GB2379105A

GB2379105A - Improvements in or relating to fast frequency-hopping modulators and demodulators

Info

Publication number: GB2379105A
Application number: GB0120642A
Authority: GB
Inventors: Martin Schwab; John Domokos; Christopher Nigel Smith
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2001-08-24
Filing date: 2001-08-24
Publication date: 2003-02-26
Anticipated expiration: 2021-08-24
Also published as: GB0220522D0; GB0120642D0; CN1547808A; WO2003019809A1; CN1314211C; GB2379105B; GB2379106B; GB2379106A

Abstract

Methods and apparatus are provided for effectively shortening the set-up time of a new receive or transmit frequency in a frequency hopping transceiver. Post-rotation of received baseband signals removes a phase error caused by a local oscillator not yet being phase locked as intended. Pre-rotation of baseband signals for transmission cancels a phase error to be introduced by a local oscillator of the transmit path not yet being phase locked as intended. The method is directed particularly towards phase-modulating a baseband signal.

Description

IMPROVEMENTS IN OR RELATING TO FAST FREQUENCY-HOPPING MODULATORS AND DEMODULATORS

Ir The present invention relates to frequency hopping synthesisers. More particularly, it relates to phase error correction of phase modulation or demodulation in fast hopping local oscillators for base stations of mobile telephone networks.

In base station transceivers used in GSM/EDGE mobile telephone networks, frequency-hopping synthesisers are required which have high switching speed, low phase noise and low spurious components simultaneously. The phase noise is important, as the baseband signal is encoded on to the RF carrier by phase modulation. In a typical carrier unit, 2-4pu transmit switching speed is necessary, typically with a signal-to-spurious frequency performance of better than-80dBc.

A known Carrier Unit (CU) for use in a mobile telephone base station is shown in Fig. 1. As shown, a transmit path 10 and a receive path 12 each communicate between a baseband processor and controller 13 and a radio transceiver 15. The transmit path 10 and receive path 12 each require a respective local oscillator signal TXLO, RXLO from a respective local oscillator 9,11. Four separate independent synthesisers 14,16, 18,20 are used for this purpose, which are switched from slot to slot in a'leap-frog'configuration. That is, while one synthesiser (e. g. 14) is in use, its complementary synthesiser 16 is tuning to the required frequency for the next slot. When the next slot begins, switch 22 will pass the output of 16 as the signal RXLO, and the synthesiser 14 will then begin tuning to the frequency of the following slot. The synthesisers 18, 20 of the transmit path operate similarly. The baseband processor and controller 13 sends frequency control signals 17 to each of the synthesisers, and a reference frequency Ref is also applied to each synthesiser.

Fig. 2 shows the phase error Ao in an output signal of a voltage controlled oscillator ('vco') being tuned to a required frequency and phase, in a local oscillator of a frequency-hopping carrier unit such as that shown in Fig. 1. Fig. 2 also represents a

typical output of a phase comparator of a phase locked loop of a local oscillator such as those shown at 9,11 in Fig. 1.

y In the first region, labelled 1, the voltage controlled oscillator is oscillating at a different frequency from the required or reference frequency, that is, Aft0. Cyclic phase errors reaching 180 are seen, representing a beat frequency being the difference between the reference frequency and the frequency generated by the voltage controlled oscillator. As the voltage controlled oscillator is controlled to reach the required frequency, the beat frequency slows down and eventually, at the end of the first region, a final zero crossing 25 occurs. The vco is providing the required frequency and Af=0. There is, however, still a phase error A (j) ?'0 between the vco output and the reference signal.

During a second region, labelled"2"in Figure 2, operation of the local oscillator draws the phase of the vco signal ever closer to the phase required to synchronise the phases of the reference signal and the feedback signal, respectively applied to the inputs of the phase comparator of the local oscillator of the present invention, as discussed in more detail below, progressively reducing Ao.

By the end of the second phase, the vco output signal has been brought into phase with the reference signal and Ao--O.

For a fast frequency hopping transducer, the first and second regions 1,2 should be made as short as possible, so that the vco signal is ready for use as soon as possible. However, an active burst, i. e. active data transmission or reception, may begin during the second phase 2, while a phase error is still present. This can cause interference with a baseband signal which is phase modulated onto its RF carrier.

The present invention relates to methods and apparatus for reducing the duration of the second region, 2, of the vco frequency and phase lock process as shown in Fig. 2. As has been mentioned above, the varying phase error in the second region

causes interference in the baseband of received signals, as they are typically encoded using phase modulation, and a phase error in the local oscillator signal will ease an offset in the detected baseband signal. The phase error may also cause interference to transmitted signals, which will contain the phase error as an apparent modulated signal.

The present invention accordingly reduces the duration and effect of the second region 2 by applying a post-rotation to the baseband signal in the receiver, and a pre-rotation to the baseband signal in the transmitter.

That is, a phase shift is applied to the received baseband signal in the opposite sense as compared to the phase error of the local oscillator signal in the receive path in order to offset the phase error caused by the local oscillator. A phase shift is applied to the transmit baseband signal before transmission, in the opposite sense as compared to the phase error of the local oscillator signal in the transmit path, in order to offset the phase error caused by the local oscillator.

Accordingly, the present invention provides a method for demodulating a phasemodulated baseband signal in a frequency-hopping demodulator, comprising the steps of : receiving an RF signal comprising a phase modulated baseband signal ; demodulating the received signal using a local oscillator itself employing a phase locked loop to provide a local oscillator signal having a time-variant phase error and, in response to the demodulating step, producing a demodulated baseband signal comprising the baseband signal offset by an error term due to the phase error. That the method further comprises the step of post-rotating the demodulated baseband signal by complex multiplication with a phase rotation term, thereby substantially removing the offset from the demodulated baseband signal to produce the baseband signal.

The present invention also provides a method for phase-modulating a baseband signal in a frequency-hopping demodulator, comprising the steps of: phase modulating the baseband signal using a local oscillator, itself employing a phase locked loop to provide a local oscillator signal having a time-variant phase error; and, in response to the phase modulating step, producing a modulated baseband signal comprising the baseband signal offset by an error term due to the phase error.

The method further comprises the step of : prior to the phase modulating step, rotating the baseband signal by complex multiplication with a phase rotation term, thereby introducing an error term of substantially equal and opposite effect to the error term introduced by the phase error, thereby substantially cancelling the offset introduced by the phase error to produce a modulated baseband signal substantially free of offset.

The phase rotation term may be calculated by a linear predictor in response to the reception of signals indicating a present value of the phase error and loop parameters characterising the phase locked loop.

A phase comparator of the phase locked loop may provide a signal indicating the present value of the phase error to the linear predictor. The signal indicating the present value of the phase error may be sampled and converted into a digital representation before being applied to the linear predictor.

The loop parameters provided to the linear predictor may be adapted according to a required frequency of operation of the local oscillator signal.

The present invention also provides a frequency-hopping phase demodulator comprising: a local oscillator itself comprising a phase locked loop providing a local oscillator signal at a predetermined frequency, with a time-variant phase error, a receive path for phase demodulating a received RF signal to produce a demodulated baseband signal comprising a required baseband signal offset by an error term corresponding to a present value of the phase error, the phase locked loop

itself comprising a phase comparator arranged to provide a phase error signal 0

indicative of a present value of the phase error. The demodulator further comprises a complex multiplier arranged to receive the demodulated baseband signal and a phase rotation signal adapted to have a substantially equal and opposite effect as compered to the error term, the complex multiplier being arranged to perform a complex multiplication of the demodulated baseband signal with the phase rotation term thereby to obtain the required baseband signal.

The present invention also provides a frequency-hopping phase modulator comprising a local oscillator itself comprising a phase locked loop providing a local oscillator signal at a predetermined frequency, with a time-variant phase error ; a transmit path for phase modulating a received baseband signal to produce a modulated RF signal comprising a modulation representing the received baseband signal offset by an error term corresponding to a present value of the phase error, the phase locked loop itself comprising a phase comparator arranged to provide a phase error signal indicative of a present value of the phase error. The modulator further comprises a complex multiplier arranged to receive a required baseband signal and a phase rotation signal adapted to have a substantially equal and opposite effect as compared to the error term, the complex multiplier being arranged to perform a complex multiplication of the required baseband signal with the phase rotation signal thereby to obtain a modulated RF signal at the output of the multiplexer which comprises a modulated version of the required baseband signal substantially free of the error term.

Such multiplexer or demultiplexer may further comprise a linear predictor and a look up table, wherein the look up table is arranged to supply values of loop parameters characterising the phase locked loop to the linear predictor, and the linear predictor is further arranged to receive signals indicating a present value of the phase error, the linear predictor being further arranged to accordingly calculate a value of the phase rotation signal.

The phase rotation signal may be calculated in response to the signal indicating the present value of the phase error. An analogue to digital converter may be provided,

to sample and convert the signal indicating the present value of the phase error into a digital representation, said digital representation being applied to the linear predictor.

The look up table may be arranged to store a plurality of sets of loop parameters each corresponding to a particular required frequency of operation of the local oscillator signal, the look up table being further arranged to receive a signal indicating which of the plurality of sets should be supplied to the linear predictor.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent with reference to the following description of certain embodiments, given by way of examples only, in conjunction with the accompanying drawings, in which: Fig. 1 shows a known architecture of carrier unit for a hopping transceiver in a base station for a mobile telephone network; Fig. 2 shows the phase error of an output of a local oscillator in a frequency hopping carrier unit, acquiring a required frequency and phase; Fig. 3 shows a frequency hopping demodulator according to a first embodiment of the present invention; Fig. 4 shows signals involved in the operation of the demodulator of Fig. 3; Fig. 5 shows signals involved in the operation of the modulator of Fig. 6; and Fig. 6 shows a frequency hopping modulator according to a second embodiment of the present invention.

Fig. 3 shows a first embodiment of the invention, in which a post-rotation is applied to received baseband signal in the receive path. Features common with those in Fig.

'carry corresponding reference numerals.

Local oscillator 9 produces a receive local oscillator frequency RXLO to the receive path 12. The local oscillator may be of any suitable type, but as shown includes a reference frequency Ref and a phase locked loop comprising a phase comparator 32 of transfer function Ko, a loop filter 33 of transfer function F (s) and a vco 34 of transfer function Kv/s providing the local oscillator signal RXLO, with a divide-byN counter 35 in the feedback path to the phase comparator 32.

According to an aspect of the present invention, a phase error signal v (t) 36 provided by the phase comparator 32 is converted into a digital version 37 by an analogue-to-digital converter (ADC) 38.

The phase error signal v (t) may be expressed as v (t) = K#*#(t)/(N+K#*F (s) *Kv/s), where z (t) is the phase error at the output of the vco and (N+KO*F (s) *Kv/s) is the loop gain of the phase locked loop.

Since the received baseband signal Srx (t) is phase modulated onto its RF carrier, the phase error (P (t) will be demodulated as a part of the baseband signal by the receive path 12, and will appear superimposed on the required baseband signal. The signal 40 produced at the output of the receive path will accordingly be Srx (t) *e l1 > (I).

Fig. 4A shows an example of the required baseband signal Srx (t) representing an audio tone.

Fig. 4B shows an example of a phase error ei (t), decaying in magnitude with time, according to the second region 2 of the vco tuning operation illustrated in Fig. 2.

Fig. 4C shows an output signal 40 Srx (t) *e') which would be produced by receive 0 chain 12 in response to the baseband signal of Fig. 4A decoded using local 1 0 oscillator signal RXLO with a decaying phase error as shown in Fig. 4B. n According to the present invention, a post-rotation of the demodulated baseband signal 40 Srx (t) *#j#(t) is performed using a complex multiplier adapted to cancel the phase shift error in the demodulated baseband signal so as to produce a required baseband signal Srx (t) as shown in Fig. 4A. This achieved by multiplying the demodulated baseband signal Srx (t) *e' (t) by a phase rotation term 44 e'' (t) as shown in Fig. 4D.

According to an aspect of the invention, phase error voltage v(t) is sampled and the corresponding phase error (D (t) is predicted on the basis of stored loop parameters.

Although the initial value of v (t) is random, as soon as frequency lock is acquired, there is a deterministic relationship between v (t) and (D (t).

Referring again to Fig. 3, according to an embodiment of the present invention, the baseband processor and controller 13 is provided with a complex multiplier 42 receiving the demodulated baseband signal Srx (t) *e (t) 40 and a phase rotation signaJe-44.

The phase rotation signal 44 is produced by a linear predictor 46 in response to the digital phase error signal 37 and values of loop parameters (F, Kv, KO) corresponding to the selected required vco frequency, which parameters are supplied by a look-up table LUT 48 in response to control signals CH indicating the required vco frequency. Preferably, steady state vco control voltages Vo are also stored for each required vco frequency. The values in the look-up table 48 are also preferably self-calibrating, receiving updated values CAL in a manner known in itself to those skilled in the art.

The phase rotation signal 44 is produced as follows.

After acquiring frequency lock, there is a finite phase error (D (t) at the output of the vco 34.

The phase error is translated into phase error voltage v (t) 36 by the phase detector 32. The phase error voltage is sampled by the ADC 38 to produce a digitised version 37, and this is applied to the baseband processor 13.

In the baseband processor, the amount of post-rotation eig (t) required can be readily

predicted from sample to sample, using the stored parameters and the instantaneous v (t) values. This may be seen from the expression :

v (t) = Ko* (D (t)/ (N+K*F (s) *Kv/s) ; which gives < (t) = v (t) (N+K ()) *F (s) *Kv/s)/K < t).

Signal v (t) is input 37 to the linear predictor ; N is a constant ; and F (s), Kv/s and Ko are provided by the look up table, enabling the linear predictor 46 to calculate (D (t), and so to produce the phase rotation signal e-j#(t) 44. The phase rotation signal 44 should have an equal but opposite magnitude as compared to the phase error introduced into the demodulated baseband signal. That is, the complex multiplier 42 performs the operation:

Srx (t) * e-= Srx (t),

to extract the required demodulated signal 50. Referring to Figs. 4A to 4D, the phase rotation signal 44 e' is, as shown in Fig. 4D, equal and opposite to the baseband offset introduced by the phase error < (t) and shown in Fig. 4B. By multiplying the demodulated baseband signal 40 (Fig.

4C) by the phase rotation signal 44 (Fig. 4D), the phase error offset (Fig. 4B) is cancelled, to provide a required baseband signal 50 (Fig. 4A).

The linear predictor calculates the value of the required phase rotation signal based on an existing sampled value 37 of v (t). It performs linear prediction to calculate an appropriate value of phase rotation signal e' (') 44 to apply at times between samples of the phase error voltage v (t). The linear predictor 46 may be replaced by any other suitable type of predictor, with possible increased cost.

The invention accordingly provides compensation for phase error in the vco output, allowing communication to take place relatively early in the second region 2 of the vco tuning operation illustrated in Fig. 2.

According to another aspect of the present invention, compensation is provided for phase error in the vco output of the transmit local oscillator signal TXLO for the transmit path 10. This allows transmission to begin relatively early in the second region 2 of the vco tuning operation illustrated in Fig. 2.

Figs. 5A-D illustrate signals relevant to this aspect of the present invention. An embodiment of the present invention is illustrated in Fig. 6 in which features corresponding to features of Fig. 3 carry corresponding reference numerals.

Similarly to the embodiment described with reference to Fig. 3, the local oscillator, in this case transmit oscillator 11, provides a local oscillator signal, here TXLO, to the transmit path 10. The phase locked loop comprising phase comparator 32, loop filter 33, vco 34 and divider 35 in the feedback path, provides local oscillator signal TXLO at a required frequency. There will, however, be certain frequency and phase offsets (Af, AO) in the local oscillator output, as shown in Fig. 2, while the vco and the phase locked loop are settling to the required frequency and phase. Once the phase locked loop is operating at the required frequency, the phase comparator 32 provides a phase error signal v (t) 36 indicating the present phase error < 1 > (t). This phase error signal is sampled and converted into a digital representation 37 by analogue-to-digital converter (ADC) 38.

As discussed with reference to the embodiment of Fig. 3, baseband processor and controller 13 comprises a linear predictor 46 receiving the digital phase error signal v (t) 37 and values of loop parameters (F, Kv, KO corresponding to the selected required vco frequency, which parameters are supplied by a look-up table LUT 48 in response to control signals CH indicating the required vco frequency. Preferably, steady state vco control voltages Vo are also stored for each required vco frequency in the look-up table 48. The values in the look-up table 48 are also preferably selfcalibrating, receiving updated values CAL in a manner known in itself to those skilled in the art.

The linear predictor 46 then generates a phase correction signal e' (t) intended to be equal and opposite to the effect of the phase error of the vco in the phase modulation performed by the transmit chain.

In the embodiment of Fig. 6, a complex multiplier 42 receives a baseband signal Stx (t) 52 (Fig. SA) for transmission, and the phase rotation signal e-j#(t) (t) 44 (Fig. 5B). Since the baseband signal Stx (t) has not been the subject of any phase error distortion, the effect of this complex multiplication is to produce a distorted baseband signal 54, Stx (t) *e' (Fig. 5C), to the transmit path 10. The transmit path 10 then attempts to modulate the baseband signal 54 onto a carrier for transmission by the transceiver 15. However, due to the phase error (D (t) in the local oscillator signal TXLO, the baseband signal 54 (Fig. 5C) will be distorted by application of a phase error term e (t) (Fig. 5D). However, according to an aspect of this invention, the distortion caused by the phase error term (Fig. 5D) is substantially equal and opposite to the distortion already caused to the original, required baseband signal Stx (t) (Fig. 5A) by the phase correction signal 44 ex (Fig. 5B), such that the signal finally applied to the transceiver 15 for transmission

contains a phase modulated baseband signal substantially corresponding to :

Stx (t) 'e'=Stx (t).

The present invention accordingly provides methods and apparatus for effectively shortening the set-up time of a new receive or transmit frequency in a frequency hopping transceiver for use in a carrier unit for a mobile telephone base station.

The invention provides post-rotation of received baseband signals to remove a phase error caused by the vco of a local oscillator not yet being phase locked as intended, thereby allowing reception to begin earlier in any particular frequency slot. Similarly, the invention provides pre-rotation of baseband signals for transmission to cancel a phase error to be introduced by the vco of a local oscillator of the transmit path not yet being phase locked as intended, thereby allowing transmission to begin earlier in any particular frequency slot. The overall effect is either to allow faster communications with faster switching between frequencies, or to allow a simpler, less costly oscillator to be used with existing communications speeds.

While the present invention has been described with reference to a limited number of particular examples, many modifications and amendments may be made within the scope of the invention. For example, the present invention is not limited to mobile telephone applications. Rather, it may be applied to other communications systems where fast modulation of a frequency-hopping carrier is required. The linear predictor described in the particular embodiments referred to may be replaced

with any other suitable type of predictor. The local oscillator may be of the type

referred to in copending UK patent application NoCt & OHO 6 agents reference

2001P15578 GB, entitled"Improvements in or relating to Fast Frequency-Hopping Synthesisers", having the same applicant and filing date as this application, or may be of any type which is capable of supplying a phase error voltage v (t).

Claims

CLAIMS: 1. A method for demodulating a phase-modulated baseband signal (50) in a frequency-hopping demodulator, comprising the steps of : - receiving (15) an RF signal comprising a phase modulated baseband signal (50); - demodulating the received signal using a local oscillator (9) itself employing a

phase locked loop (32, 33, 34, 35) to provide a local oscillator signal (RXLO) having a time-variant phase error ( < (t)) ; and - in response to the demodulating step, producing a demodulated baseband signal (40) comprising the baseband signal (50) offset by an error term due to the phase error, characterised in that the method further comprises the steps of : - post-rotating the demodulated baseband signal (40) by complex multiplication (42) with a phase rotation term (44), thereby substantially removing the offset from the demodulated baseband signal (40) to produce the baseband signal (50).
2. A method for phase-modulating a baseband signal (52) in a frequencyhopping demodulator, comprising the steps of : - phase modulating (10) the baseband signal (52) using a local oscillator (11), itself employing a phase locked loop (32,33, 34,35) to provide a local oscillator signal (TXLO) having a time-variant phase error ( (D (t) ); - in response to the phase modulating step, producing a modulated baseband signal (56) comprising the baseband signal (52) offset by an error term due to the phase error, characterised in that the method further comprises the steps of : - prior to the phase modulating step, rotating the baseband signal (52) by complex multiplication (42) with a phase rotation term (44), thereby introducing an error term of substantially equal and opposite effect to the error term introduced by the phase error, thereby substantially cancelling the offset introduced by the phase error to produce a modulated baseband signal (56) substantially free of offset.

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3. A method according to claim 1 or claim 2 wherein the phase rotation term is calculated by a linear predictor (46) in response to the reception of signals (37) indicating a present value (# (t)) of the phase error and loop parameters characterising the phase locked loop.
4. A method according to claim 2 wherein a phase comparator (32) of the phase locked loop provides a signal (36) indicating the present value ( < (t) ) of the phase error to the linear predictor.
5. A method according to claim 4 wherein the signal (36) indicating the present value ( < (t)) of the phase error is sampled and converted (38) into a digital representation (37) before being applied to the linear predictor.
6. A method according to any of claims 3-5 wherein the loop parameters provided to the linear predictor are adapted according to a required frequency of operation of the local oscillator signal.
7. A frequency-hopping phase demodulator comprising: - a local oscillator (9) itself comprising a phase locked loop (32,33, 34,35) providing a local oscillator signal (RXLO) at a predetermined frequency, with a time-variant phase error; - a receive path (12) for phase demodulating a received RF signal to produce a demodulated baseband signal (40) comprising a required baseband signal (50) offset by an error term corresponding to a present value of the phase error ( (D (t) ), the phase locked loop itself comprising a phase comparator (32) arranged to provide a phase error signal (36) indicative of a present value ( < P (t)) of the phase error characterised in that the demodulator further comprises: - a complex multiplier (42) arranged to receive the demodulated baseband signal and a phase rotation signal (44) adapted to have a substantially equal and opposite effect as compered to the error term, the complex multiplier being arranged to

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perform a complex multiplication of the demodulated baseband signal with the

phase rotation term thereby to obtain the required baseband signal (50).

ID
8. A frequency-hopping phase modulator comprising: - a local oscillator (11) itself comprising a phase locked loop (32, 33, 34,35) providing a local oscillator signal (TXLO) at a predetermined frequency, with a time-variant phase error ( < (t) ); - a transmit path (10) for phase modulating a received baseband signal (54) to produce a modulated RF signal (56) comprising a modulation representing the received baseband signal (54) offset by an error term corresponding to a present value of the phase error (O (t) ), the phase locked loop itself comprising a phase comparator (32) arranged to provide a phase error signal (36) indicative of a present value ( (t) ) of the phase error, characterised in that the modulator further comprises: - a complex multiplier (42) arranged to receive a required baseband signal (52) and a phase rotation signal (44) adapted to have a substantially equal and opposite effect as compared to the error term, the complex multiplier being arranged to perform a complex multiplication of the required baseband signal with the phase rotation signal thereby to obtain a modulated RF signal (56) at the output of the multiplexer which comprises a modulated version of the required baseband signal (50) substantially free of the error term.
9. A multiplexer or demultiplexer according to claim 7 or claim 8 further comprising a linear predictor (46) and a look up table (48), wherein the look up table is arranged to supply values of loop parameters characterising the phase locked loop to the linear predictor, and the linear predictor is further arranged to receive signals (37) indicating a present value ( < (t) ) of the phase error, the linear predictor being further arranged to accordingly calculate a value of the phase rotation signal (44).

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10. A modulator according to any of claims 7-9 wherein the look up table is arranged to store a plurality of sets of loop parameters each corresponding to a particular required frequency of operation of the local oscillator signal (TXLO, RXLO), the look up table being further arranged to receive a signal (CH) indicating which of the plurality of sets should be supplied to the linear predictor.

10. A multiplexer or demultiplexer according to any of claims 7-9 wherein the ID phase rotation signal is calculated in response to the signal (36) indicating the present value ( (t)) of the phase error.

11. A multiplexer or demultiplexer according to claim 10 further comprising an analogue to digital converter (38) arranged to sample and convert the signal (36) indicating the present value ( < P (t) ) of the phase error into a digital representation (37), said digital representation being applied to the linear predictor.

12. A multiplexer or demultiplexer according to any of claims 9-11 wherein the look up table is arranged to store a plurality of sets of loop parameters each

corresponding to a pawicu ! t required frequency of operation of the local oscillator signal (TXLO, RXLO), the look up table being further arranged to receive a signal (CH) indicating which of the plurality of sets should be supplied to the linear predictor.

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Amendments to the claims have been filed as follows CLAIMS :

1. A method for phase-modulating a baseband signal (52) in a frequencyhopping modulator, comprising the steps of : - phase modulating (10) the baseband signal (52) using a local oscillator (11), itself employing a phase locked loop (32, 33, 34, 35) to provide a local oscillator signal (TXLO) having a time-variant phase error ( < (t)) ; - in response to the phase modulating step, producing a modulated baseband signal (56) comprising the baseband signal (52) offset by an error term due to the phase error ; characterised in that the method further comprises the steps of : - prior to the phase modulating step, rotating the baseband signal (52) by complex multiplication (42) with a phase rotation term (44), thereby introducing an error term of substantially equal and opposite effect to the error term introduced by the phase error, thereby substantially cancelling the offset introduced by the phase error to produce a modulated baseband signal (56) substantially free of offset.

2. A method according to claim 1 wherein the phase rotation term is calculated by a linear predictor (46) in response to the reception of signals (37) indicating a present value (q) (t)) of the phase error and loop parameters characterising the phase locked loop.

3. A method according to claim 1 wherein a phase comparator (32) of the phase locked loop provides a signal (36) indicating the present value ( < (t)) of the phase error to the linear predictor.

4. A method according to claim 3 wherein the signal (36) indicating the present value ( (D (t)) of the phase error is sampled and converted (38) into a digital representation (37) before being applied to the linear predictor.

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5. A method according to any of claims 2-4 wherein the loop parameters provided to the linear predictor are adapted according to a required frequency of operation of the local oscillator signal.

6. A frequency-hopping phase modulator comprising: - a local oscillator (11) itself comprising a phase locked loop (32,33, 34,35) providing a local oscillator signal (TXLO) at a predetermined frequency, with a time-variant phase error (C (t) ); - a transmit path (10) for phase modulating a received baseband signal (54) to produce a modulated RF signal (56) comprising a modulation representing the received baseband signal (54) offset by an error term corresponding to a present value of the phase error ( < (t)), the phase locked loop itself comprising a phase comparator (32) arranged to provide a phase error signal (36) indicative of a present value ( < (t) ) of the phase error; characterised in that the modulator further comprises: - a complex multiplier (42) arranged to receive a required baseband signal (52) and a phase rotation signal (44) adapted to have a substantially equal and opposite effect as compared to the error term, the complex multiplier being arranged to perform a complex multiplication of the required baseband signal with the phase rotation signal thereby to obtain a modulated RF signal (56) at the output of the multiplexer which comprises a modulated version of the required baseband signal (50) substantially free of the error term.

7. A modulator according to claim 6 further comprising a linear predictor (46) and a look up table (48), wherein the look up table is arranged to supply values of loop parameters characterising the phase locked loop to the linear predictor, and the linear predictor is further arranged to receive signals (37) indicating a present value ( < 1 > (t) ) of the phase error, the linear predictor being further arranged to accordingly calculate a value of the phase rotation signal (44).

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8. A modulator according to any of claims 6-7 wherein the phase rotation signal is calculated in response to the signal (36) indicating the present value ( < (t) ) of the phase error.

9. A modulator according to claim 8 further comprising an analogue to digital converter (38) arranged to sample and convert the signal (36) indicating the present value ( < (t) ) of the phase error into a digital representation (37), said digital representation being applied to the linear predictor.