Circuit for driving a voltage controlled oscillator for frequency modulation

A circuit providing an output signal for modulating the oscillation frequency of a voltage-controlled oscillator (VCO) in response to a control signal, the VCO including a first voltage-controlled variable capacitance device coupled to an output node of the VCO for varying the oscillation frequency of the VCO in response to the output signal, the circuit comprising:

• a charge pump coupled to receive a reference current and providing a charge current to a first node in response to the control signal;
• a first capacitor having a fixed capacitance value and coupled between a first reference voltage and the first node;
• a second capacitor coupled between the first reference voltage and the first node and comprising a second voltage-controlled variable capacitance device, the second capacitor having the same voltage-controlled variable capacitance device structure as the first voltage-controlled variable capacitance device of the voltage-controlled oscillator;
• and
• an amplifier having an input terminal coupled to the first node and an output terminal providing the output signal,
• wherein a voltage change at the first node is generated in response to the charge current and the output signal of the amplifier has a value corresponding to the voltage change at the first node, and when the output signal is coupled to the VCO for modulating the oscillation frequency, the output signal causes the same voltage change to be applied across the first voltage-controlled variable capacitance device of the VCO.