GB2337382A - Voltage controlled oscillators - Google Patents

Abstract

A voltage controlled oscillator, operable within a predetermined operating frequency range (Fmin-Fmax), comprises: a resonant circuit (Lp, Cp) a first variable capacitor (Ct3) connected via a first coupling capacitor (Cc3) in parallel to the said resonant circuit (Lp, Cp), the value of the said first variable capacitor (Ct3) being variable by means of a first variable control signal (VS1), whereby the value of the resonant frequency (F vco ) of the circuitry connected between the terminals (2,4) can be varied only within a sub-range (Fminus- Fplus) of the operating frequency range of the voltage controlled oscillator (Fmin-Fmax) by varying the value of the first variable control signal (VS1); and a second variable capacitor (Ct2) connected via a second coupling capacitor (Cc2) between the said pair of terminals (2,4) in parallel to the said resonant circuit (Lp, Cp), the value of the said second variable capacitor (Ct2) being variable by means of a second variable control signal (VS2), whereby the centre value of the said frequency sub-range (Fminus- Fplus) can be set by varying the value of the said second variable control signal (VS2). In a modification (fig. 6 not shown), the voltage VS2 is dependent on an output of filter (16) fed to a microprocessor (24).

Description

1 1 Voltage ControUed Oscillator

Technical Field

The invention concerns a voltage controlled oscillator, Background

2337382 A voltage controlled oscillator (VCO) is a circuit which is designed to produce an output signal of a particular frequency. This frequency is determined by the level of a voltage which is input to the oscillator. The output frequency of the oscillator can be varied by varying the magnitude of the voltage which is input to the oscillator. Typically, a voltage controlled oscillator may be used to provide a signal for transmission by the transmitter of a communications system. Such a transmitter may be part of, for example, a portable or a mobile radio.

The output frequency of the WO can be varied reliably between a certain maximum frequency and a certain minimum frequency. The frequencies between these limits are referred to as the WO's frequency range. There are however particular requirements for phase noise performance and transmitter modulation linearity over a WO's frequency range. Prior art designs of WO often meet these requirements for phase noise performance and transmitter modulation linearity only with great difficulty and expense, often requiring relatively high control voltages to do this.

A typical WO design includes a parallel circuit comprising an inductor and a capacitor in the oscillator. Such a design is illustrated in figure 1. Figure 1 includes an oscillator 10, which includes an inductor Lp and a capacitor Cp connected in parallel. The frequency of the oscillations produced by the oscillator, for the moment disregarding the other components shown in figure 1, would simply be the resonant frequency of this parallel circuit. The parallel circuit comprising inductor Lp and Cp is shown in figure 1 to be connected between a pair of terminals which are labelled 2 and 4 respectively.

2 Also connected to terminals 2 and 4 is a series circuit comprising a first varactor Ctl and a coupling capacitor Ccl. Varactor Ctl functions as a tuning capacitor, which can be tuned to the desired capacitance value by applying a tuning voltage VS via the lead which bears reference 20. Line 20 is a line which supplies the varactor's tuning voltage, otherwise termed the 'steering' voltage. Although Ctl is a varactor, a WO can actually be constructed using any tuneable capacitor.

The voltage VS determines the capacitance value of the varactor CU. This capacitance value changes the total capacitance between terminals 2 and 4. This change in capacitance changes the resonant frequency of the circuitry connected between terminals 2 and 4, and thereby changes the frequency of the signal output by the W0. The components illustrated in figure 1 therefore provide an oscillator whose output frequency can be varied by varying the voltage VS. This is one basic design of VCO.

The arrangement of figure 1 shows a radio-frequency output from the oscillator 10, which is fed to a synthesiser 12. A further input to the synthesiser 12 is provided by a reference oscillator 14. The output of the synthesiser is fed to a loop filter 16, which provides the control voltage VS.

A further development of this basic WO design is shown in figure 2. Figure 2 shows the main elements of one of the applicant's prior art WO circuits.

Figure 2 comprises similar elements to those shown in figure 1. Additionally, figure 2 comprises a further varactor W. A lead 22 supplies a voltage VT to the varactor Ct2. The voltage VT provides compensation for the manufacturing tolerances of the components Lp, Cp and Ctl. Capacitor W is a coupling capacitor which connects varactor Ct2 to terminal 2, thus connecting it in parallel with the basic components Lp, Cp and Ct l of the VCO.

3 Components Lp, Cp and Ctl can only be manufactured sufficiently precisely that their values and characteristics fall within certain margins of the intended, optimal values. These margins are the manufacturing tolerances. In a theoretical circuit, a voltage VS of, for example, Vnominal volts would need to be supplied in order to cause the WO to produce a given output signal with a frequency of Fnominal. However, in a practical circuit the manufacturing tolerances result in the actual voltage VS required to produce the output signal of frequency Friominal probably being slightly above or below the theoretical value Vnominal.

The arrangement of figure 2 allows a compensation voltage W to be fed to varactor W on line 22 in order to compensate for these manufacturing tolerances. This voltage W controls the capacitance value of varactor W, and can therefore introduce an offset capacitance between the terminals 2 and 4. The effect of this compensation is to change the frequency output by the WO for a particular input voltage VS. By choosing the correct value of VT, the WO can be made to produce the particular output frequency Friominal in response to a voltage input VS which is very close to the theoretical voltage Wominal. Thus compensation for manufacturing tolerances can largely be achieved, In the arrangement of figure 2, the voltage VS does not need to be variable over a large enough voltage range both to provide the desired output frequency range in the ideal case and to compensate for the manufacturing tolerances.

In contrast, the voltage VS in figure 1 must perform both of these functions. It is therefore clear that the arrangement of figure 2 is an improvement over that of figure 1.

The arrangement of figure 2 includes circuitry to derive the signal VT. This circuitry includes a microcontroller 18, having a microprocessor 24 and a digital-analogue converter to derive the actual voltage level W for line 22.

4 In the applicant's prior art design according to figure 2, line 22 could be supplied with any of eight different values of compensation voltage. Each value of this voltage provided optimum compensation for manufacturing tolerances when the WO was operating in a different part of its operating 5 frequency range.

The operating characteristic of a WO takes the general form shown in figure 3. Figure 3 shows a typical graph of the output frequency F of the WO plotted against the steering voltage VS.

As figure 3 shows, the characteristic of the WO is a curve, rather than a straight line. The operating frequency range of the WO is the portion of the curve between the frequency points marked as Fmin and Fmax on figure 3. These frequency points correspond to the voltage VS taking the values Vmin and Vmax respectively.

Figure 3 also shows two dotted lines at voltage values Wolmin and Wolmax. These indicate respectively the minimum and maximum voltages VS which might need to be supplied to real circuits of the design shown in figure 1 to provide output frequencies from Fmin to Fmax. The compensated arrangement of figure 2 largely eliminates the variation which these dotted lines indicate. This shows the superior accuracy of the arrangement of figure 2 in comparison to the 'un-compensated' arrangement of figure 1.

Summary of the Invenflon

A voltage controlled oscillator (VCO) in accordance with the invention is operable within a predetermined operating frequency range, Fmin-Fmax. The WO comprises: a resonant circuit connected between a pair of terminals; a first variable capacitor connected via a first coupling capacitor between the said pair of terminals in parallel to the said resonant circuit, whereby the value of the first variable capacitor influences the value of the resonant frequency of the circuitry connected between the said pair of terminals, the value of the said first variable capacitor being variable by means of a first variable control signal, whereby the value of the resonant frequency of the circuitry connected between the terminals can be varied only within a subrange, Fminus- Fplus, of the operating frequency range of the voltage controlled oscillator, Fmin-Frriax, by varying the value of the first variable control signal; and a second variable capacitor connected via a second coupling capacitor between the said pair of terminals in parallel to the said resonant circuit, whereby the value of the second variable capacitor influences the value of the resonant frequency of the circuitry connected between the said pair of terminals, the value of the said second variable capacitor being variable by means of a second variable control signal, whereby the centre value of the said frequency sub-range, Fminus- Fplus, can be set by varying the value of the said second variable control signal.

A portable or a mobile radio may incorporate a voltage controlled oscillator in accordance with the invention.

The arrangement in accordance with the invention provides particular advantages. Amongst these are very good phase-noise performance of the WO, low-voltage operation and very good modulation linearity.

Brief description of the drawinkrs

Figure 1 shows a schematic view of a basic voltage controlled oscillator (VCO) 25 circuit of the prior art.

Figure 2 shows a schematic view of a further WO circuit of the prior art.

Figure 3 shows a graph of WO output frequency plotted against the WO 30 steering voltage VS.

Figure 4 shows an illustrative embodiment of a circuit arrangement in accordance with the present invention.

1 1 - 1 1 6 Figure 5 shows a graph of the output signals of the WO of figure 4 plotted against the steering voltage VS 1.

Figure 6 shows a WO in accordance with an alternative embodiment of the invention.

Detailed description of the preferred embodiment

A circuit arrangement in accordance with the present invention is shown in figure 4. Components of the arrangement of figure 4 which correspond to components of the arrangement of figure 2 are not described again here.

The function and advantages of the circuit arrangement of figure 4 can best be understood after a more precise analysis of the function of the prior art arrangement of figure 2.

In the circuit arrangement of figure 2, the voltage VS supplied to the varactor Ctl varies over a very wide range. The value of VS must vary sufficiently to drive the WO to produce all frequencies between Fmin and Fniax.

The variable parameter KvcO is critical to understanding what this means for the circuit design. KvcO is the slope of the curve shown in figure 3. This is the rate of change of the WO's output frequency with voltage VS, and is dependent on the characteristics of the varactor CU. Since the characteristic shown is a curve, KvcO is a variable. If the characteristic in figure 3 had been linear, then KvcO would have been a constant.

The value of Kveo must be large in order for the WO's output frequency to vary over the whole operating frequency range from Vmin to Vmax, if Vmax is to be kept low as required in low voltage radio designs. However, a large value of Kvco requires that the coupling capacitor Cel be large.

7 However, use of a large coupling capacitor Cel brings severe disadvantages: (i) In a practical circuit, there will be noise signals on line 20. These may arise, for example, simply because line 20 acts as an aerial and picks up stray signals. Noise voltage on line 20 will cause the capacitance of the varactor Ctl to jitter. This in turn causes frequency jitter on the output of the VCO. The larger the value of capacitor Cc l, the more jitter will be seen on the output frequency of the WO, and the worse the phase-noise performance of the VCO. (ii) A varactor itself will also introduce phase noise into the VCO. The larger the value of capacitor Ccl, the worse the phase noise due to the varactor Ctl.

Some prior art arrangements in fact make use of a voltage multiplier to increase the value of Vmax which can be applied to the varactor, in order to avoid using an arrangement with a large Kveo and large capacitor Cc l.

A practical use of a WO is as the synthesiser of a radio. The standards to which radios must operate on criteria such as adjacent channel performance are set by international agreement. In most synthesisers, the phase noise performance of the WO is however the factor which limits the adjacent channel performance of the radio.

Thus there is a ma3dmilm allowable phase noise for the WO if used in a radio. If the bandwidth Frnin-Frnax which the radio is designed to operate over is so large that the varactor Ctl must be coupled in too tightly to achieve the phase noise requirement, i.e. coupled with too large a capacitor Ccl, then the operating bandwidth of the radio must be reduced. Reducing the radio's operating bandwidth may in practical terms mean building two or more diflerent versions of the radio, each operating over only part of the section of frequency spectrum reserved for that class of radio. Such radios clearly cannot then communicate with each other, only with other radios built for the same part of the frequency spectrum. This adds comple3dty to the use, maintenance and administration of radio systems btfilt in accordance with the 8 arrangement of figure 2. This compleidty increases, the narrower the range of the frequency spectrum over which the radio can operate.

The arrangement of figure 4 provides a wide WO operating frequency range, with very low phase noise on the WO's output signal.

Considering figure 4 in detail, line 20 supplies a variable voltage VS l to a first varactor W. However, voltage VS1 only varies over a limited range. This variation is sufficient to vary the VCO's output signal over a limited frequency sub-range from Fminus to Fplus. This frequency sub-range is narrower than the VCO's whole operating frequency range from Fmin to Fmax. Capacitor W is a coupling capacitor which connects varactor W to terminal 2.

Figure 5 shows a graph of the output frequency of the WO of figure 4. The portion of variation over the sub-range from Fminus to Fplus has been shown as the solid line towards the bottom of figure 5. The range of steering voltages required to vary the output frequency over the range Fplus-Faiinus is clearly not as great as that required in the arrangement of figure 2 to vary the output frequency over the range Fmax-Fmin.

The arrangement of figure 4 also shows a second variable control signal VS2, which varies the value of a second varactor Ct2. The amount of capacitance provided by the varactor W acts as an offset to the resonant circuit of the WO. Therefore the centre value of the frequency sub-range Fminus- Fplus can be set by varying the value of the second variable control signal VS2.

Clearly the arrangement of figure 4 provides two ways of influencing the output voltage of the WO. One way is by varying the value of VS1, the other is by varying the value of VS2.

In a preferred embodiment, signal VS2 can take one of a number of discrete values. These discrete values determine the 'amount' of capacitance that M adds to the resonating circuitry between terminals 2 and 4. By changing the 9 signal VS2 by one increment, the centre value of the frequency sub-range Fminus- Fplus can be shifted along the output characteristic to a new frequency value. In efFect, a further increment of capacitance is added to the circuitry between terminals 2 and 4 each time that the signal VS2 is changed 5 by one increment.

The influence of incremental changes to the value of the signal VS2 is represented on figure 5 by the series of dotted curves. Each dotted curve shows the variation of the WO's output frequency resulting from variation of the signal VS l at a particular discrete value of signal VS2.

In the prefer-red embodiment, the control signal VS1 is continuously variable. Furthermore, in this embodiment, successive discrete values of the control signal VS2 diller by an amount which varies the centre value of the frequency sub-range (Fminus- Fplus) by not more than an amount equal to the fun width of the frequency sub-range.

With this embodiment, control signal VS1 provides continuously variable control of the VCO's output frequency over the range Fplus-Fminus. Control signal VS2 provides stepwise shifting of the VCO's output frequency. The step width is of a size such that one step moves the frequency sub-range Fminus to Fplus a distance along the frequency axis such that there is no gap between the ranges of WO output frequencies obtainable. There may however be slight overlap in these ranges. This would mean that the WO's output frequency for the highest values of VS l at one value of VS2 might also be obtainable at the lowest values of VS I at the next, higher incremental value of VS2.

The voltage controlled oscillator's predetermined operating frequency range Frnin-Fmax can therefore be divided into smaller subsections. These are essentially frequency bins. The discrete voltage value of VS2 sets the WO to provide an output signal within a particular bin. The continuously variable 1) 1 voltage value of VS l can vary the output frequency continuously within that particular bin.

The embodiment of the invention described above ensures that all WO output frequencies between Fmax and Fmin are obtainable by at least one combination of VS l and VS2 values. However, other embodiments of the invention are conceivable. One possibility would be that the step size of the output frequency change caused by variation of the value of VS2 would be greater than the frequency range Fplus-Fminus. The radio would then be able to operate continuously within several frequency bands, each of width FplusFminus, but these bands would be separated by frequency bands where the radio could not operate.

Notably, the voltage values VS2 can be chosen to compensate for the tolerances of the components in the WO of figure 4. This is done in an analogous way to that described above in connection with the arrangement of figure 2.

The frequency range or %in B l between Fminus and Fplus, in which the W0 is currently operating, is marked as the solid line towards the bottom in Figure 5. Figure 5 also shows various other frequency bins B2-B9. In the preferred embodiment of the invention, the bins B1-B9 would either represent contiguous or overlapping frequency ranges. There would be no gaps in the frequency ranges covered by these bins. Clearly for the VCO's operating frequency range to be broken down into the nine bins shown in figure 5, VS2 must be able to take nine digerent discrete values. The frequency range Fminus to Fplus for the arrangement of figure 4 would therefore be a frequency span equal to or greater than one ninth of the width of the VCO's frequency output range Fmin to Fmax.

The arrangement of figure 4 has a major advantage with respect to that of figure 2. A varactor W can be employed in the arrangement of figure 4 which has a substantially lower value of Kvco. This means that for a change in VS1 11 1 ) 11 111 1 1 1 1 1 of a given magnitude at any particular WO output frequency in the arrangement of:figure 4, the resulting change in the WO's output frequency is less than the frequency shift that a change of the same magnitude to signal VS would cause in the arrangement of figure 2. The lower value of KvCO in the arrangement of figure 4 means that capacitor W can have a lower value than the corresponding capacitor Cel in figure 2. This smaller capacitor provides weaker coupling of any noise on line 20 into the WO than a large capacitor would provide. This in turn results in less phase noise in the WO's output. Similarly, less of the varactor's inherent noise will be coupled into the WO by capacitor W, because of its smaller size.

As explained earlier, there is a maximum acceptable phase-noise on the output signal of a WO which is used in a radio. The reduced phase-noise in the arrangement of figure 4 results in the WO of figure 4 being useable over a wider bandwidth than that of figure 2, when both must meet the same phasenoise limit.

The arrangement of figure 4 shows a lower gradient of its characteristic curve, because of the lower value of K,,0. Despite this, the span of values of voltage VS l required to produce the output frequency range Fplus-Fminus at each value of VS2 are still substantially smaller than the span of values of voltage VS required in the arrangement of figure 2 to provide the entire output frequency range Fmax-Fmin. Therefore the circuit of figure 4 is well suited to implementation as a low voltage circuit.

The circuit of figure 4 has other advantages. The lower gradient characteristic curve of figure 4 is also less curved than that in figure 2. This lower curvature leads to improved modulation linearity. The modulation voltage applied to varactor CW will produce frequency modulation in the WO's output signal.

The linearity of this conversion depends on the degree of curvature of the varactor W's characteristic at the particular operating frequency. The lower the curvature, the greater the linearity.

12 The voltage controlled oscillator of figure 4 may be employed in the synthesiser section of a mobile or a portable radio. This provides a radio which is useable over a wider bandwidth than prior art radios employing, for instance, VC0s of the form shown in figure 2. A single design of radio can therefore be used within a very wide bandwidth, allowing fewer versions of the radio to cover the whole available bandwidth than was the case with prior art radios built to the design of figure 2. A typical phase noise which is achievable with the arrangement of figure 4 would be - 1 14dBc/Hz.

In an example of the circuit of figure 4 which the applicant has constructed, the entire frequency band in which the radios are to operate was from 444-464 Mhz in the UHF band. This means that the range Fmax-Fmin is 20 Mhz. This range was divided into two halves, one version of the radio being constructed for each half. Each version of the radio was designed to operate in 16 frequency bins. The portion of the spectrum marked as Fplus -Fminus on figure 5 was therefore no more than 1 Mhz, even allowing for the provision of overlap between successive frequency bins B. In a further example of the circuit of figure 4, the WO was designed to operate in the range of approximately 136-164 Mhz in the VHF band.

The performance of a WO can be further enhanced by circuitry of the general design shown in figure 6. Elements of figure 6 which have already been described in conjunction with figures 1-5 will not be described again here.

Figure 6 shows a second output from the loop filter 16 onto a line 50. A voltage VL is output by loop filter 16 onto line 50. Line 50 is connected to buffer 52. Buffer 52 buffers the voltage VL. The buffered output voltage from buffer 52 is fed to an A-D converter 54. A-D converter 54 digitises the value of the buffered voltage fed to it and supplies this digitised value to the microprocessor 24 within microcontroller 18.

13 In operation, loop filter 16 provides a voltage output VL on line 50 which is representative of the steering line voltage VS 1. The value of VL is measured and fed to the microprocessor by circuit elements 52 and 54.

Microprocessor 24 uses the value of VL in calculating the voltage output VS3 which the microcontroHer 18 generates in operation. Voltage VS3 in the arrangement of figure 6 is the voltage output by the microcontroller's D-A converter on line 22 to adjust the value of varactor W.

In setting the value of VS3 during operation of the arrangement of figure 6, microprocessor 24 makes use of pre-stored values which VS3 should take for particular WO operating frequencies. These values of VS3 are the empirically determined values which cause the WO to output the correct frequency F for a given control signal VS1.

The values of VS3 are pre-stored in the microcontroller 18, and result from a measuring algorithm to be described below. The algorithm can be performed in the factory where the radio is manufactured, via a special test system command. Alternatively or additionally, the radio can perform the algorithm each time it is powered up. The algorithm can even be performed at routine intervals after power up. As performance of the algorithm results in the radio being tuned, tuning can thus be performed repeatedly during the life of the radio. This is an improvement over merely tuning the radio at the time of initial manufacture and, possibly, when it is returned for maintenance.

The example of the algorithm below is for the oscillator of a radio where the radio's wanted output frequency span is divided into a number of segments. Each of these segments requires a different value of voltage to be applied to the varactor W. This is the voltage VS3 in figure 6. The algorithm below is particularly advantageous when used in such a radio, but can also be used in a radio which does not employ such segmentation of its operating frequency range.

1 1 1 1 14 The algorithm could be as follows for each segment of frequencies:

Step 1. Set synthesiser to desired lowest frequency. Step 2. Measure VC for this frequency. Step 3. If VC< target value then decrement VS3 by 1 increment and go to step 2. Step 4. Set synthesiser to desired highest frequency. Step 5. Cheek value of synthesiser's lock detect line. If synthesiser is not locked then fail, and put out error tone.

Step 6. VC is optimally adjusted for the WO save value, until the algorithm is nextinvoked.

This algorithm of steps 1-6 above is then repeated for each frequency segment in the radio's operating frequency range.

There are numerous advantages to the arrangement of figure 6. Amongst these are:

1. The WO is tuned automatically. No factory test system measurements are required. 2. Tuning the radio would only take a very short time. This is because the tuning is performed under the control of the radio, with no remote control by a test system being required. 3. Because the WO is adjusted optimally every time, the elrects of component ageing tolerances are eliminated. 4. Because the WO is adjusted optimally every time, the effects of component variations with temperature are eliminated. 5. The elimination of the efFects of both component ageing tolerances and component variations with temperature allows a much narrower WO range to be designed, whilst still guaranteeing that the synthesiser will lock. This means that the synthesiser-to-WO steering line 20 can be very lightly coupled, since it allows for a lower Kveo and consequentially only needs a small coupling capacitor W. This will improve the synthesiser phase noise and thus the performance of the radio.

The embodiment of the invention illustrated in figure 6 enjoys both the advantages of the arrangement of figure 4 and those provided by the additional components shown in figure 6. However, the sIdned person could apply the inventive features explained in connection with figure 6 to other arrangements than that shown in figure 4. For instance, the features and method described in connection with figure 6 could be applied to the prior art 10 arrangement described in connection with figure 2.

16 Cl 1. A voltage controlled oscillator, operable within a predetermined operating frequency range (Fmin-Fmax), comprising:

a resonant circuit (Lp, Cp) connected between a pair of terminals (2,4); a first variable capacitor (C0 connected via a first coupling capacitor (Cc3) between the said pair of terminals (2,4) in parallel to the said resonant circuit (Lp, Cp), whereby the value of the first variable capacitor (Ct3) influences the value of the resonant frequency Tvco) of the circuitry connected between the said pair of terminals (2,4), the value of the said first variable capacitor (Ct3) being variable by means of a first variable control signal (VS1), whereby the value of the resonant frequency Tvco) of the circuitry connected between the terminals (2,4) can be varied only within a sub-range (Fminus- Fplus) of the operating frequency range of the voltage controlled oscillator (Fmin-Fmax) by varying the value of the first variable control signal (VS1); and a second variable capacitor (Ct2) connected via a second coupling capacitor (Cc2) between the said pair of terminals (2,4) in parallel to the said resonant circuit (Lp, Cp), whereby the value of the second variable capacitor influences the value of the resonant frequency (Fvco) of thecircuitry connected between the said pair of terminals (2,4), the value of the said second variable capacitor (Ct2) being variable by means of a second variable control signal (VS2), whereby the centre value of the said frequency sub-range (Fminus- Fplus) can be set by varying the value of the said second variable control signal (VS2).

2. A voltage controlled oscillator in accordance with claim 1, wherein the first variable control signal (VS 1) is continuously variable.

3. A voltage controlled oscillator in accordance with claim 1 or claim 2, wherein the second variable control signal TS2) takes discrete values.

17 4. A voltage controlled oscillator in accordance with claim 3, wherein successive discrete values of the second variable control signal TS2) differ by an amount corresponding to the voltage required to vary the centre value of the frequency sub-range Tminus- Fplus) by an amount equal to or less than the width of the frequency sub-range (Fminus- Fplus).

5. A voltage controlled oscillator in accordance with any previous claim, wherein the first variable capacitor (Ct3) andfor the second variable capacitor (Ct2) are varactors.

6. A voltage controlled oscillator in accordance with any previous cl i, wherein the second variable control signal (VS3) is set to a value which has been selected in advance to provide the correct voltage controlled oscillator output frequency W,0) for the instantaneous value of the first variable control signal (VS1).

7. A voltage controlled oscillator in accordance with claim 6, comprising a buffer (52), analogue- digital converter (54) and a microprocessor (24) for measuring the instantaneous value of the first variable control signal (VS 1).

8. A voltage controlled oscillator in accordance with claim 7, wherein the set of values of the second variable control signal TS3) for use in the voltage controlled oscillator in operation are selected on power-up of the voltage controlled oscillator by determining the values of the second variable control signal (VS3) which provide the correct WO output frequency (Fvco) for various test values of the first variable control signal (VS1).

9. A voltage controlled oscillator substantially as hereinbefore described with reference to, or as illustrated by, any of figures 4, 5 or 6 of the drawings.

10. A portable or mobile radio comprising a voltage controlled oscillator in accordance with any previous claim.