Frequency fine tuning

1. An oscillator comprising:

an oscillating circuit for generating an output signal having an output signal frequency, the oscillating circuit comprising an inductive element and a capacitive element, the capacitive element having a variable capacitance for coarsely tuning the output signal frequency;

a first pair of cross coupled transistors, a first transistor of the first pair being coupled to a first terminal of the inductive element and a second transistor of the first pair being coupled to the second terminal of the inductive element;

a second pair of cross coupled transistors, a particular transistor of the second pair of transistors being coupled to said first terminal and said first transistor, and the other transistor of the second pair being coupled to the second terminal and the second transistor; and

a current supply for supplying current to the oscillating circuit, the current supply being variable for finely tuning the output signal frequency, the current supply being coupled to the first pair of cross coupled transistors, the fine tuning of the output signal frequency being dependent on the current supplied to the first and second pairs of cross coupled transistors.

2. An oscillator as claimed in claim 1 , the current supply being coupled to the first pair of cross coupled transistors, the fine tuning of the output signal frequency being dependent on the bias across each of the transistors.

3. An oscillator as claimed in claim 1 , each of the cross coupled transistors having a parasitic capacitance, the fine tuning of the output signal frequency being dependent on the parasitic capacitances.

4. An oscillator as claimed in claim 1 , the first pair of transistors being PMOS transistors and the second pair of transistors being NMOS transistors.

5. An oscillator, comprising:

an oscillating circuit for generating an output signal having an output signal frequency, the oscillating circuit comprising an inductive element and a capacitive element, the capacitive element having a variable capacitance for coarsely tuning the output signal frequency; and

a current supply for supplying current to the oscillating circuit, the current supply being variable for finely tuning the output signal frequency, the current supply comprising a first current source and a second current source, the first current source being configured to supply a current greater than current supplied by the second current source, the second current source being variable so as to vary the current supply for finely tuning the output signal frequency.

6. An oscillator as claimed in claim 5 , wherein the first and second current sources are digital-to-analogue converters.

7. An oscillator as claimed in claim 5 , the current supply further comprising a switch coupling the first current source to the second current source and being controllable so as to enable the fine tuning of the output signal frequency.

8. An oscillator, comprising:

an oscillating circuit for generating an output signal having an output signal frequency, the oscillating circuit comprising an inductive element and a capacitive element, the capacitive element having a variable capacitance for coarsely tuning the output signal frequency; and

a current supply for supplying current to the oscillating circuit, the current supply being variable for finely tuning the output signal frequency,

the oscillator being configured such that varying the current fine tunes the output signal frequency at a resolution of between 100 kHz/μA and 300 kHz/μA.