National Research Development Corporation

British Technology Group Limited

University of Bristol

Andrew Corporation

Btg International Limited

Fairfield Industries, Inc.

Unique Wireless Developments, L.l.c.

Andrew Bateman

Joseph P McGeehan

Peter Blakeborough Kenington

Mark Anthony Beach

Kam Yuen Chan

Peter Kenington

Richard Michael Bennett

Ross James Wilkinson

Richard M Bennett

Wycliffe Timothy Slingsby

A method and apparatus is provided for locking a local oscillator to a reference oscillator. The reference oscillator is arranged to transmit a signal containing two reference pilot tones in a signal sideband. The apparatus samples the received signal and makes a frequency shifted copy where the shift is equal to the expected separation between the reference tones. The non-shifted and shifted signals are then combined in the frequency domain to constructively reinforce one of the reference pilot tones which is then searched for. Once the tone has been identified, a correction to the local oscillator frequency is calculated. The apparatus may be of particular use in a radio telemetry system in which it is important that the frequency of the local oscillators in the transmitters and receivers are very accurately matched to each other and where, in order to reduce interference, all the clocks used in the system are locked to a master clock.

Apparatus for and method of synchronising oscillators within a data communication system

The predistortion circuit ( ) predistorts the input signal to a non-linear signal processing device in order to linearise its output. The predistortion circuit ( ) derives the predistortion by sampling the input at ( ) and multiplies it with itself using mixers (multipliers) ( etc.) in order to generate various orders of distortion ( , etc.) which are summed to provide the predistortion. Filter/time delay means ( ) are incorporated in the paths which generate the orders of distortion in order to introduce a phase and/or amplitude variation with frequency into the distortion. The filter/time delay means can be implemented by adaptive filters in digital signal processing circuits (DSPs). The DSPs can sample the output of the signal processing device being linearised in order to obtain feedback for adapting the filter(s).

Signal processing apparatus

A Cartesian loop transmitter for transmitting baseband signals is disclosed. The disclosed Cartesian loop includes a forward path having a first input for receiving the baseband signals and a second input and a feedback path having an input from the forward path. The forward path including an upconverter to upconvert the input signals to a signal to be transmitted, an attenuator having selectable attenuation and a plurality of power amplifier stages. The feedback path including a pin diode attenuator having selectable attenuation and a downconverter to downconvert the input from the forward path. The feedback path provides an error signal indicative of the non-linearity of the forward path to the second input of the forward path.

Cartesian loop transmitter

A full-duplex radio transceiver, in which cancellation of transmission signals in the receive path is achieved by sampling the transmission signals, and controllably adjusting the gain and phase of the sampled signal, before subtracting the adjusted signal from the received signals in order to minimise the component of the transmission signals appearing in the received signals. Processing is done in a DSP at or near baseband.

Full-duplex radio transmitter/receiver

An apparatus is disclosed for processing an input signal. The apparatus includes two feedback loops for generating output signal components from the input signal. Each loop contains an oscillator, which has a frequency or phase which is variable in response to a control signal, and a comparator for generating the control signal. The oscillator generates a loop output signal which forms one of the components of the output signal. The apparatus also includes a combiner for combining the loop output signals to produce the output signal. For each loop, the apparatus also produces a feedback loop operating signal, these signals being dependent on the output signal and in phase quadrature with one another. One input of the comparator in each loop receives the feedback loop operating signal, and the other input of the comparator receives a component of the input signal. The apparatus also continuously ensures that the output signal is stable whatever the phase of the output signal relative to that of the input signal.

Methods and apparatus for modulating, demodulating and amplifying

A system for creating multiple subchannels in a single two-way radio channel and a power control system for a mobile radio transmitter are described. Each subchannel in the system is capable of carrying voice or data. Each subchannel is split into an upper and a lower band with the upper band placed in a particular frequency range above the center of the channel and the lower band is placed in a corresponding frequency range below the center of the channel. Either one or two pilot tones placed at or adjacent to the center of the channel are used to provide a reference for all of the subchannels to allow for correction of distortions when the signal is received. The system includes base station with a transmitter and receiver and a plurality of mobile units each with a transmitter and a receiver. The base station transmits a signal which is the sum of all the subchannels. The mobile unit receives the signal, separates and processes the particular subchannel for that mobile unit. The mobile unit transmits only the desired subchannel or subchannels with a pilot tone located adjacent to one of the bands of the subchannel. Two or more subchannels can be used in conjunction to achieve faster data rates or higher quality voice transmissions. The mobile transmitter power control system for the mobile transmitter allows the mobile unit to monitor the pilot tone from the base station and to calculate a transmission power based on a desired signal strength. Additionally, the transmission power may be altered during transmission as commanded by the base station, or if the mobile unit measures a change in the strength of the pilot tone from the base station.

Method and apparatus for creating multiple subchannels in a single

Method and apparatus for amplifying, modulating and demodulating

Broadband amplifiers which are intended to be linear suffer from distortion and although many techniques have been devised to overcome this problem significant improvements are required especially at frequencies above 100 MHz. An error signal is formed from the output of the amplifier 4 by comparison in a subtractor 11 with its input and the error signal is combined in a coupler 17 with the amplifier output to reduce distortion. The present invention provides automatic amplitude and phase control of the signals forming the error signal by means of components 24 and 25 in a multi-channel input and in the error signal by means of components 13 and 14. These components receive control signals from feedback networks 18' and 20' which each process two of three input signals from: couplers 34, 36 before the subtractor 11, a coupler 22 which passes the error signal and a coupler 21 after the coupler 17. Preferably the two input signals used by a feedback network are used to form in-phase and quadrature signals by a 90.degree. phase shifter 41 and mixers 42 and 43. These signals are each amplified 46, 47 and integrated 48, 49 to provide the control signals. The input signals from the feedback networks may be reduced in frequency and applied to a DSP which then provides the control signals.

Apparatus and method for reducing distortion in amplification

Broadband amplifiers which are intended to be linear suffer from distortion and although many techniques have been devised to overcome this problem significant improvements are required especially at frequencies above 100 MHz. An error signal is formed from the output of an amplifier by comparison in a subtractor with its input and the error signal is combined in a coupler with the amplifier output to reduce distortion. The present invention provides automatic amplitude and phase control of the signals forming the error signal by two groups of adjustment means, one group in a multi-channel input path and one group in the error signal path. These groups receive control signals from respective feedback networks which each process two of three input signals from: couplers before the subtractor, a coupler which passes the error signal and a coupler situated after the coupler which combines the amplifier output signal and the error signal. Preferably the two input signals used by a feedback network are used to form in-phase and quadrature signals using a 90.degree. phase shifter and two mixers. These signals are each amplified and integrated to provide the control signals. The input signals from the feedback networks may be reduced in frequency and applied to a DSP which then provides the control signals.

In deriving a control signal for recombining the parts of the spectrum separated by a frequency notch in transparent tone-in-band signals, cross-correlation products give rise to undesirable frequency and phase jitter. In a receiver which overcomes this problem an input signal E containing the notch is passed to two first mixers and then portions corresponding to the lower and upper portions of the input spectrum are selected by two mirror filters before being translated in frequency by two second mixers to a final output band in which the notch is eliminated. Two selection filters select parts of the spectra at the outputs of the first mixers which correspond to an overlap region of the said lower and upper portions when derived in a transmitter and apply them to a control-signal mixer. A control signal for the second mixers is derived from the output of the control-signal mixer by noise filtering and frequency division by two. The above mentioned problem is overcome by giving the mirror filters transfer functions H.sub.1- (f) and H.sub.1+ (f) which are the same as the transfer functions of mirror filters in the transmitter which produced the notch and by giving the selection filters transfer functions H.sub.2- (f) and H.sub.2+ (f) which satisfy the relationships H.sub.2- (f)=.vertline.H.sub.1- (f).vertline..sup.2 and H.sub.2+ (f)=.vertline.H.sub.1+ (f).vertline..sup.2.

Method and apparatus for transparent tone-in-band transmitter, receiver

In transparent tone-in-band communication systems a notch in the frequency band is usually formed and then the resulting spectrum is translated in frequency to an intermediate frequency range as part of the transmission process. In a transmitter of the present invention a more simple arrangement is used in which the notch is formed directly in an intermediate frequency range by using mirror filters to divide an input signal into two portions and supply respective mixers, each having one output sideband in the intermediate frequency range. The mixers receive different reference frequencies and the mixer outputs are supplied to a summing circuit whose output is passed to a band pass filter to remove the unwanted sidebands. The original frequency spectrum is restored at the receiver by mixer processes of shifting the two selected sidebands similar to, but the inverse of, those used in the transmitter.

Transparent tone-in band transmitters, receivers and systems

In previous transparent tone-in band systems the notch has been removed from the frequency spectrum before a carrier signal carrying data has been demodulated. The present specification describes an arrangement in which data is recovered by first and second mixers from a signal having a spectrum containing a notch. Phase and frequency locking is obtained: either by using a third mixer fed from the inputs to the first and second mixers to provide, indirectly, one signal for a fourth mixer which also receives the carrier signals and provides demodulating signals for the first and second mixers; or by a correction system coupled to the outputs of the first and second mixers for correcting demodulating signals supplied to these mixers.

Data transmission using a transparent tone-in band system

In those transparent tone-in-band systems which employ a single oscillator in the transmitter to generate a frequency notch between upper and lower portions of a band which is to be transmitted, there is not complete freedom to choose the width of the notch. In the present invention which also uses a single transmitter oscillator, one portion of the band transmitted is selected using a filter. A mixer is used to change the frequency of the input band in a way which does not allow the lower sideband to 'fold' back at zero frequency and interfere with the lower transmitted portion. A further filter selects the other portion for transmission from the output of the mixer.

Transmitters and systems for tone-in-band transmission

A communication system which uses a transmitter and a receiver. The transmitter divides a band of interest in the frequency spectrum into upper and lower portions, and frequency translates one of these portions in order to provide a frequency notch between the portions. The receiver of the system includes a receiver processor which receives the upper and lower portions, and which restores the original frequency spectrum. The receiver processor at least partially determines the final position of the restored portion.

When modems are used in SSB systems a problem arises in controlling variable data rates. If a transparent tone-in band system is used, the notch in the band transmitted will be required for a control signal for the SSB receiver. The present invention allows a further possibility for transmitting control signals, for instance one representative of data rate, by varying the width of the notch in the transmitter and determining notch width in the receiver. The notch width is controlled by frequencies applied to two mixers and reference signals for these mixers are derived by respective multipliers from a clock signal. In a receiver the process of restoring the original frequency spectrum also employs two mixers receiving signals from two oscillators. The frequency of one of the oscillators is controlled to ensure that lower and upper portions of the restored spectrum have the correct frequency and phase relationship. The width of the notch is determined by mixing the frequencies of the oscillators in the receiver, using a filter to select one side band, and a multiplier.

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Bath, United Kingdom

Andrew Bateman