Low phase noise wide tuning range digitally-controlled LC oscillator using switchable inductor

By adding a switched-loop around the inductor of a traditional LC oscillator, a single oscillator with 36% tuning range covers all American and European GSM cellular bands and meets the stringent phase noise requirements under +/-25% capacitor process variation. With the switch in the 'open' state, the measured phase noise at a 20 MHz offset from a 915 MHz carrier is -165 dBc/Hz, while at the same frequency with the switch 'closed' the phase noise performance only degrades by 2 dB. This circuit technique improves the tuning range of an LC oscillator dramatically without increasing die area.