GB2295748A

GB2295748A - Spillover prevention in signal repeaters

Info

Publication number: GB2295748A
Application number: GB9423829A
Authority: GB
Inventors: Christopher Keith Richardson
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1994-11-25
Filing date: 1994-11-25
Publication date: 1996-06-05
Anticipated expiration: 2014-11-25
Also published as: GB2295748B; GB9423829D0

Abstract

A signal repeater system comprises a receiver fed via signal combiner means with respective received signals from two directional receiver antennas to provide a resultant signal which is amplified by the transmitter and re-radiated from a transmitter antenna. The transmitter antenna is positioned and arranged with respect to the receiver antennas so that signals transmitted from the transmitter antenna and received by the receiver antennas are substantially nullified in the signal combiner means. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO SIGNAL REPEATERS This invention relates to signal repeater systems and more especially although not exclusively it relates to signal repeater systems for digital audio broadcasting (DAB) signals, as used in a DAB system of the kind currently proposed by the British Broadcasting Corporation (BBC).

In order to provide effective DAB signal coverage throughout the country, a plurality of DAB signals transmitters is required suitably geographically spaced so that each transmitter covers a pre-determined region of the country. It will however be readily appreciated that unless an uneconomically large number of DAB signal transmitters are used, there will be comparatively small regions of the country wherein the DAB signal strength will be unacceptably small, due to local geographical features for example which obstruct the original. In order to provide a suitable DAB signal in the small regions cost effectively, one solution is to provide repeater signal transmitter systems which will simply serve to amplify the frequency spectrum within the DAB band, and rebroadcast it on the same frequency.There is no necessity to demodulate or remodulate the signal and thus a detailed specification of the input signal is not required in order to design the repeater.

One of the problems associated with this kind of repeater however, is that instability can occur due to contemporaneous reception of a large repeated locally transmitted signal along with a small signal to be repeated, especially since both are at the same frequency. Additional problems may also result from multi-path effects which can be particularly troublesome in wide-band systems.

It is therefore an object of the present invention to provide cost effectively, a wide-band on-channel repeater system wherein the problems outlined briefly above are largely obviated.

According to the present invention a signal repeater system comprises a receiver and a transmitter, the receiver being fed via signal combiner means with respective received signals from two directional receiver antennas thereby to provide a resultant signal which is amplified by the transmitter and re-radiated from a transmitter antenna, the transmitter antenna being positioned and arranged with respect to the receiver antennas so that signals transmitted from the transmitter antenna and received by the receiver antennas are substantially nullified in the signal combiner means.

It will be appreciated that the system is such that a wanted signal to be repeated produces unequal signals at the two receiver antennas whereas an unwanted locally radiated signal produces balanced signals, and thus the unwanted locally radiated signal only is cancelled or nullified.

The system may include phase and amplitude control means effective to control the relative phase and amplitude of the signals from the two receiver antennas so that the signals transmitted from the transmitter antenna and received by the receiver antennas are substantially nullified.

The phase and amplitude control means may be controlled by an auto-correlator arrangement which serves automatically to control the said relative phase and amplitude so that the signals transmitted from the transmitter antenna and received by the receiver antennas are substantially nullified in the presence of changing signal propagation conditions.

The auto-correlator may comprise a sub-audio frequency oscillator which provides modulation signals which are used to modulate the said relative phase and amplitude, a signal sample of the signals transmitted from the transmitter antenna containing the modulation being fed back to the phase and amplitude control means whereby the said relative phase and amplitude are controlled appropriately so that the signals transmitted from the transmitter antenna and received by the receiver antennas are substantially nullified.

The auto-correlator may comprise a pair of mixers each fed with the signal sample via a low pass filter which passes frequencies below the sub-audio frequency and each fed also from the sub-audio frequency oscillator thereby to provide phase and amplitude feedback signals which are combined with the modulation signals to provide phase and amplitude control signals, which control signals are applied to the phase and amplitude control means.

The mixers may be fed from the sub-audio frequency oscillator in relative phase quadrature from phase quadrature means.

The modulation signals may be fed via the phase quadrature means to signal combiner means fed also from the mixers via very low pass filter means thereby to provide at output terminals of the combiner means the control signals.

The signal combiner means may comprise a 90 degree hybrid coupler to which signals from respective receiver antennas are fed.

The system may be arranged to afford a frequency band of operation suitable for DAB signals.

A plurality of repeater sub-systems may be provided each arranged to cover a different frequency band, the sub-systems being coupled in parallel between the receiving antennas and the transmitting antenna, the frequency bands being chosen so that in combination they cover a DAB frequency band of interest.

One embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a generally schematic diagram showing the aerial arrangement of a signal repeater system; Figure 2 is a somewhat schematic block circuit diagram showing in more detail the system of Figure 1; and, Figure 3 is a somewhat schematic block circuit diagram of a multi channel system suitable for covering a DAB frequency band.

Referring now to Figure 1 an on-channel repeater system comprises a pair of receiving antennas 1 and 2 which are arranged to feed received signals to a signal repeater 3 wherein they are processed and amplified by an amplifier 4, which forms a part of the repeater, and then transmitted from a transmitting antenna 5.

The receiving antennas 1 and 2 are positionally arranged with respect to the transmitting antenna 5 so that signals transmitted by the transmitting antenna 5 are received in a balanced manner by the receiving antennas 1 and 2 whereby each antenna receives substantially the same signal. Thus the signals received by the antennas 1 and 2 on lines 6 and 7 which originate from the transmitting antenna 5 are fed to the signal repeater 3 wherein they are arranged to mutually cancel or nullify. It is arranged that one of the receiving antennas, 1 or 2, is directed towards a source of wanted signal to be repeated, and this wanted signal is thus received in an unbalanced manner between the receiving antennas 1 and 2.Since this wanted signal appears on the lines 6, 7, as two unbalanced signals, it does not cancel in the signal repeater 3 and is accordingly amplified by the amplifier 4 and transmitted by the transmitting antenna 5 as the repeated signal.

Referring now to Figure 2, wherein parts corresponding to Figure 1 bear the same numerical designations, the signal repeater 3 comprises a pair of input amplifiers 8 and 9 fed from the receiving antennas 1 and 2 respectively. The amplifiers 8 and 9 are arranged to feed the input ports 10 and 11 respectively of a 90 degree hybrid coupler 12. The hybrid coupler 12 has an isolation port 13 which is conventionally connected to earth via an isolation resistive component 14. The hybrid coupler 12 is arranged to feed the transmitting aerial 5 via the serial combination of band pass filters 15 and 16, the amplifier 4, and an additional amplifier 17 which is connected between the band pass filters 16 and 17.A sample of the signals transmitted from the transmitting aerial 5 is fed via a rectifying diode 18, a low pass filter 19 and an amplifier 20 to input ports 21 and 22 of mixers 23 and 24 respectively. Further input ports 25 and 26 of the mixers 23 and 24 respectively are fed with a 17 Hz signal from a low frequency oscillator 27 via a phase quadrature device 28. The phase quadrature device 28 is arranged also to feed a pair of signal combiners 29 and 30 via impedance matching resistors 30 and 31. The signal combiners 29 and 30 are fed also with output signals from the mixers 23 and 24 respectively via very low pass filters 32 and 33 respectively. The signal combiner 29 is arranged to feed a control signal to a phase controller 34 and the signal combiner 30 is arranged to feed a signal to the amplifier 9 to control its gain.The phase controller 34 and the gain controlled amplifier 9 therefore in combination comprise phase and amplitude control means. The phase controller 34 may simply comprise a controllable variable impedance, the impedance of which is varied to produce an appropriate phase shift.

In operation of the system, a sample of the signal transmitted by the transmitting aerial 5, which sample contains modulation produced by the low frequency oscillator 27, is applied to the mixers 21 and 22 to provide feedback signals on lines 35 and 36 to signal combiners 29 and 30 respectively, which feedback signals are combined with samples of the low frequency oscillator signal, derived via resistors 30 and 31, thereby provide on lines 37 and 38 control signals which are effective to control the phase and amplitude respectively of received signals fed to the 90 degree hybrid coupler 12, so that pick up by the aerials 1 and 2, of signals from the transmitting aerial 5 tend to be nullified.

It will be appreciated that by utilising an auto-correlator as described hereinbefore with reference to Figure 2, and by using careful positioning of the receiving antennas with respect of the transmitting antennas, the fabrication of a relatively inexpensive and particularly efficient signal repeater system is facilitated.

In order to provide for operation of the signal repeater in the DAB system, the components such as amplifiers and filters in a signal path between the receiving aerials 1 and 2 and the transmitting aerial 5, are arranged to facilitate amplification of a frequency of 226.25 MHz with a 1.5 MHz band width. In this system a sub-audio frequency of 17 Hz is chosen since it does not conflict with DAB signal requirements. Additionally, in order to define a suitable time constant for the system, the filters 32 and 33 are arranged to have a pass band of about 1 Hz or less.

Although a repeater system as hereinbefore described will be satisfactory for many applications, in order to cater for situations where multi-path signals in different bands can be a problem, an arrangement may be utilised as shown in Figure 3.

Referring now to Figure 3, wherein parts corresponding to Figure 2 bear the same numerical designations, an on-channel repeater system comprises a plurality of signal repeater sub-systems only two of which 39 and 40 are shown wherein each repeater subsystem is arranged to cover a different frequency band the repeater sub-systems being coupled in parallel, as shown, so that in combination they cover a DAB frequency band of interest. Each of the signal repeater sub-systems, which are fed from amplifiers 41 and 42, comprise a cancellor 43, an amplifier 44, a band pass filter 45, a correlator 46, and a detector 47. The signal repeater sub-systems are substantially identical and correspond substantially with the repeater as therein before described with reference to Figure 2, the repeater sub-systems being distinguished from each other only by the band of operation.

Output signals from the repeaters are fed on lines 48 and 49 to a signal combiner 50 which feeds the transmitting aerial 5 via a delay device 51 and a power amplifier 52.

Various modifications may be made to the arrangements just before described without departing from the scope of the invention and for example although the repeater described is very suitable for use as a DAB signal repeater, such systems are eminently suitable for other applications , and especially applications which are required to operate over a wide band.

Claims

1. A signal repeater system comprising a receiver and a transmitter, the receiver being fed via signal combiner means with respective received signals from two directional receiver antennas thereby to provide a resultant signal which is amplified by the transmitter and re-radiated from a transmitter antenna, the transmitter antenna being positioned and arranged with respect to the receiver antennas so that signals transmitted from the transmitter antenna and received by the receiver antennas are substantially nullified in the signal combiner means.

2. A signal repeater system as claimed in Claim 1, including phase and amplitude control means effective to control the relative phase and amplitude of the signals from the two receiver antennas so that the signal transmitted from the transmitter antenna and received by the receiver antennas are substantially nullified.

3. A system as claimed in Claim 2 wherein the phase and amplitude control means is controlled by an auto-correlator arrangement which serves automatically to control the said relative phase and amplitude so that the signals transmitted from the transmitter antenna and received by the receiver antennas are substantially nullified in the presence of changing signal propagation conditions.

4. A system as claimed in Claim 3 wherein the auto-correlator comprises a sub-audio frequency oscillator which provides modulation signals which are used to modulate the said relative phase and amplitude, a signal sample of the signals transmitted from the transmitter antenna containing the modulation being fed back to the phase and amplitude control means whereby the said relative phase and amplitude are controlled appropriately so that the signals transmitted from the transmitter antenna and received by the receiving antennas are substantially nullified.

5. A system as claimed in Claim 4 wherein the auto-correlator comprises a pair of mixers each fed with the signal sample via a low pass filter which passes frequencies below the sub-audio frequency and each fed also from the sub-audio frequency oscillator thereby to provide phase and amplitude feedback signals which are combined with the modulation signals to provide phase and amplitude control signals, which control signals are applied to the phase and amplitude control means.

6. A system as claimed in Claim 5 wherein the mixers are fed from the sub-audio frequency oscillator in relative phase quadrature from phase quadrature means.

7. A system as claimed in Claim 6 wherein the modulation signals are fed via the phase quadrature means to signal combiner means fed also from the mixers via very low pass filter means, thereby to provide at output terminals of the signal combiner means the control signals.

8. A system as claimed in any preceding Claim wherein the signal combiner means comprises a 90 degree hybrid coupler to which signals from respective receiver antennas are fed.

9 A system as claimed in any preceding Claim arranged to afford a frequency band of operation suitable for DAB signals.

10. A system as claimed in any preceding Claim comprising a plurality of repeater sub-systems each arranged to cover a different frequency band, the sub-systems being coupled in parallel between the receiving antennas and the transmitting antenna, the frequency bands being chosen so that in combination they cover a DAB frequency band.

11. A signal repeater system as claimed in Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.