Phase locked loop

1. A phase locked loop for generating a frequency chirp, the phase locked loop comprising:

a phase frequency detector configured to receive a reference frequency signal at a first input;

a low pass filter configured to receive a current from the phase frequency detector at a filter input, and to output a control voltage;

a voltage controlled oscillator configured to generate the frequency chirp at an output in response to receiving the control voltage;

a feedback path connecting the output of the voltage controlled oscillator to a second input of the phase frequency detector, the feedback path comprising a frequency divider; and

a timing module configured to generate a reset pulse;

wherein the low pass filter comprises:

a plurality of capacitors connected in parallel between the filter input and a common voltage line; and

a voltage source configured to generate an initial control voltage, wherein the voltage source is switchably connected to a node between a first capacitor of the plurality of capacitors and the common voltage line, wherein the voltage source is connected to the node during a chirp reset mode defined by the reset pulse such that the voltage at the node is substantially equalised to the initial control voltage.

2. The phase locked loop of claim 1 , wherein the voltage source acts as a current sink, or wherein the voltage source acts as a current source.

3. The phase locked loop of claim 1 , wherein the voltage source comprises a digital to analogue convertor.

4. The phase locked loop of claim 3 , wherein the digital to analogue convertor is further switchably connected to an output of the low pass filter to provide an initial control voltage to the voltage controlled oscillator.

5. The phase locked loop of claim 3 , wherein the voltage source further comprises a unity gain amplifier.

6. The phase locked loop of claim 1 , wherein the capacitance of the first capacitor is larger than the capacitance of any other capacitor of the plurality of capacitors.

7. The phase locked loop of claim 1 , wherein the phase locked loop comprises a switch configured to switch the voltage source into connection with the node in response to receiving the reset pulse.

8. The phase locked loop of claim 1 , further comprising a lock detector connected between the first input of the phase frequency detector and the second input of the phase frequency detector, the lock detector configured to identify a cycle slip during the chirp reset mode.

9. The phase locked loop of claim 8 , wherein the lock detector is configured to adjust a duration of the chirp reset mode.

10. A method of generating a chirp in a phase locked loop, the method comprising:

generating an output signal at an output of a voltage controlled oscillator of the phase locked loop;

entering a frequency sweeping mode in which the frequency of the output signal is swept from an initial frequency to a final frequency by sweeping a control voltage applied to the voltage controlled oscillator from an initial control voltage to a final control voltage by sweeping a division ratio of a frequency divider in a feedback path of the phase locked loop;

entering a chirp reset mode in which the control voltage is reset to the initial control voltage by switching a voltage source into connection with a node between a first capacitor of the low pass filter and a common voltage line, such that the voltage at the node is substantially equalised to an initial control voltage generated by the voltage source; and

re-entering the frequency sweeping mode.

11. The method of claim 10 , wherein the duration of the chirp reset mode is less than 50%, or less than 20%, or less than 10% of the duration of the frequency sweeping mode.

12. The method of claim 10 , wherein switching the voltage source into connection comprises sinking current from the node across the voltage source, or applying current from the voltage source to the node.

13. The method of claim 10 , further comprising initialising the control voltage by connecting the voltage source to an output of the low pass filter.

14. The method of claim 10 , wherein the phase locked loop comprises a lock detector connected between an input of a phase frequency detector of the phase locked loop and the output of the voltage controlled oscillator; and

wherein the method further comprises identifying, with the lock detector, a cycle slip during the chirp reset mode; and

in response to identifying a cycle slip, adjusting the duration of the chirp reset mode.