EP1330881A2

EP1330881A2 - Radio transmitters

Info

Publication number: EP1330881A2
Application number: EP01978599A
Authority: EP
Inventors: Dennis Culling; Geoffrey Sidney Sparks
Original assignee: Sepura Ltd
Current assignee: Sepura Ltd
Priority date: 2000-10-23
Filing date: 2001-10-23
Publication date: 2003-07-30
Also published as: WO2002035712A2; WO2002035712A3; GB0025902D0; GB0125455D0; GB2372163A

Abstract

A signal synthesiser arrangement includes a signal generator in the form of a voltage controlled oscillator (VCO) (10) which generates a high frequency source signal under the control of a control unit (16). The synthesiser control unit (16) controls the oscillator (10) to produce an output signal of a given frequency. The output signal of frequency F1 from the oscillator (10) is provided to a signal mixer (12). It is also input to a frequency divider (15), which divides it by a value N1 to produce an 'offset frequency' signal F2 which is also provided to the mixer (12). The mixer (12) combines these signals and provides the combined signal to a bandpass filter (11) which filters the combined signal to select and provide one of the 'sideband' signals produced by the mixer, i.e. F1 + F2, or F1 - F2, as its output signal F3, while suppressing the unwanted, other sideband signal. The output signal F3 from the filter (11) is further frequency divided by a second value N2 in a further frequency divider (17) to produce a signal of frequency Fout that can be used as a carrier frequency signal for a radio transmitter.

Description

Radio Transmitters

The present invention relates to radio transmitters and in particular to a method of and an apparatus for generating the RF carrier wave that is to be used for transmission (and reception) by a radio transmitter.

As is known in the art, in a radio transmitter, the RF carrier wave that is to be used for transmission is typically generated by a synthesiscd signal source included in the radio. This synthesiscd signal source will include a signal generator, typically an oscillator (which is often referred to as the "local oscillator" of the radio unit), which generates a source high frequency signal from which the actual radio frequency signal (i.e. earner wave) used for transmission and reception is derived. This oscillator is typically a voltage controlled oscillator (VCO). In radio transmitters using such a synthεsised signal source, it is desirable to avoid the frequency of the source signal from the oscillator being an integer multiple of the actual transmission carrier frequency being used. This is because oscillators, in particular voltage controlled oscillators, are especially susceptible to interference from frequencies having an integer relationship to their frequency of oscillation. Such interference can cause unwanted modulation of the oscillator's output signal. Thus where a radio transmitter's earner frequency has an integer relationship to its oscillator's output frequency, the harmonics of the RF transmitter output can tend to interact with the oscillator and cause unwanted modulation of the oscillator's output signal. This effect is particularly pronounced where the radio transmitter output has a significant amount of amplitude modulation (such as would be the case, for example, with a TETRA (TErrestrial Trunked RAdio) signal). This problem can be alleviated by operating the radio transmitter at an output carrier frequency that does not have an integer relationship with its oscillator's (VCO's) operating frequency. One known way to achieve this is to add an "offset" frequency signal to the source signal generated by the radio unit's oscillator, with the combined signal then being used as or to derive the transmitter's overall operating carrier frequency. The offset frequency is arranged to ensure that the combined frequency signal (and hence the transmitter carrier frequency derived from it) does not have an integer relationship with the oscillator's operating frequency. This arrangement is usually referred to as an "offset loop synthesiser".

Figure 1 illustrates a typical offset-loop synthesiser architecture. The system includes a high frequency signal source oscillator 1 in the form of voltage controlled oscillator which generates a signal having a frequency Fa. A second signal source oscillator 3 generates an offset frequency Fb which is mixed with the output Fa of the voltage controlled oscillator 1 in a mixer 2 to generate a combined frequency Fc = Fa ± Fb. The combined frequency Fc is then passed through a bandpass filter

4, which filter selects either the upper or lower sideband of the signal Fc, i.e. either the signal at the frequency of the oscillator 1 plus the offset frequency (Fa + Fb), or the signal at frequency of the oscillator 1 minus the offset frequency (Fa - Fb). The selected frequency (sideband) is then used as or to derive the radio transmitter's output (operating) carrier frequency. The frequency not selected by the filter 4 (i.e. the unwanted sideband (image frequency)) appears as a spurious output and so the filter 4 should ensure adequate suppression of this frequency component at the chosen offset frequency. This means that care has to be taken with the choice of the offset frequency and the design of the filtering, so as to suppress these spurious signals in the filter output to an adequate level.

It is also known in offset-loop synthesis techniques to use superheterodyne arrangements to try to ensure that the combined frequency signal does not have an integer relationship with the initial oscillator's operating frequency. However, such superheterodyne arrangements are comparatively complex. An alternative approach to an offset-loop synthesiser is described in

US-A-5535247. In this arrangement, the frequency of the source signal generated by the radio unit's oscillator is modified by multiplying it by a factor 1/k (which can be non-integer and greater than or less than one), and the so-modified signal then used as or to derive the transmitter's carrier frequency. This arrangement is analogous to the frequency multipliers used in FM equipment in conjunction with a crystal oscillator.

According to a first aspect of the present invention, there is provided a signal generating apparatus for a radio transmitter, comprising: a signal generator for generating a source frequency signal; means for deriving an offset frequency signal from the source frequency signal generated by the signal generator; and means for mixing the source frequency signal generated by the signal generator with the offset frequency signal to provide a combined signal which can be used as or to derive a carrier frequency signal for a radio transmitter. According to a second aspect of the present invention, there is provided a method of operating a signal generating apparatus for a radio transmitter, which apparatus includes a signal generator that generates a source frequency signal, the method comprising: deriving a frequency offset signal from the source frequency signal generated by the signal generator; and mixing the derived frequency offset signal with the source frequency signal generated by the signal generator to provide a combined signal which can be used as or to derive the carrier frequency signal of a radio transmitter.

In the present invention, a signal which is to be used by a radio transmitter as or to derive its operating (carrier) frequency is formed by mixing the signal generated by a signal generator with an offset frequency signal, i.e. as in an offset loop synthesiser. However, the offset frequency signal is derived from the output signal of the signal generator itself, rather than, for example, being generated by a second signal source (e.g. oscillator). This avoids the need for an additional signal generator for generating the offset frequency, and allows the offset frequency to be generated using, for example, relatively easily available frequency dividers. Deriving the offset frequency in this way has also been found to be beneficial in the suppression of spurious signals, which allows, for example, for more simple filtering arrangements to be used to select the wanted signal and reject unwanted frequency components.

The signal generator in this arrangement will, as noted above, typically be an oscillator, and most typically a voltage controlled oscillator. However, the invention is also applicable to other foπns of signal generator that can be operated in this way and in particular for which the problem discussed above can arise.

The offset frequency signal should be such that the final carrier frequency signal will not have an integer relationship with the source signal generated by the signal generator. In practice, it should be such that the mixed signal does not have an integer relationship with the source signal generated by the signal generator.

The offset frequency can be derived as desired from the output signal generated by the signal generator. It is preferably derived by dividing the signal generator's output signal (i.e. the source frequency signal) by a given factor, most preferably by an integer value. The dividing factor preferably has the value 2", where n is an integer greater than zero, i.e. the factor equals 2 or 4 or 8, etc. Integer division of the signal generator's signal can be achieved with easily available integer frequency dividers, and yet achieves the requirement that the signal generator's output frequency does not have an integer relationship to the combined signal generator and offset frequency signal (and hence the radio transmitter's output frequency).

Where the signal generator (e.g. oscillator) is, as would typically be the case, controlled via a closed loop, feedback, control arrangement, the offset frequency is preferably mixed with the signal generator's output signal within the closed loop of the signal generator's control arrangement, preferably by adding it into the forward path of the control loop. By applying the frequency offset inside the signal generator's control loop, it is transparent to the control loop's programming, such that the signal generator's control programming does not need to be modified to take account of the frequency offset being added, e.g. to target a given frequency to which the frequency offset will then be added, and the effect of the frequency offset is automatically present in the feedback signal of the control loop. Thus, for example, where the signal generator is controlled via a phase locked loop, the mixing of the signals is preferably carried out inside the phase locked loop.

In a particularly preferred embodiment of the present invention, the mixed signal comprising the signal generator source frequency output signal and the frequency offset signal is filtered so as to select one of the two different frequency signals (i.e. sidebands) produced by the mixing, which filtered and selected signal is then used as or to derive the carrier frequency to be used by the radio transmitter. In other words, the mixed signal is filtered to select either a signal having a frequency equal to the frequency of the signal generator source frequency signal plus the offset frequency, or a signal having a frequency equal to the frequency of the signal generator source frequency signal minus the offset frequency. The filtering should also act to suppress the unwanted, other sideband (image) frequency. Where appropriate, this filtering preferably takes place within the closed control loop of the signal generator's control circuit (i.e. in the forward path of the control loop after the signal mixer and before the feedback path). The combined signal resulting from the mixing of the signal generator signal and the frequency offset signal, after filtering where appropriate, can be used directly as the operating carrier frequency signal to be used by the radio transmitter.

However, in a particularly preferred embodiment, the (filtered) combined signal is further modified in frequency and that modified signal then used as the carrier frequency signal for the radio transmitter. This further frequency modification of the output combined signal is preferably carried out by dividing that signal by a given factor, and most preferably by an integer value, so as to produce a carrier frequency that is less than the frequency of the combined signal. Again, this dividing factor preferably has a value 2", where n is an integer greater than zero, i.e. the factor is 2, 4, or 8, etc. A further division of the combined source and offset signals in this way has been found to allow even better suppression of spurious output signals in the so-generated carrier frequency signal, thereby, for example, allowing a simplification of the filtering, and a reduction in the amount of filtering, needed to achieve a given overall specification at the final output of the system.

The present invention is applicable to any radio transmitter that derives its carrier frequency from a signal generator, e.g. oscillator, signal source and can be applied in, for example, cartesian loop transmitters, F transmitters, polar loop transmitters, etc. It is particularly, but not exclusively, applicable to "direct-conversion" transmitters, i.e. transmitters in which the earner wave is modulated directly, as it allows such transmitters to avoid the problem of oscillator interference and feedback discussed above. Thus in a particularly preferred embodiment, the modulation with the wanted signal is performed after the signal mixing stage. In such an arrangement, the combined signal produced by mixing the source frequency signal and the offset frequency signal would be produced in an unmodulated form, i.e. the combined signal for use as or to derive the carrier frequency signal would be unmodulated when it is first produced, and neither the source frequency signal nor the offset frequency signal would be modulated (with any wanted information) before they are mixed. The unmodulated combined or carrier signal would then be provided to a modulation stage for modulation with the wanted information prior to transmission. The present invention also extends to a radio transmitter incorporating the apparatus of any of the above aspects of the invention and to a method of operating such a radio transmitter in accordance with the methods of any of the above aspects of the invention.

Thus, for example, according to a further aspect of the present invention, there is provided a radio transmitter comprising: a signal generator for generating a source frequency signal; means for deriving an offset frequency signal from the source frequency signal generated by the signal generator; means for mixing the source frequency signal generated by the signal generator with the offset frequency signal to provide a combined signal; and means for using the combined signal as or to derive the carrier frequency signal to be used by the radio transmitter.

This aspect of the invention can, and preferably does, include one or more or all of the preferred features of the invention discussed above. Thus it is, for example, preferably a direct-conversion transmitter. The methods in accordance with the present invention may be implemented at least partially using software e.g. computer programs. It will thus be seen that when viewed from further aspects the present invention provides computer software specifically adapted to carry out the methods hereinabove described when installed on data processing means, and a computer program element comprising computer software code portions for performing the methods hereinabove described when the program element is run on data processing means. The invention also extends to a computer software carrier comprising such software which when used to operate a signal generating apparatus comprising a digital computer causes in conjunction with said computer said apparatus to carry out the steps of the method of the present invention. Such a computer software carrier could be a physical storage medium such as a ROM chip, CD ROM or disk, or could be a signal such as an electronic signal over wires, an optical signal or a radio signal such as to a satellite or the like.

It will further be appreciated that not all steps of the method of the invention need be carried out by computer software and thus from a further broad aspect the present invention provides computer software and such software installed on a computer software carrier for carrying out at least one of the steps of the methods set out hereinabove.

A number of preferred embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 illustrates a known offset-loop synthesiser architecture; Figure 2 shows schematically a signal synthesiser architecture in accordance with a first embodiment of the present invention; and

Figure 3 shows schematically a signal synthesiser architecture in accordance with a second embodiment of the present invention.

Figure 2 shows a signal synthesiser in accordance with the present invention that provides an output signal having a frequency Fout that can be used as a carrier wave for a radio transmitter. The signal Fout can be provided, for example, as the carrier frequency input signal to a cartesian loop, polar loop, or FM radio transmitter arrangement (not shown).

The signal synthesiser arrangement of Figure 2 includes a signal generator in the form of a voltage controlled oscillator (VCO) 10 which generates a high frequency source signal under the control of control unit 16 (which control unit receives the oscillator's control loop feedback signal 13 and provides, via a loop filter, a loop error control signal to the oscillator 10, as known in the art). The synthesiser control unit 16 controls the oscillator 10 to produce an output signal of a given frequency.

The output signal of frequency Fl from the oscillator 10 is provided to a signal mixer 12. It is also input to a frequency divider 15, which divides it by a - 1 - value Nl (which value, as discussed above, is preferably an integer, preferably of the form 2") to produce an "offset frequency" signal F2 which is also provided to the mixer 12. The mixer 12 combines these signals (i.e. Fl and F2) and provides the combined signal to a bandpass filter 1 1 which filters the combined signal to select and provide one of the "sideband" signals produced by the mixer, i.e. Fl + F2, or

Fl - F2, as its output signal F3, while suppressing the unwanted, other sideband signal.

A (feedback) part 13 of the output signal F3 of the filter 1 1 is also fed back to the control unit 16 to provide closed loop, feedback control of the signal synthesiser arrangement, i.e. oscillator 10. Thus the oscillator 10, mixer 12, filter

1 1 and control unit 16 form a closed synthesiser loop. (The feedback signal 13 could instead be taken from the output signal, Fout, of the second frequency divider 17 (see below), if desired.)

The output signal F3 from the filter 1 1 , i.e. from the oscillator control loop, is further frequency divided by a second value N2 (which value, as discussed above, is preferably an integer, preferably of the form 2") in a further frequency divider 17 to produce the signal of frequency Fout that is to be used as the carrier frequency signal of the radio transmitter. The signal Fout would be provided to the remainder of the radio transmitter as is normal in the art, e.g. as an input to a cartesian loop transmitter architecture.

It should be noted that at this stage the signal Fout (and the other signals Fl, F2, F3) are not modulated with any wanted information. Rather the signal Fout would be provided to a modulation stage for modulating with the wanted information prior to transmission. Thus the arrangement is a simple (direct conversion) transmitter arrangement, i.e. one in which the carrier is modulated directly, rather than a superheterodyne arrangement.

In this embodiment, the offset frequency F2 is derived from the voltage controlled oscillator's output Fl by division by Nl . After mixing this offset frequency F2 with the voltage controlled oscillator's output Fl and filtering, the resulting signal F3 is frequency divided by a second value N2 to produce the required carrier frequency Fout for use by the radio transmitter. In general in this arrangement, the offset frequency F2 is less than the carrier frequency Fout, and is less than the frequency of the mixed, combined signal F3. (In general F3 = Fl ± kF2, where k is an integer. As will be explained further below, where Nl is an integer value, the frequency F3 = kF2, where k is an integer.)

The frequency relationships of the various signals produced in the arrangement of Figure 2 will now be considered. In the arrangement shown in Figure 2, the offset frequency:

F2 = F 1/N 1

and the output frequency of the filter 1 1 :

F3 = Fl + F2 (or F l - F2, depending on which frequency the filter 1 1 is arranged to pass)

Fl Fl

= Fl + — (or Fl - —X

Nl Nl

This satisfies the requirement that frequency Fl of the signal generated by the oscillator 10 should not have an integer relationship to the transmitter output (carrier) frequency (which is based on F3).

The present invention also helps the suppression of spurious signals in the transmitter output. For example, if the filter 1 1 selects the upper sideband of the mixed signal, i.e. Fl + F2, then:

1

F3 = Fl 1 + Nl

but Fl = N1.F2

F3 = F2 (1 + Nl).

Due to the mixing process, outputs from the mixer 12 will also occur at frequencies N1.F2 and (Nl + 2).F2. However, the frequency range defined by:

F2.N1 i frequency «, F2(N1 + 2)

will be free of spurious products apart from the wanted component at F2(N1 + 1). Furthermore, when Nl is an integer value, since the frequencies Fl and F2 are correlated, it is possible to arrange that the various components produced in the mixing process which fall at the wanted frequency add constructively.

It can be seen from the above that in this arrangement the value of the frequency F2 determines the frequency spacing of the unwanted products F2.N1 and F2(N 1 + 2) from the wanted signal F2(N1 + 1 ), and hence the required performance of the bandpass filter 1 1 so as to pass only the wanted product. The higher the value of the offset frequency F2, the greater this frequency spacing will be. Thus a higher offset frequency F2 will allow the unwanted products F2.N1 and F2(N1 + 2) to be more easily removed without the need for great complexity in the bandpass filter arrangement 1 1. The use of higher values for the offset frequency F2 is facilitated in the present embodiment by the further frequency division step in the frequency divider 17 (which divides the filter output F3 by a factor of N2), as that division allows higher absolute frequency values for the source signal Fl and the offset frequency signal F2 to be used for a given desired carrier frequency output.

(The situation if the lower sideband:

is selected is analogous. In that case the spurious products which must be rejected are Nl .F2 and F2(Nl - 2).)

As well as facilitating the use of offset frequencies that help to ensure that spurious frequencies present before filtering are sufficiently displaced in frequency from the wanted signal so as to be attenuable by a relatively simple filter arrangement, the frequency divider 17 also helps itself to suppress unwanted signals, thereby further relaxing the requirements imposed upon the bandpass filter 1 1.

To illustrate this, the situation where two signals, a wanted frequency, Fa, and an unwanted signal, Fb (at a lower level), are simultaneously applied to the input of frequency divider 17, will be considered.

The unwanted signal Fb will cause phase modulation of the wanted signal Fa around the frequency divider's input threshold, i.e. Fa will become phase modulated with Fb. After division by the factor N2 in the frequency divider 17, the wanted signal is at frequency Fa/N2. The sidebands caused by the phase modulation will be at frequency (Fa/N2)±Fb.

It can be shown (by examination of a Bessel function of the first kind) that for a small modulation index, the amplitude of the first sidebands of a phase (or frequency) modulated signal are proportional to the modulation index (J_t(mp) tends to mp/2 for small mp).

Thus at the output of the frequency divider 17, the amplitude level of the sidebands due to the unwanted signal Fb is reduced in proportion to the frequency division ratio when compared to the input. Furthermore, the relative distance between the wanted signal and the modulation sidebands has been increased since the wanted output is now at frequency Fa/N2 whilst the unwanted components are still displaced from it by the frequency Fb.

Thus as well as the presence of frequency divider 17 assisting the suppression of unwanted frequencies by means of the bandpass filter 1 1 at the input of frequency divider 17, the second frequency divider 17 also helps any remaining unwanted products in its output to be more easily removed by a further filter at its output, due to the increased relative frequency separation of the wanted and unwanted products in the second frequency divider's output.

Figure 3 shows schematically the implementation of the present invention in an 800 MHz dual-band synthesiser. The synthesiser includes a voltage controlled oscillator 21, which is controlled by a synthesiser control unit 22. The oscillator 21 also has a band-switching input 23. This could, for example, switch the output frequency Fout of the synthesiser between the frequency bands 805-825 MHz and 850-870 MHz. In this embodiment, the oscillator's signal output Fl is divided by four in divide-by-4 frequency divider 24 (i.e. Nl above is 4), to produce an offset frequency signal F2. The oscillator's output signal Fl and the offset frequency signal F2 are then mixed by mixer 25 and provided to a bandpass filter 26 which passes one of the sidebands created by the signal mixing. The filtered signal, i.e. the selected sideband, is provided to a buffer amplifier 27. A portion of the output of the amplifier 27 is then split off as a feedback signal to the synthesiser loop control unit 22 by a 6dB splitter 28.

The remainder of the mixed output signal passes as an output signal of the oscillator's control loop to a further frequency division stage, in divide-by-2 frequency divider 29, where it is divided by 2 (i.e. N2 above is 2) to provide the signal Fout that is to be used as the carrier frequency for the transmitter (and transmitted after modulation).

The various frequency relationships in this arrangement are as follows, assuming that the filter 26 selects the lower sideband, i.e. F1-F2: _{c t} Fl - F2 Fout =

1

Fl - 7-7/4 T out =

Fl Fl

Fout =

^■tpout - 3 + E/

Fout - — -*F1

Where in this case, the desired earner frequency, i.e. Fout, is 805-825 MHz, then as N 1=4 and N2=2, Fl (i.e. the oscillator's frequency) should be 2146.67-2200 MHz and F2 (i.e. the offset frequency) will be 536.67-550 MHz. It can be seen from the above that the present invention provides an offset loop synthesiser architecture in which the offset signal is derived from the signal generating oscillator's output signal itself, preferably by (integer) division of the oscillator's output signal. The offset signal ensures a non-integer relationship between the oscillator's operating frequency and the carrier frequency used by the transmitter, i.e. eliminates the problem of harmonics of the carrier frequency interfering with the oscillator's operation. Furthermore, the present invention permits such operation in a simple (direct-conversion) transmitter architecture and so can avoid the need to use, for example, more complex superheterodyne topology to avoid these problems.

In preferred embodiments of the present invention, there is a further frequency division after the mixing with the offset signal. This reduces significantly the filtering needed to achieve adequate suppression of spurious signals in the overall output carrier frequency signal.

The invention provides, in its preferred embodiments at least, a system which cannot generate unwanted outputs separated from the wanted signal by less than the output frequency of the offset signal generating frequency divider.

Claims

1. A signal generating apparatus for a radio transmitter, comprising: a signal generator for generating a source frequency signal; means for deriving an offset frequency signal from the source frequency signal generated by the signal generator; and means for mixing the source frequency signal generated by the signal generator with the offset frequency signal to provide a combined signal which can be used as or to derive a carrier frequency signal for a radio transmitter.

2. The apparatus of claim 1 , wherein the signal generator comprises a voltage controlled oscillator.

3. The apparatus of claim 1 or 2, wherein the offset frequency signal is derived such that the combined signal does not have an integer relationship with the source frequency signal generated by the signal generator.

4. The apparatus of claim 1 , 2, or 3, wherein the means for deriving the offset frequency signal comprises means for dividing the source frequency signal generated by the signal generator by an integer value.

5. The apparatus of claim 4, wherein the dividing factor has a value 2", where n is an integer greater than zero.

6. The apparatus of any one of claims 1 to 5, wherein the signal generator is controlled via a closed loop, feedback, control arrangement, and the offset frequency is mixed with the signal generator's output signal within the closed loop of the signal generator's control arrangement.

7. The apparatus of claim 6, wherein the offset frequency is mixed with the signal generator's output signal by adding it into the forward path of the control loop.

8. The apparatus of any one of claims 1 to 7, further comprising a filter for filtering the combined signal so as to select either a signal having a frequency equal to the frequency of the signal generator source frequency signal plus the offset frequency, or a signal having a frequency equal to the frequency of the signal generator source frequency signal minus the offset frequency, which filtered and selected signal is then used as or to derive the carrier frequency to be used by the radio transmitter.

9. The apparatus of claim 8, wherein the signal generator is controlled via a closed loop, feedback, control arrangement, and the filtering takes place within the closed control loop of the signal generator's control circuit.

10. The apparatus of any one of the preceding claims, further comprising means for modifying the combined signal in frequency, and means for using the modified signal as the carrier frequency signal for the radio transmitter.

1 1 . The apparatus of claim 10, wherein the means for modifying the combined signal comprises means for dividing that signal by an integer value, so as to produce a carrier frequency that is less than the frequency of the combined signal.

12. The apparatus of claim 1 1 , wherein the dividing factor has a value 2", where n is an integer greater than zero.

13. The apparatus of any one of the preceding claims, wherein the combined signal is produced in an unmodulated form.

14. A radio transmitter incorporating the apparatus of any one of the preceding claims.

15. A radio transmitter comprising: a signal generator for generating a source frequency signal; means for deriving an offset frequency signal from the source frequency signal generated by the signal generator; means for mixing the source frequency signal generated by the signal generator with the offset frequency signal to provide a combined signal; and means for using the combined signal as or to derive the carrier frequency signal to be used by the radio transmitter.

16. A method of operating a signal generating apparatus for a radio transmitter, which apparatus includes a signal generator that generates a source frequency signal, the method comprising: deriving a frequency offset signal from the source frequency signal generated by the signal generator; and mixing the derived frequency offset signal with the source frequency signal generated by the signal generator to provide a combined signal which can be used as or to derive the carrier frequency signal of a radio transmitter.

17. The method of claim 16, comprising deriving the offset frequency signal such that the combined signal does not have an integer relationship with the source frequency signal generated by the signal generator.

18. The method of claim 16 or 17, comprising dividing the source frequency signal generated by the signal generator by an integer value to derive the offset frequency signal.

19. The method of claim 18, wherein the dividing factor has a value 2ⁿ, where n is an integer greater than zero.

20. The method of any one of claims 16 to 19, wherein the signal generator is controlled via a closed loop, feedback, control arrangement, the method further comprising mixing the offset frequency signal with the signal generator's output signal within the closed loop of the signal generator's control arrangement.

21. The method of claim 20, comprising adding the offset frequency signal into the forward path of the control loop within the closed loop of the signal generator's control arrangement to mix it with the signal generator's output signal.

22. The method of any one of claims 16 to 21 , further comprising filtering the combined signal so as to select either a signal having a frequency equal to the frequency of the signal generator source frequency signal plus the offset frequency, or a signal having a frequency equal to the frequency of the signal generator source frequency signal minus the offset frequency, and then using the filtered and selected signal as or to derive the carrier frequency to be used by the radio transmitter.

23. The method of claim 22, wherein the signal generator is controlled via a closed loop, feedback, control arrangement, and the filtering takes place within the closed control loop of the signal generator's control circuit.

24. The method of any one of claims 1 to 23, further comprising modifying the combined signal in frequency, and using the modified signal as the earner frequency signal for the radio transmitter.

25. The method of claim 24, comprising modifying the combined signal by dividing that signal by an integer value, so as to produce a carrier frequency that is less than the frequency of the combined signal.

26. The method of claim 25, wherein the dividing factor has a value 2", where n is an integer greater than zero.

27. The method of any one of claims 16 to 26, further comprising modulating the combined signal or the carrier frequency signal with information for transmission after the combined signal has been formed.

28. A method of operating a radio transmitter, comprising operating the radio transmitter in accordance with the method of any one of claims 16 to 27.

29. A computer program element comprising computer software code portions for performing the method of any one of claims 16 to 27 when the program element is run on data processing means.

30. A signal generating apparatus for a radio transmitter substantially as hereinbefore described with reference to Figure 2 or 3 of the accompanying drawings.

31. A radio transmitter substantially as hereinbefore described with reference to Figure 2 or 3 of the accompanying drawings.

32. A method of operating a signal generating apparatus for a radio transmitter substantially as hereinbefore described with reference to Figure 2 or 3 of the accompanying drawings.

33. A method of operating a radio transmitter substantially as hereinbefore described with reference to Figure 2 or 3 of the accompanying drawings.