GB2323227A - Synthesiser - Google Patents

Abstract

A synthesiser is arranged to generate frequency hopping signals which change frequency without a settling time. The arrangement comprises high-band and low-band signal generators 5, 6 which are alternatively multiplied with an intermediate frequency signal using a mixer 3 acting in combination with a selection switch 4. By arranging for the frequency of the signals generated by the high-band and low-band signal generators in combination with the intermediate frequency to generate mirror frequency signals within a common frequency bandwidth, the synthesiser may be arranged to change frequencies without a requirement for a settling time, and without the frequency of the high-band and low-band signal generators having substantially the same frequency contemporaneously. The synthesiser may form part of a DECT (Digital Enhanced Cordless Telephone) system. The IF signal may be FSK or PSK modulated.

Description

SYNTHESISER The present invention relates to synthesisers which operate to generate signals with a desired frequency. More particular, the present invention relates to frequency hopping synthesisers.

Synthesisers are used in a variety of applications to generate signals with a desired frequency in a controlled way. For example, in order to effect modulation and demodulation of radio signals, frequency synthesisers are used in order to generate signals with a frequency related to a carrier frequency of the radio signals. In the case of modulation, a frequency synthesiser generates signals at an intermediate frequency, or at a carrier frequency, which are combined with signals modulated by data to be communicated to generate the radio signals. In the case of demodulation, frequency synthesisers are used to generate signals which are arranged to be multiplied by received radio signals, and thereafter filtered, to effect down conversion of the received signal to a baseband representation.

In some radio systems, the carrier frequency is arranged to hop over a plurality of predefined frequencies so as to mitigate, inter alia, effects of co-channel interference. To effect frequency hopping of a carrier signal, synthesisers in transmitters of the radio system are arranged to rapidly change from generating signals at one frequency to generating signals at another frequency in accordance with a predetermined hopping pattern.

At receivers in the radio system, synthesisers are arranged correspondingly to hop between the same predetermined frequencies, in order to effect demodulation, and thereby recover communicated data. Such synthesisers are known as frequency hopping synthesisers.

In radio systems which employ frequency hopping, a settling time is provided to allow a frequency hopping synthesiser to settle at a new frequency having hopped from an old frequency.

However, there are situations where it is desirable to have zero settling time. For example, in a digital cordless telephone system such as the Digital Enhanced Cordless Telephone (DECT) system, data is communicated between mobiles and base stations by transmitting bursts of data in predetermined time slots. The DECT system is therefore a Time Division Multiple Access (TDMA) system. To mitigate the effects of co-channel interference, the DECT system also provides for bursts of data to be communicated on different frequencies in accordance with a frequency hopping scheme. To allow a frequency hopping synthesiser to settle to a new frequency, a guard time is provided between time slots, before a subsequent burst of data is transmitted. As well as a settling time, the guard time incorporates a time associated with the propagation of radio signals from mobile units to base stations.

The guard time allocated to the propagation delay, therefore determines a maximum range at which mobiles may transmit data bursts to a base station. However, if frequency hopping synthesisers in the transmitters of the base and mobile stations could be arranged to hop in zero time, the portion of the guard time allocated for settling time of the frequency hopping synthesiser could be used to extend the propagation delay accommodated by the system, thereby extending the operating range of the DECT system.

Known techniques for providing a frequency hopping synthesiser with zero settling time, include direct digital synthesis.

However, direct digital synthesis is comparatively expensive, and therefore inappropriate for mass production applications. Another known technique for effecting zero hopping time, is to provide a transmitter with twin synthesisers. With such an arrangement, one of the twin synthesisers can be arranged to switch to a new frequency whilst the other of the twin is generating signals at the old frequency. The arrangement is known as 'leap-frog' synthesisers. However, the twin synthesiser frequency hopping arrangement suffers from a technical problem in that as the frequency of the tuning synthesiser crosses the frequency at which the static synthesiser is generating signals, a disturbance in the frequency of signals generated by the static synthesiser will occur, such that the frequency of the signals generated by the static synthesiser will drift as a result of injection locking and tuning synthesiser leakage.

The technical problem of generating a frequency hopping synthesiser with zero settling time using a twin synthesiser arrangement is addressed by the present invention.

According to the present invention there is provided a synthesiser for generating frequency hopping signals of a desired frequency, comprising high-band and low-band signal generators, which respectively operate to generate signals with frequencies from a high frequency bandwidth and a low frequency bandwidth, a selection switch which operates to selectively feed signals from either said high-band or said low-band signal generators to a first input of a mixer, said mixer being arranged to scale signals fed from either said high-band or said low-band signal generators with signals generated by an intermediate frequency signal source, and a filter coupled to said mixer which operates to attenuate signals with frequencies outside a predetermined common frequency band, wherein said high-band and low-band signal generators act in combination with said selection switch in dependence upon mirror frequency signals generated by said mixer to effect a change in frequency of a frequency hopping signal generated at an output of the filter, without a requirement for a settling time.

The frequency hopping signal may be generated in association with one of said high-band or said low-band signal generators during a predetermined period, and the other of said high-band or said low-band signal generators may be tuned to a subsequent frequency for use in generating said frequency hopping signal during a subsequent predetermined period.

The high frequency bandwidth and low frequency bandwidth may be mutually exclusive, so that a change in frequency of said frequency hopping signal may be effected without a requirement for the frequency of signals generated by said high-band signal generator and said low-band signal generator to be substantially similar contemporaneously.

By providing a frequency synthesiser with high and lowband signal generators to effect frequency hopping, and by alternatively switching between the high-band and low-band signal generators, one of either the high-band or low-band signal generators may be arranged to generate signals with a desired frequency, whilst the other of the high-band or low-band signal generators is tuning to a new frequency. In this way, a time for a resulting frequency hopping signal to change from a first frequency to a second frequency is determined by the time to switch between one of the low and high-band frequency signal generators to the other of the low and high-band signal generators.

When signals at different frequencies are scaled in frequency by a frequency mixer or multiplier, the resulting scaled signal will be comprised of signals at a frequency substantially equal to the sum of the frequencies of the original signals, and signals at a frequency substantially equal to the difference between the frequencies of the original signals. The signals at a frequency substantially equal to the sum of the frequencies of the original signal are a mirror of the signals at the difference of the frequencies of the original signals. Typically, therefore, one of the mirror signals is removed from the resulting mixed signals by a bandpass filter.

Signals at a frequency substantially equal to the sum of the frequencies of the original signals are referred to herein as high frequency mirror signals, whereas those at a frequency substantially equal to the difference of the frequencies of the original signals are referred to as low frequency mirror signals.

By arranging for a frequency synthesiser to be comprised of a high-band signal generator and a low-band signal generator in an arrangement hereinbefore described, and by selecting the high frequency bandwidth and low frequency bandwidth in combination with a frequency of an intermediate frequency signal mixed with signals from either the high or the low-band frequency signal generators, the resulting mixed signals may be arranged to be comprised of one of the high or low mirror frequency signals within a common frequency band. Furthermore, when the frequency hopping signal is being generated in association with the high-band signal generator, it is the low frequency mirror signals which fall within the common frequency band to provide the hopping frequency signal. When the lowband signal generator is arranged to generate the hopping frequency signal within the common frequency band, it is the high frequency mirror signals which fall within the common frequency band to provide the hopping frequency signal. In this way, the frequency synthesiser may be arranged to generate a frequency hopping signal within the common frequency band which may hop between frequencies without a requirement for settling time, and without a requirement for either of the high-band or low-band signal generators to generate signals contemporaneously with substantially the same frequency.

The intermediate frequency signal source may be modulated by a baseband signal. The intermediate frequency signal source may be phase shift key modulated. Intermediate frequency signal source may be frequency shift key modulated.

Advantageously, digital data modulating the intermediate frequency signal source may be differentially encoded.

If the intermediate frequency signal source is modulated in accordance with a frequency or phase shift key modulation scheme, a shift in phase or frequency of the intermediate frequency signal will generate a corresponding phase or frequency shift in the frequency hopping signal, but a directional sense of the change in frequency or phase will be reversed when the lowband signal generator is being used as compared with the highband signal generator. By differentially encoding the modulating data, and correspondingly differentially decoding the modulating data at a receiver, the effects of this reversal in the directional sense of a change in frequency or phase can be obviated.

According to an aspect of the present invention there is provided a method of generating a frequency hopping signal, comprising the steps of, (i) generating a high-band signal, which high-band signal has a frequency from a high bandwidth of frequencies, (ii) multiplying said high-band signal with an intermediate frequency signal, the frequency of said intermediate frequency signal and said high bandwidth of frequencies being such that first mirror frequency signals formed by the multiplication fall within a predetermined common frequency band, (iii) generating a low-band signal, which low-band signal has a frequency from a low bandwidth of frequencies, (iv) multiplying said low-band signal with said intermediate frequency signal, said low bandwidth of frequencies and said intermediate frequency being such that second mirror frequency signals associated with said low-band signal fall within said predetermined common frequency band, (v) changing alternately the frequency of said high and lowband signals, and, (vi) selecting first or second mirror frequency signals, in accordance with the alternating change in frequency of said highband or low-band signal so that a frequency hopping signal representative of signals from either said first or said second mirror frequency signals is effected.

One embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, wherein, FIGURE 1 is a schematic block diagram of a transmitter embodying a frequency synthesiser, FIGURE 2 represents an illustration of frequency bands of signals generated in accordance with the operation of the transmitter shown in Figure 1.

An example embodiment of a frequency synthesiser which operates in accordance with the present invention is shown embodied within a transmitter of radio signals shown in Figure 1.

Although the frequency synthesiser is shown in Figure 1 embodied within a radio frequency transmitter, it will be appreciated that a frequency synthesiser operating in accordance with the principles of the present invention may find application in a receiver of a radio system, or in any system requiring a signal to be generated with a frequency which may be switched from one predetermined frequency to another predetermined frequency without a requirement for a settling time.

In Figure 1, an intermediate frequency signal source 1 is shown to be connected to a first input 2 of a frequency mixer 3.

Connected to a second input of the signal mixer 3, is an output of a selection switch 4, which is arranged to feed signals from either a high-band signal generator 5, or a low-band signal generator 6.

Coupled to an output of the mixer 3, is a bandpass filter 7, which is arranged to filter signals fed from the mixer 3 to an aerial 8.

Selection of either signals from the high-band frequency generator 5 or the low-band frequency generator 6, is effected by the selection switch 4 in combination with a frequency hopping controller 9. For the example embodiment, the frequency hopping synthesiser is shown to be all elements within the box 10.

However, as will be appreciated from the following description, in other applications the intermediate frequency signal source 1, may form part of the frequency synthesiser 10.

A schematic block diagram shown in Figure 3 provides an illustrative example of parts of a transmitter embodying the frequency hopping synthesiser 10. In this way, the intermediate frequency signal generator 1, is arranged to represent all parts of a radio transmitter associated with the modulation and generation of baseband signals, up-converted to an intermediate frequency fIF. Therefore, f( t), is provided to represent data modulating the intermediate frequency signal generated by the signal source 1, in accordance with the operation of a radio system in which the transmitter or parts of the transmitter shown in Figure 1 are embodied. The intermediate frequency signals may therefore be modulated by a digital signal using, for example, frequency shift keying or phase shift keying. As will be appreciated, further elements may be present in the transmitter shown in Figure 1, for example, a power amplifier may be connected to the frequency synthesiser between the aerial 8 and the bandpass filter 7 in order to effect amplification of the frequency hopping signal.

The operation of the frequency synthesiser shown in Figure 1 will now be described. The high-band signal generator 5, which may be a local oscillator, and the low-band signal generator, which may be a further local oscillator, are arranged to operate in combination to effect frequency hopping of a signal generated at the output of the bandpass filter 7, in which a change of frequency is effected without a requirement for a settling time. This is achieved by arranging for one of the high-band or low-band signal generators to provide a signal at a first frequency, during which the other of the high-band or low-band signal generators tunes to a new frequency. Once the other signal generator has acquired the new frequency, the selection switch 4 serves to feed signals at the new frequency from the other signal generator instead of signals from the first of the high-band or low-band signal generators, generating signals at the old frequency. Thereafter, operation of the two signal generators is reversed, in that the signal generator which was providing the input to the frequency mixer 3, now becomes tuned to a subsequent frequency.

Signals generated at an output of the mixer 3 fed to the bandpass filter 7, will be comprised of signals at a frequency substantially equal to the sum of the frequencies of either the low-band fL, or high-band fH signal generators and the frequency of the intermediate frequency signal fIF, and signals at a frequency substantially equal to the difference between the frequencies of either the low-band fL or high-band fH signal generators and the frequency of the intermediate frequency signal fIF. Therefore, there are four possible frequencies at which signals may appear at the output of the mixer 3. These are the high and low frequency mirror signals associated with the lowband signal generator fL + fIF, and fL - fIF, and the high and low frequency mirror signals associated with the high-band signal generator fH + fIF and fH - fIF. However, the selection switch 4, in combination with the frequency hop controller 9, ensure that during any period signals from only one of either the high-band signal generator or low-band signal generator are fed to the second input of the mixer 3. Correspondingly, signals at the output of the mixer 3, will be either at a frequency fL - IF and fL + IF, or fH - IF, and fH + IF.

An illustration of the effect of the arrangement of the lowband and high-band signal generators, in combination with the intermediate frequency signal source, on the signals produced at the output of the mixer 3, may be seen in Figure 2. In Figure 2, the frequency band associated with the high-band signal generator is shown to be the region of frequencies D, whereas the frequency band associated with the low-band signal generator is associated with the frequency band B. As an illustrative example, band D is shown to be between 990 MHz and 1010 MHz, whereas band B is shown to be 790 MHz to 810 MHz. In this example the frequency of the intermediate frequency signal, fIF is100 MHz.

When the selection switch 4 is arranged to connect signals from the high-band signal generator 5 to the second input of the frequency mixer 3, a corresponding version of the baseband signal f(t) will appear as two versions, one at each of high and low mirror frequencies, at fH - fIF and fL - fIF. Therefore, the mirror versions of the baseband signal f( t) will appear in bands C and E.

Similarly, in the case of the low- band signal generator, versions of the baseband signal f(t) will appear at each of the high and low mirror frequencies fL + IF and fL - IF. Therefore, the high and low mirror frequency signals associated with the low-band signal generator will appear within bands A and C. Hence, by selecting the frequency of the intermediate frequency signal fIF, and the frequency bands of the high-band and low-band signal generators, it is possible to arrange that the high frequency mirror signal resulting from multiplication of the low-band signal generator with the intermediate frequency signal will produce a version of the baseband signal f(t) within the common frequency hopping band C, and the low frequency mirror signal generated by the high-band signal generator will also fall within the common frequency band C. By arranging for the high-band and low-band signal generators to change frequency in accordance with a desired frequency hopping pattern, a frequency hopping signal representative of the baseband signal f(t) may be generated at an output of the bandpass filter 7, which is arranged to pass signals within the common frequency band C, without a requirement for either of the low-band or high-band signal generators 5, 6, to have substantially similar frequencies contemporaneously. Therefore, frequency hopping may be effected without the requirement for a settling time.

The arrangement of the frequency synthesiser hereinbefore described is particularly suitable for mass production applications.

This because the arrangement of high-band and low-band signal generators, wherein the high and low-bands are mutually exclusive, obviates a requirement for substantial radio frequency shielding.

One consequence of a frequency synthesiser arrangement according to Figure 1, is that where the baseband signal f(t) is arranged to modulate the intermediate frequency signal in accordance with, for example, frequency shift keying or phase shift keying, changes in phase or frequency in the frequency hopping signal will occur in an opposite directional sense in dependence upon which of the high-band or low-band signal generators are used to generate the frequency hopping signal.

This is because the frequency hopping signal is generated in association with the low mirror frequency signal of the high-band signal generator, whereas the high frequency mirror signal is used to provide the frequency hopping signal in association with the low-band signal generator. Thus, a phase or frequency change in the baseband signal will cause a change in phase or frequency in an opposite directional sense, independence upon which of the high-band or low-band signal generators is generating the frequency hopping signal. One technique for mitigating this problem where data is communicated using digital modulation, is to differentially encode the digital data, such that data is only represented by a change in the frequency or phase state of the baseband signal. Alternatively, other techniques for mitigating the reversal of the phase or frequency sense rotation may be employed, such as arranging for data to be reversed in a receiver of a radio system in accordance with a frequency hopping pattern.

The frequency synthesiser hereinbefore described, finds application in various systems. For example, a cordless telephone system arranged to operate in accordance with the Digital Enhanced Cordless Telephone (DECT) standard, must be arranged to hop between a plurality of predetermined frequencies between transmissions of bursts of data in predetermined time slots. The DECT system provides a settling time for frequency hopping synthesisers as part of a guard time between data transmissions.

However, a frequency synthesiser arrangement as hereinbefore described may effect frequency hopping without a settling time.

As such, the settling time may be reused by providing an increase in a propagation delay between a transmitter embodied within a mobile terminal of the DECT system, and a receiver embodied within a base station operating in accordance with the DECT system.

As will be appreciated by those skilled in the art, various modifications to the arrangement and embodiments hereinbefore described may be effected without departing from the scope of the present invention. In particular the frequency hopping synthesiser may be comprised of more than two signal generators in a situation where fast frequency hopping is required.

Furthermore, the invention finds application in other radio systems other than DECT, for example, cellular radio systems such as the Global System for Mobiles (GSM), wherein frequency hopping is used.

Claims (14)

1. A synthesiser for generating frequency hopping signals of a desired frequency, comprising high-band and low-band signal generators, which respectively operate to generate signals with frequencies from a high frequency bandwidth and a low frequency bandwidth, a selection switch which operates to selectively feed signals from either said high-band or said lowband signal generators to a first input of a mixer, said mixer being arranged to scale signals fed from either said high-band or said low-band signal generators with signals generated by an intermediate frequency signal source, and a filter coupled to said mixer which operates to attenuate signals with frequencies outside a predetermined common frequency band, wherein said high-band and low-band signal generators act in combination with said selection switch in dependence upon mirror frequency signals generated by said mixer to effect a change in frequency of a frequency hopping signal generated at an output of the filter, without a requirement for a settling time.

2. A synthesiser as claimed in Claim 1, wherein the frequency hopping signal is generated in association with one of said highband or said low-band signal generators during a predetermined period, and the other of said high-band or said low-band signal generators is tuned to a subsequent frequency for use in generating said frequency hopping signal during a subsequent predetermined period.

3. A synthesiser as claimed in Claims 1 or 2, wherein the high frequency bandwidth and low frequency bandwidth are mutually exclusive, so that a change in frequency of said frequency hopping signal may be effected without a requirement for the frequency of signals generated by said high-band signal generator and said low-band signal generator to be substantially similar contemporaneously.

4. A synthesiser as claimed in any preceding Claim, wherein said mirror signals generated by said mixer in association with said intermediate frequency signal source and said high-band and said low-band signal generators, comprise low frequency mirror and high frequency mirror signals, and said intermediate frequency and said high frequency bandwidth and said low frequency bandwidth are arranged so that either said low frequency mirror signals or said high frequency mirror signals fall within said predetermined common frequency band.

5. A synthesiser as claimed in Claim 4, wherein the low frequency mirror signals associated with said high-band signal generator, and the high frequency mirror signals associated with said low-band signal generator fall within said predetermined common frequency band.

6. A radio communications system comprising a transmitter and a receiver, each of which transmitter and receiver include a synthesiser as claimed in any preceding Claim.

7. A radio communications system as claimed in Claim 7, wherein the synthesiser included in said transmitter forms part of a modulator, and the synthesiser included in the receiver forms part of a demodulator, and the modulator and demodulator in combination with said synthesisers operate to communicate data via frequency hopping radio signals.

8. A radio communications system as claimed in Claim 6, wherein the intermediate frequency signal source of the synthesiser in said transmitter comprises a digitally modulated baseband signal source and an up-converter means which operates to up-convert digitally modulated baseband signals, generated by said signal source, to signals at said intermediate frequency, and the intermediate frequency signal source in said receiver is a demodulating signal generator, which operates in combination with the filter of the synthesiser associated with said receiver to generate signals representative of said baseband signal.

9. A radio communications system as claimed in Claim 8, wherein said baseband signal is modulated by digital data in accordance with phase or frequency shift keying.

10. A radio communications system as claimed in Claim 9, wherein the digital data is differentially encoded, and the demodulated data is differentially decoded in said receiver, thereby mitigating the effects of phase or frequency reversal in dependence upon operation of said synthesiser.

11. A radio communications system as claimed in Claims 6 to 10, wherein said transmitter and said receiver operate in accordance with a Digital Enhanced Cordless Telephone system.

12. A synthesiser as hereinbefore described with reference to the accompanying drawings.

13. A method of generating a frequency hopping signal, comprising the steps of, (i) generating a high-band signal, which high-band signal has a frequency from a high bandwidth of frequencies, (ii) multiplying said high-band signal with an intermediate frequency signal, the frequency of said intermediate frequency signal and said high bandwidth of frequencies being such that first mirror frequency signals formed by the multiplication fall within a predetermined common frequency band, (iii) generating a low-band signal, which low-band signal has a frequency from a low bandwidth of frequencies, (iv) multiplying said low-band signal with said intermediate frequency signal, said low bandwidth of frequencies and said intermediate frequency being such that second mirror frequency signals associated with said low-band signal fall within said predetermined common frequency band, (v) changing alternately the frequency of said high and lowband signals, and, (vi) selecting first or second mirror frequency signals, in accordance with the alternating change in frequency of said highband or low-band signal so that a frequency hopping signal representative of signals from either said first or said second mirror frequency signals is effected.

14. A method of generating a frequency hopping signal as claimed in Claim 13, wherein said high bandwidth of frequencies are mutually exclusive, and signals from said first mirror frequency signals and from said second mirror frequency signals fall within a common predetermined frequency band.