GB2065421A - Improvements in or relating to duplex transceivers - Google Patents

Abstract

A rebroadcast transceiver comprises a receiver aerial 1, a first pair of multiplicative mixers 4, 5, an oscillator 6 for providing a local oscillator signal for the said mixers, a first phase quadrature device 8 via which the mixer 5 is fed with the local oscillator signal, the mixer 4 being fed directly therefrom, first and second low pass filters 9, 10, a second pair of multiplicative mixers 11, 12 fed one from each mixer 4, 5 via respective first and second low pass filters 9, 10, a second phase quadrature device 17 via which the mixer 12 is fed from the oscillator 6 the other mixer 11 being fed directly therefrom, a transmitter aerial 2 to which output signals from the mixers 11, 12 are fed, a first delay device 7 via which signals from the oscillator 6 are fed to the mixers 4, 5, a second delay device 14 via which signals from the low pass filter 9 are fed to the mixer 11, a third delay device 16 via which signals from the low pass filter 10 are fed to the mixer 12, a first correlator responsive to signals obtaining across the delay device 14 for applying a feedback signal to the input of the low pass filter 9, a second correlator responsive to signals obtaining across the delay device 16 for applying a feedback signal to the input of the low pass filter 10, a third correlator responsive to signals obtaining between the input of the delay device 14 and the output of the delay device 16 for applying a feedback signal to the input of the low pass filter 9 and a fourth correlator responsive to signals obtaining between the input of the delay device 16 and the output of the second delay device 14 for applying a feedback signal to the input of the low pass filter 10, the feedback signals in aggregate being effective to cancel unwanted feedback between the transmitter aerial and the receiver aerial. <IMAGE>

Description

SPECIFICATION Improvements in or relating to duplex transceivers This invention relates to duplex transceivers.

More especially it relates to transceivers suitable for simultaneous co-herent rebroadcast of all signals within the received band width of the transceiver.

According to the present invention a rebroadcast transceiver comprises a receiver aerial, a first pair of multiplicative mixers fed from the receiver aerial, an oscillator for providing a local oscillator signal for the said mixers, a first phase quadrature device via which one mixers of a first pair is fed with the local oscillator signal from the oscillator, the other mixer of the first pair being fed directly therefrom, first and second low pass filters, a second pair of multiplicative mixers fed one from each mixer of said first pair of multiplicative mixers via respective first and second low pass filters, a second phase quadrature device via which one of the multiplicative mixers of the second pair is fed from the oscillator with a local oscillator signal the other mixer of th'e second pair being fed directly therefrom, a transmitter aerial to which output signals from the said second pair of multiplicative mixers are fed, a first delay device via which signals from the oscillator are fed to the mixers of the first pair, a second delay device via which signals from the said first low pass filter are fed to the said other multiplicative mixer of the said second pair, a third delay device via which signals from the second low pass filter are fed to the said one mixer of the said second pair, first correlator means having two input terminals and one output terminal and responsive to signals obtaining across the said second delay device for applying a feedback signal to the input of the first low pass filter, second correlator means having two input terminals and one output terminal and responsive to signals obtaining across the third delay device for applying a feedback signal to the input of the said second low pass filter, a third correlator means having two input terminals and one output terminal and responsive to signals obtaining between the input of the second delay device and the output of the third delay device for applying a feedback signal to the input of the first low pass filter and a fourth correlator means having two input terminals and one output terminal and responsive to signals obtaining between the input of the third delay device and the output of the second delay device for applying a feedback signal to the input of the second low pass filter, the feedback signals in aggregate being effective to cancel unwanted feedback between the transmitter aerial and the receiver aerial.

The multiplicative mixer of the second pair which is fed via the second phase quadrature device may be that mixer of the second pair which is fed with signals derived from the said one mixer of the said first pair.

Each correlator means may comprise two multiplicative mixers, one of these multiplicative mixers being fed from the two input terminals of the correlator means and being arranged to feed the other of these mixers via a low pass filter and the said other of these mixers being fed also from one of the input terminals thereby to provide an output signal at the output terminal of the correlator constituting a signal which is produced by correlation between the signal fed to the two input terminals.

The multiplicative mixers of the said second pair may be arranged to feed an additive combiner which feeds the transmission aerial via a power amplifier.

The receiving aerial may be arranged to feed the multiplicative mixers of the first pair via a duplexer.

The low pass filters may each be arranged to feed their associated delay device via an amplifier, one amplifier to each low pass filter.

One embodiment of the invention will now be described by way of example with reference to the accompanying drawing which is a generally schematic block diagram of a wide band co-herent rebroadcasting transceiver.

Referring now to the drawing, the transceiver comprises a receiving aerial 1 which is arranged for receiving a signal to be rebroadcast from a transmitter aerial 2. The receiving aerial is coupled via a signal splitter or a duplexer 3 to feed a pair of multiplicative mixers 4 and 5. Received signals are converted in the mixers 4 and 5 to base band or audio frequency signals by means of a local oscillator signal derived from an oscillator 6 which is nominally at the carrier frequency of signals to be rebroadcast. The mixer 4 is fed via a delay device 7 the function of which will later be described and the mixer 5 is fed from the delay device 7 via a quadrature device 8.

Output signals from the mixers 4 and 5 are fed via low pass filters 9 and 10 at base band or audio frequency to a further pair of mixers 11 and 1 2 via an amplifier and a delay device. The filter 9 is arranged to feed the mixer 11 via an amplifier 1 3 and a delay device 14, whereas the filter 10 is coupled in a similar manner to the mixer 1 2 via an amplifier 1 5 and a delay device 1 6. The base band frequencies fed via the delay devices 14 and 1 6 to the mixers 11 and 1 2 respectively are "up converted" in the mixers 11 and 1 2 which have applied to them local oscillator signals from the oscillator 6.The local oscillator signal applied to the mixer 11 is applied thereto directly whilst the local oscillator signal applied to the mixer 1 2 from the oscillator 6 is fed thereto via a 90 phase shift device 1 7. Output signals from the mixers 11 and 1 2 are fed to an additive combiner 1 8 the output from which is fed via a power amplifier 19 to the transmission aerial 2.

It will be appreciated that the signals received by the aerial 1 are converted in the mixers 4 and 5 to base band or audio frequencies in dependence upon the type of signal received by reason of the fact that the frequency of the oscillator 6 is arranged to be nominally the same as the carrier frequency received at the aerial 1. The base band signals are fed via the filters 9 and 10 and up converted to R.F. frequency in the mixers 11 and 1 2 for re-transmission via the transmission aerial 2.

It is clearly necessary to compensate in some way for feedback of signals from the transmission aerial 2 to the receiver aerial 1 and for this purpose four correlators 20, 21, 22 and 23 are provided which are constructed in a similar manner. The correlators 20, 21, 22 and 23 operate to correlate the unwanted frequencies sensed after filtering by the filters 9 and 10 with the unwanted frequencies present at the output of the mixers 4 and 5 in such a manner as to effect cancellation. The correlator 20 senses the signal across the delay device 1 4 and provides a feedback signal which is fed to the input of the low pass filter 9.In a similar manner the correlator 23 senses the signals developed across the delay line 1 6 and provides a correlation feedback signal which is fed to the input of the low pass filter 10. in order to cancel quadrature components the correlators 21 and 22 are cross coupled between the delay devices 14 and 1 6 as shown, to provide feedback signals for the filters 9 and 10 respectively. It will be appreciated that the delay devices 14 and 1 6 are necessary in order to distinguish between the wanted signals and the feedback signal so that the correlators operate to cancel the unwanted feedback signal only. The delay device 7 is necessary in order to provide compensation for the signal delay between the transmission aerial 2 and the receiving aerial 1.

Since the correlators 20, 21, 22 and 23 are similar, for simplicity one correlator 20 only has been shown in detail and the construction of this correlator will now be described. The correlator comprises two multiplicative mixers 24 and 25 and has two input terminals which are coupled to respective input ports of the mixer 24 one of the input ports of the mixer 24 being coupled to one input port of the mixer 25. The output port of the mixer 24 is coupled to the other input port of the mixer 25 via a low pass filter 26 and the output port of the mixer 25 is coupled to the low pass filter 9 to provide a cancelling feedback signal and it constitutes the output terminal of the correlator.

It will be appreciated by those skilled in the art that the form of the correlators may be changed and that other known correlators may be used to produce a similar result. The system just before described will operate satisfactorily as a multi-channel radio rebroadcast equipment. All the received signals within the band width of the equipment are amplified and rebroadcast on the same frequency as which they are received. Both the phase and amplitude characteristics of the signals are maintained. For example the system is capable of the simultaneous co-herent rebroadcast of AM, FM, SSB and Digital type signals on adjacent channels.

It will be apparent that the band width of the receiver is determined by the oscillator frequency plus and minus the band width of the low pass filters. In operation of the transceiver the received signals are converted to low frequencies ranging from DC to half the total band width of the system by the first pair of mixers 4 and 5, and as hereinbefore described the two low frequency signals which are phase quadrature related are filtered amplified and fed to the second pair of mixers 11 and 1 2. It can be shown that the combined output of the second pair of mixers 11 and 12, is an amplified replica of the frequency spectrum received at the input of the transceiver.If the total gain of the transceiver were not more than the isolation between the aerials 1 and 2, a cancellation arrangement utilising the correlators 20, 21, 22 and 23 would not be required, but in practice a system wherein the gain is less than the aerial isolation is not practicable and so the correlators are in practice essential. The correlator circuits 20, 21, 22 and 23 attenuate by a conventional cancellation technique all signals (i.e. unwanted signals) appearing at the output of the first pair of mixers 4 and 5 which correlate with signals present at the input of the second mixers 11 and 1 2. As just before explained the delay devices 14 and 1 6 present correlation of the wanted signals and the wanted received signals are not therefore affected by operation of the correlators.

If the system is connected as shown in the diagram, the received spectrum of signals are amplified and re-transmitted on the same frequency. If however, the 90" phase shift circuit 8 is connected to mixer 4 instead of mixer 5, the re-transmitted spectrum would be reversed in frequency to that of the received spectrum.

For example, a signal received X Khz below the local oscillator frequency generated at 6 would be re-transmitted X Khz above that frequency.

The same condition would result if phase shift circuit 8 were connected to mixer 11 instead of mixer 1 2.

Claims (7)

1. A rebroadcast transceiver comprising a receiver aerial, a first pair of multiplicative mixers fed from the receiver aerial, an oscilla tor for providing a local oscillator signal for the said mixers, a first phase quadrature device via which one mixer of the first pair is fed with the local oscillator signal from the oscillator, the other mixer of the first pair being fed directly therefrom, first and second low pass filters, a second pair of multiplicative mixers fed one from each mixer of said first pair of multiplicative mixers via respective first and second low pass filters, a second phase quadrature device via which one of the multiplicative mixers of the second pair is fed from the oscillator with the local oscillator signal, the other mixer of the second pair being fed directly therefrom, a transmitter aerial to which output signals from the said second pair of multiplicative mixers are fed, a first delay device via which signals from the oscillator are fed to the mixers of the first pair, a second delay device via which signals from the said first low pass filter are fed to the said other multiplicative mixer of the second pair, a third delay device via which signals from the second low pass filter are fed to the said one mixer of the second pair, first correlator means having two input terminals and one output terminal and responsive to signals obtaining across the said second delay device for applying a feed-back signal to the input of the first low pass filter, second correlator means having two input terminals and one output terminal and responsive to signals obtaining across the third delay device for applying a feed-back signal to the input of the said second low pass filter, a third correlator means having two input terminals and one output terminal and responsive to signals obtaining between the input of the second delay device and the output of the third delay device for applying a feed-back signal to the input of the first low pass filter and a fourth correlator means having two input terminals and one output terminal and responsive to signals obtaining between the input of the third delay device and the output of the second delay device for applying a feed-back signal to the input of the second low pass filter, the feed-back signals in aggregate being effective to cancel unwanted feed-back between the transmitter aerial and the receiver aerial.

2. A rebroadcast transceiver as claimed in claim 1 wherein the multiplicative mixer of the second pair which is fed via the second phase quadrature device is that mixer of the second pair which is fed with signals derived from the said one mixer of the said first pair.

3. A rebroadcast receiver as claimed in claim 1 or claim 2 wherein each correlator means comprises two multiplicative mixers one of these multiplicative mixers being fed from the two input terminals of the correlator means and being arranged to feed the other of these mixers via a low pass filter and the other of these mixers being fed also from one of the input terminals thereby to provide an output signal at the output terminal of the correlator constituting a signal which is produced by correlation between the signal fed to the two input terminals.

4. A receiver as claimed in any preceding claim wherein the multiplicative mixers of the said second pair are arranged to feed an additive combiner which feeds the transmission aerial via a power amplifier.

5. A transceiver as claimed in any of the preceding claim wherein the receiving aerial is arranged to feed the multiplicative mixers of the first pair via a duplexer.

6. A transceiver as claimed in any preceding claim wherein the low pass filters are arranged to feed their associated delay device via an amplifier one amplifier to each low pass filter.

7. A transceiver substantially as hereinbefore described with reference to the accompanying drawings.