Patent GB-1060137-A (GB)
Title: Multiple receiver selector
IPC: H04B7/08
Filed: 1964-06-26
Abstract: 1,060,137. Radio receiving systems; transistor gating circuits. STANDARD TELEPHONES & CABLES Ltd. June 26, 1964 [June 28, 1963 (2)], No. 26499/64. Headings H3T and H4L. In a diversity receiving system for use with a mobile transmitter, each receiver is connected to signal comparing means for switching the receiver providing the best signal to a common output circuit, and means are provided for preventing such switching until a predetermined difference in signal levels from the respective receivers is reached. As shown, remote receivers RRE1 to RREn are-arranged to be selectively connected to pass the received audio-frequency signal via a line a and gated amplifier Q4a, Q4n to a central exchange CE and also provide on line b a negative potential which varies in accordance with the level of the received signal. Normally with no input signal, this negative potential is one volt and maintains transistors Q1a, Q1n conducting so that their collectors assume near ground potential, these collectors being connected via lead AC to the base of a transistor Q201 so that it is normally non-conducting. Failure of a remote receiver will remove the negative potential on the corresponding line b and the corresponding transistor Q1a &c. will be cut-off and transistor Q201 will conduct and operate an alarm via relay R1. Each receiver is associated with a gating circuit, that for receiver RRE1 comprising transistor Q2a whose base is connected to the line b and which is normally non-conductive due to the 2 volts bias provided by resistor 203 which is common to the emitter circuits of all the transistors Q2. Under these conditions the complementary transistor Q3a is cut-off and the potential on line b is such that the amplifier transistor Q4a is cut-off. Assuming that receiver RRE1 is now providing the best signal and that a negative potential of 6 volts appears on line b which is greater than that appearing on any of the other lines b, the transistor Q2a will conduct and the remaining transistors Q2 will be cut-off due to the potential appearing across the resistor 203. Thus the transistor Q3a will conduct and increase the negative potential on the base of amplifier transistor Q4a so that the signal on line a is amplified and supplied to the exchange CE. A portion of the voltage on the collector of Q3a is fed back to the base of Q2a via resistor 105a to speed up the initial switching operation and also biases transistor Q2a further into conduction to prevent rapid switching over between receivers providing approximately equal signals. The signal on the collector of Q3a is also supplied via a diode CR204a to the base of a transistor Q202 common to all the gating circuits which conducts and alters the potential at the junction of resistors 204, 205. This bias potential is adjusted by potentiometer 206 to a predetermined threshold release level for the operated gating transistor Q2a and thus determines the minimum signal level for continued conduction of the transistor. In order to prevent these gating operations from taking place during the digit signals used for calling a relay K1 connected to the exchange via line FR is operated at the start of the digit signal and remains operated for the dialling period, contacts K1a to K1n shunting the feedback resistors 105a &c. by low-valued resistors 120a &c. to prevent the switching of receivers during this period.