IE44371B1 - Improvements in or relating to radio-relay systems - Google Patents

Description

The invention relates to radio-relay systems having go and return channels and a circuit provided for the input-coupling and output-coupling of additional signals on the link equipment, in particular at least two intermediate stations, where wider modulation bands are normally transmitted in frequency modulation form, for example and wherein additional signals are added in the receiver mixers of both directions by means of.the modulation of the local oscillator signals of the respec tive receivers.

In radio-relay systems it is a common practice to transmit, in addition to actual information signals of normal type, frequency-modulated additional signals such as service conversations and control signals.

Thus, for example, in a radio-relay system for 1,800 speech channels, a sub-base band range from 0.3 to approximately 54 KHz is reserved for 13 additional service channels each having a band width of approximately 3.1 KHz, and is used for any required additional transmission of the type described above.

At intermediate stations at which demodulation is not normally performed the receiving oscillator in each channel is designed to be modulatable in order to be able to feed in such service signals. The output coupling of the additional signals is effected at a second intermediate frequency output of the receiver 4 3 71 in each channel, which is followed by a junction circuit to provide a common output at the local station for any such additional signals. Provision must be made for the transmission of service signals in both transmission directions and for reception of such signals in both directions. This requirement leads to a configuration such as that shown schematically as a block circuit diagram in Figure 1, in which a receiver El receives radio frequency signals from one channel direction and any additional signals ZS, and by subsequent mixing converts them into an intermediate frequency level ZF, which is supplied to a transmitter Si that contains a further mixer to raise the frequency to that required for emission via an antenna RF. The other direction of the link operates in a similar manner using a receiver E2 and a transmitter S2. As a result of the feed-in process, the signals ZS that are injected are unavoidably coupled out again in their own station, as well as being transmitted,as each receiver has a secondary i.f. output connected to a common junction unit G, the receiver El being connected to an input 1 and the receiver E2 being connected to an input 2 of this junction unit G. The locally injected signals 2S are applied to the local oscillators of the receiver El and E2. At an output 3 of the junction unit G there thus appears both the locally injected additional signal and any additional signal received via the link, and the local signal can be a serious source of disturbance, which may even result in acoustic feed-back producing cross-talk and/or oscillations, so rendering the system unusable for the additional signals. 437 1 - 4 One object of the present invention is to provide a system that substantially avoids these disadvantages.

The invention consists in a radio relay system having go and return channels, in which system at least two stations of a link are adapted to convey frequency modulation signals and additional signals that are fed into the link at said stations via respective receiver mixers of both channel directions by modulation of the local oscillator signals of the respective receivers, the receivers of both channel directions at said stations separating out any thus inserted additional signals whether inserted at the local station or elsewhere on the link, and passing them to respective channel inputs of an electronic hybrid circuit in which separate transistor stages feed a common load to give a phaseopposing cancellation of like-frequency and like-phase components that arise from the insertion of said additional signals fed in at that station, and permitting undistorted reception of any additional signal inserted elsewhere on the link.

Advantageously, said electronic hybrid circuit comprises a common base transistor stage to whose input is fed any additional signal from one channel and a common emitter transistor stage to whose input is fed any additional signal from the other channel, the collectors 4 3 71 factors of said stages at equal values.

Advantageously, the common power amplifier stage is connected in a common emitter configuration using a transistor of opposite conductivity type, from whose collector negative feedback paths lead to the emitters of the two preceding transistors, each path forming part of a network that includes adjustable means by which the respective amplification of each said preceding transistor stage can be set.

In this way it can advantageously be ensured that signals arriving simultaneously via both channels are added in phase-opposing fashion in the hybrid circuit and extinguished. Advantageously the electronic hybrid circuit is formed by a circuit arrangement of the type described below, by which the two local inputs for the service signals are decoupled from one another.

The invention will now be described with reference to the drawings, in which:Figure 1 is a block schematic circuit diagram of a known arrangement for the insertion and extraction of additional signals at an intermediate station of a link; and Figure 2 is a circuit diagram of one exemplary embodiment of an electronic hybrid circuit that may be used in place of the junction G of Figure 1 in an exemplary embodiment of the invention.

In the known arrangement shown in Figure 1, the two signal outputs 1 and 2 for the additional signals are fed to the junction unit G which forms an output signal available at the terminal 3.

As the construction of a conventional hybrid form of junction circuit using coils can be technically difficult and may be impractical afc high frequency ranges, in an exemplary embodiment of the invention the junction G is replaced by an electronic circuit as illustrated in detail in Figure 2.

Figure 2 shows a transistor circuit in which one additional signal is fed via input 1 to a transistor stage Tsl connected as a common base stage. The second additional signal is fed via the input 2 to a second transistor stage Ts2 connected as a common emitter stage, the two transistors being of the same conductivity type, and having their collectors directly connected to one another, and to.a common output impedance K1S which is in parallel with the input impedance of a subsequent power transistor stage Ts3 provided for amplification and other functions. The transistor Ts3 is of the opposite conductivity type in this embodiment, and is connected as a common emitter stage which feeds both signals to an output 3 via the series combination of a capacitor Cll and a resistor S23. Its emitter is connected to earth via a capacitor Cl5 and to a supply terminal via a resistor R21. The inputs of the two circuits are different to one another on account of their different characteristics, and in order to match the input impedances the input of the stage Tsl contains a coupling capacitor C3 followed by a series resistor R5 which serves to increase the input impedance of this common base stage, whilst a coupling capacitor C9 and a resistor R14 arranged in a parallel arm form the input of the common emitter stage with the transistor Ts2, in order to reduce the latter’s relatively high input impedance so that the two input impedances have the same value Z. . The transistor Tsl has an emitter - 7 resistor R6, whilst its base is connected to the junction of two resistors R7 and R8, and to a decoupling capacitor C6.

The input of the transistor Ts2 includes a pair of resistors R15 and R16 to provide a base bias, with a blocking capacitor CIO.

As a result of the use of a common base stage for one signal and a common emitter stage for the other, if equal amplification is provided in both paths, then it is ensured that any equal-phase, equal-level components presented at the two inputs 1 and 2 are eliminated at the output 3.

In order to increase the linearity and constancy of the system, negative feedback is provided from the common power stage Ts3 to the input stages. One component of the output energy of the transistor Ts3 is fed-back via a variable resistor R9 from the collector of the transistor Ts3 to the emitter of the transistor Tsl. A further component of this output energy is fed back via a voltage divider R20, C14, R18, to serve as negative feedback to the emitter of the transistor Ts2.z in parallel to the latter1s emitter resistor R17. The resistor R18 is variable, so that it is possible to adjust the negative feed-back in the two arms to enable equal amplification to be set up in the two paths.

Phase adjustment, which may become necessary at higher frequencies, can be effected by the provision of variable reactive elements in the appropriate arms.

In this way a very good decoupling of the type required can be achieved.

Claims (5)

1. CLAIMS : 1. A radio relay system haying go and return channels', in xvhich system at least two stations of a link are adapted to convey frequency modulation signals and additional signals that are fed into the link at said stations via respective receiver mixers of both channel directions by modulation of the local oscillator signals of the respective receivers, the receivers of both channel directions at said station separating out any thus inserted additional signals, whether inserted at the local station or elsewhere on the link and passing them to respective channel inputs of an electronic hybrid circuit In which separate transistor stages feed a common load to give a phase-opposing cancellation of likefrequency and like-phase components that arise from the insertion of said additional signals fed in at that station and permitting undistorted reception of any additional signal inserted elsewhere on the link.

2. A system as claimed in Claim 1, in which said electronic hybrid circuit comprises a common base transistor stage to whose input is fed any additional signal from one channel and a common emitter stage to whose input is fed any additional signal from the other channel, the collectors of the respective transistors in said stages being connected to one another and to a common load impedance, impedance matching means being provided to set the input impedances of both stages to equal values by the connection of parallel and series resistors, and gain control means being provided to set the respective amplification factors of said stages at equal values. 4 4 3 71

3. A system as claimed in Claim 2, in which the input of a common power amplifier stage is connected to said common load impedance.

4. A system as claimed in Claim 3, in which said common power amplifier stage is a common emitter stage, and the collector of said power amplifier stage is connected via respective negative feed-hack paths to the emitters of the two preceding transistor stages, said feed-back paths each including said gain control means by which the amplification of the respective stage can be adjusted,

5. A radio relay system including at least two stations substantially as described with reference to Figure 1 and Figure 2.