The influence of Kvco in PLL
Post by Berti on Feb 17^th, 2008, 10:31pm

Hi,

Kvco directly influences the tuning range and phase noise with opposite sign.
A large Kvco results in a large tuning range, but any noise (or ripple) on the loop filter voltage will be amplified by the large Kvco (large tuning range, but higher noise).
When choosing a small Kvco the phase noise will be smaller, but also the tuning range.

Ring oscillator designs often have a relatively high Kvco (e.g. 1GHz/V) because the jitter performance is moderate and often dominated by the VCO. Low phase noise (LC-based) PLL (e.g. for GSM transceivers) often have a low Kvco. But to cover the tuning range it might then be necessary to add some kind of coarse tuning (dividing the overall tuning range in coarse bands).

However, I think this trade-off will is covered in most of the PLL-books (Gardner, Best).

Regards

Title: Re: The influence of Kvco in PLL
Post by Monkeybad on Feb 18^th, 2008, 2:40am

Thanks! Berti! Your answer is very helpful.
As I know, the jitter comes from two sources.
One comes from the reference clock, PFD, charge pump, and it passes the loop-filter so it is a low-pass signal.
Another one comes from the VCO directly, and it can't be filtered by loop filter so it is a high-pass signal.
So what you mean is that:
(1)For ring oscillator, the jitter in VCO is dominated so I set the Kvco higher to suppress the jitter.

(2)For LC tank PLL, like GSM transceivers, the jitter is low in VCO because we use the LC tank
structure. The jitter from reference clock is dominated so I set the Kvco lower for not to
disturbance the LC tank VCO. But the tuning range is smaller.
Is it correct?

Title: Re: The influence of Kvco in PLL
Post by Berti on Feb 18^th, 2008, 4:39am

Sorry, I think my explanation was not so clear.

Reference, VCO and loop filter noise are amplified by Kvco. Therefore you always want
the Kvco as small as possible.
However, a small Kvco results in a small tuning range. There is a trade-off.

Since ring-oscillators usually aim at moderate phase noise performance, those designs often use a larger Kvco (compared
to very low phase noise LC-oscillators) to cover a larger tuning range. But this is more a feeling/impression from my side
rather than a general rule.

Regards

Title: Re: The influence of Kvco in PLL
Post by Monkeybad on Feb 18^th, 2008, 6:04pm

Thanks for your explanation,Berti.
Now I get much clear about my confusion.