Issue: how to enlarge VCO's tuning range?

Besides those traditional methods: varactor, switch cap. Is there any good suggestion to enlarge LC VCO's tuning range? (CMOS IC design)

And how about the trade-off of your idea?

http://www.iet.ntnu.no/groups/krets/...ing%20range%22

You may try by tuning the VCO by adjusting the L in the LC-tank VCO. I know that there are several articles describing the impact on the VCO by varrying the inductance. One thing to keep in mind is that the L is usually the main Q-limiting factor of the tank. The backgate approach in the above mentioned link provides verry limited tuning range.

I am not a CMOS designer, but generally the following guideline applies:

the tuning range depends on the design of your resonator. Mostly this is done with vacators (tunable capacitor diodes) or alikes, with certain tuning range

C. The varactor has lower Q than fixed capacitors, so:
* If you want a large tuning range, you don"t put any fixed cap in parallel to the varactors, and use a large inductance
* If you don't want a large tuning range (eg DECT VCO) you do put a capacitor in parallel with the varactor. The resonator inductance becomes lower, and overall Q-factor will be substantially higher.

Globally speaking, de design tradeoff involved here is Q-factor (and resulting lower phase noise) against tuning range.