Wireless communication unit, integrated circuit comprising a voltage controlled oscillator and method of operation therefor

TLDR: In this paper, a phase-locked loop (PLL) circuit with a tuning port for controlling a frequency of a signal output from the VCO is described. But the number of switches required to switch in one or more varactors associated with the tuning port is fixed.

Abstract: A wireless communication unit (400) comprises a frequency generation circuit (423) employing a phase locked loop (PLL) circuit comprising a voltage controlled oscillator (VCO) (1100) having a modulation port for directly modulating a signal output from the VCO. The VCO is operably coupled to at least one switch (1121, 1122) and a capacitor bank (1105) comprising one or more varactors. A controller (1130) is arranged to switch-in one or more varactors associated with the modulation port of the PLL circuit the number of switched-in varactors varying proportionally to a piecewise approximation of 1/f2. This allows the modulation port gain KmOd of the VCO to remain substantially constant when the frequency varies. In addition, or alternatively, the PLL circuit may comprise a VCO having a tuning port for controlling a frequency of a signal output from the VCO. The controller (1130) is arranged to switch in one or more varactors associated with the tuning port of the PLL circuit, the number of switched-in varactors varying proportionally to a piecewise approximation of 1/f3. This allows the ratio Kv/N to remain substantially constant when the frequency varies, Kv being the turning port gain of the VCO and N is a division ratio of a divider (305) in the feedback loop of the PLL.