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A variable frequency oscillator comprising: an oscillatory circuit for generating a periodic output dependent on the capacitance between a first node and a second node of the circuit, and having a capacitative element connected between the first node and the second node; the capacitative element com

A variable frequency oscillator comprising: an oscillatory circuit for generating a periodic output dependent on the capacitance between a first node and a second node of the circuit, and having a capacitative element connected between the first node and the second node; the capacitative element comprising: a variable capacitance unit, the capacitance of which is variable for varying the frequency of the output and a plurality of finite capacitances each being selectively connectable in parallel with the variable capacitance unit between the first node and the second node to trim the frequency of the output.

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1. A variable frequency oscillator comprising:an oscillatory circuit for generating a periodic output dependant on the capacitance between a first node and a second node of the circuit, and having a capacitative element connected between the first node and the second node; the capacitative element c

1. A variable frequency oscillator comprising:an oscillatory circuit for generating a periodic output dependant on the capacitance between a first node and a second node of the circuit, and having a capacitative element connected between the first node and the second node; the capacitative element comprising:a variable capacitance unit, the capacitance of which is variable by means of a voltage applied to a variable capacitance input for varying the frequency of the output;a plurality of trimming capacitances each being selectively connectable to the variable capacitance unit to trim the frequency of the output;a memory for storing information defining one or more sets of the trimming capacitances; andcontrol apparatus coupled to the memory for causing a set of the trimming capacitances to be connected to the variable capacitance unit by the steps of:retrieving from the memory information defining a set of the trimming capacitances;connecting that set of the trimming capacitances to the variable capacitance unit;comparing the voltage at the variable capacitance input with a first preset voltage range; andif that voltage is outside the first preset voltage range, determining, based on the voltage at the variable capacitance input, an adjusted set of the trimming capacitances and storing in the memory information defining that adjusted set of the trimming capacitances.2. The variable frequency oscillator of claim 1, wherein the trimming capacitances are each selectively connectable between the first node and an intermediate node, and wherein the variable capacitance unit is connected between the second node and the intermediate node.3. The variable frequency oscillator of claim 1, wherein the trimming capacitances are each selectively connectable in parallel with each other.4. The variable frequency oscillator of claim 1, wherein a switch is connected in series with each trimming capacitance for selectively connecting the respective trimming capacitance to the variable capacitance unit in response to a respective switching signal.5. The variable frequency oscillator of claim 4, wherein each switch is a switching transistor.6. The variable frequency oscillator of claim 1, wherein the control apparatus is capable of generating the switching signals.7. The variable frequency oscillator of claim 1, wherein each of the said one or more sets corresponds to a respective operating frequency of the oscillator.8. The variable frequency oscillator of claim 1, wherein at least one of the trimming capacitances has a different capacitance value from another of the trimming capacitances.9. The variable frequency oscillator of claim 1, comprising feedback apparatus connected between the output and the variable capacitance input for stabilising the oscillator.10. The variable frequency oscillator of claim 9, wherein the feedback apparatus is a phase-locked loop.11. The variable frequency oscillator of claim 1, wherein the variable capacitance unit is a variable capacitance diode.12. A radio terminal comprising a variable oscillator as claimed in claim 1.13. The method of claim 1, wherein the step of determining is performed only if the voltage at the variable capacitance input is inside a second preset voltage range.14. The method of claim 1, wherein, in the step of storing, the information defining the adjusted set of the trimming capacitances is stored so as to replace in the memory the said information defining a set of the trimming capacitances.15. The variable frequency oscillator of claim 1, wherein the trimming capacitances are each selectively connectable in series with each other.