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

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