PLL system and method for controlling a gain of a VCO circuit

A phase locked loop system, comprising:

• a voltage controlled oscillator circuit, comprising:
• a frequency tuning input;
• an output providing a first clock signal with a synthesized frequency;
• a first plurality of switchable varactors for selecting a frequency band of said VCO, wherein said VCO has a gain that changes with frequency band;
• and
• a second plurality of switchable varactors for varying said VCO gain in said selected frequency band of said VCO;
• a PLL feedback circuit comprising:
• a first input receiving said first clock signal with said synthesized frequency,
• a second input receiving a second clock signal with a reference frequency, and
• an output providing a tuning voltage to said frequency tuning input of said VCO;
• a switching device for switching said PLL feedback circuit to either an open loop state or a closed loop state, wherein in said open loop state a plurality of different predefined tuning voltages can be applied to said frequency tuning input of said VCO;
• a frequency measurement device for measuring said synthesized frequency of said first clock signal;
• and
• a control unit operable to determine said VCO gain with respect to said synthesized frequency and said predefined tuning voltages, wherein said control unit is operable to search for and select a frequency band of said VCO by applying a first plurality of sets of setting values to said first plurality of varactors, apply a second plurality of sets of setting values to said second plurality of varactors for said selected frequency band and measure said VCO gain for said selected frequency band and said setting values for said second plurality of varactors, and select a best VCO gain for said frequency band.