Dual mode PLL for phase coherent application

An apparatus comprising:

• a phase detector configured to determine a phase difference between a reference frequency and a feedback signal derived from a VCO frequency;
• a first Low Pass Filter (LPF) configured to filter a first output of the phase detector and to control a first set of variable capacitance devices;
• a Proportional and Integral Controller (PIC) that includes a second LPF and a time variant proportional gain and integrator, the second LPF configured to filter the first output of the phase detector and the time variant proportional gain and integrator configured to modify a second output of the second LPF to control a second set of variable capacitance devices;
• a Voltage Controlled Oscillator (VCO) configured to generate the VCO frequency from a resonance between a plurality of capacitors, the first set of variable capacitance devices, the second set of variable capacitance devices and an inductor;
• and
• a feedback divider configured to generate the feedback signal by dividing the VCO frequency by a comparison frequency of the phase detector.

A method of operation for a dual mode Phase-Locked Loop (PLL) for phase coherent application comprising:

• calibrating the PLL to determine a best combination of a plurality of capacitors of a Voltage Controlled Oscillator (VCO) to minimize a residual frequency error between a target frequency and a VCO frequency generated by the VCO;
• controlling the VCO with a first error voltage and a second error voltage;
• determining the first error voltage by a first Low Pass Filter (LPF), the first LPF configured to filter a first output of a phase detector, the phase detector configured to determine a phase difference between a reference frequency and a feedback signal derived from the VCO frequency;
• and
• determining the second error voltage by a second LPF of a Proportional and Integral Controller (PIC) configured to filter the first output of the phase detector and by a time variant proportional gain configured to modify a second output of the second LPF, wherein the first error voltage and the second error voltage modify a respective capacitance of a respective set of variable capacitance devices configured to resonate with the best combination of capacitors and an inductor to generate the VCO frequency.