In order to increase the VCO frequency tuning range without a significant phase noise degradation, we use sub-bands.
The implementation is usually a digital controled bank of switched capacitors.
The digitally conrolled bands should have enough overlap to support all Process Voltage and Temperature (PVT) variations.
The tuning range should support all PVT.
Some definitions:
OL - Over Lap between two bands
TR - Frequency Tuning Range of a single band
Gap - The frequency distance of two bands
Bands Over Lap [%] = OL/TR *100
Bands Gap [%] = Gap/TR *100
The switch design
The switch should have low enough resistance so it will not degrade the quality facator Q of the tank.
This implies a large W/L ratio. on the hather hand increasing W will increase the capacitance of the switch at "Off" state.
Here you have one of the important tradeoffs in the VCO design: Q of the switch at "On" vs. C of the switch at "Off".
