Patent GB-888533-A (GB)
Title: Improvements in selective call systems
Inventor: SNEATH OSWALD BARBER
IPC: H04B1/16
Filed: 1958-02-28
Abstract: 888,533. Inductive signalling. MULTITONE ELECTRIC CO. Ltd. Feb. 27, 1959 [Feb. 28, 1958], No. 6536/58. Addition to 855,347. Class 40(5). A selective receiver for consecutive, electromagnetic, inductive signals of different frequencies, includes a magnetic pick-up coil, an amplifier terminating in a limiter, two or more selective filters, one filter passing a signal of a given frequency to open a gate, whereby the other filter becomes operative to pass the signal to which it is tuned. In the embodiments described a receiver as described in the parent Specification has a limiter stage Tl. In the collector circuit of T1, Fig. 1, are included two tuned circuits C1, L1 and C2, L2. Signals in the secondary of L1 are rectified by diode X1, and bias Xl less negative with respect to the emitter of T2. L2 supplies an alternating voltage to X2, but no current flows when X2 is biased negative. When a signal from L1 is followed by a signal from L2 pulses pass through X2 and T2 producing a signal in transducer Q, which continues until operation of switch S. The oscillator T2, L4 will not restart until excited by pulses through rectifier X2 because there is not negative bias on the base of T2. The maximum operating interval between two pulses is determined by the time constant of C4 and R2, R3, R4. When a suitable low thermal capacity high negative temperature coefficient resistor R5 is connected across switch S, and oscillation begins the resistance of R5 will fall until the oscillation is damped out. 3 gate circuit. In the three gate circuit, Fig. 2, the first pulse passing at L1 and rectified by X1 reduces the negative charge with respect to the positive supply on C5. The second pulse, passing L2 causes X2 to reduce the charge on C4 below that of C5 and the third pulse causes current to pass X3 triggering transistor T2. In a modification of this three pulse receiver the first pulse is applied to a transistor T3, Fig. 3, and on conclusion of the pulse T3 is cut off. The second pulse changes the charge on C4 so that the junction of C4 and X2 becomes positive with respect to the junction of R2 and R3. The third pulse then starts the oscillator as before. When the call is to be continued only for the duration of the third oscillation, the first pulse cuts off T3, Fig. 4, but pulse 2 is rectified by X2 biasing its junction with C6 negative, this negative charge remaining only so long as T3 is cut off, thereby rendering T4 operative. The signal from L3, amplified by T4 makes T5 oscillate. Due to the damping provided by resistance R19 oscillation only continues so long as a signal from L3 continues. In Fig. 5 the signal from T4 is further amplified in T6 part of the output being fed through C9 and rectifiers X4, X5 to keep the gates open. When the last calling tone serves as a carrier for speech, the output from T6, Fig. 5, maintains the gates open, and is also rectified by X6, and after passing a filter is amplified by T8. When the call is continued only for the duration of the second of two signals the coil L1, Fig. 7, applies bias through X6 to C4 powering T3. No current passes under no signal conditions, and the condenser is charged for a time determined by R6. T4 forms a relaxation oscillator rendered active by a signal applied thereto. When this ends, damping by R7, C5 stops oscillation. When the frequency to which L2 is tuned is received, whilst a charge remains on C4, T4 oscillates, and by feeding back a signal through C5, which is rectified by T3, C4 remains charged. If C4 is discharged T4 will not oscillate until both pulses are again received. An additional transistor before its oscillator stage and a crystal transducer are used in a further embodiment, Fig. 8, where a very loud calling signal is required. Where two pulses are followed by a carrier frequency common to all receivers the-first and second pulses are applied to gates including tuned circuits L1, L2, Fig. 9, the charge appearing on C9 providing the supply voltage for transistor T8 which amplifies the H.F. carrier. The carrier is amplified and rectified in T9. So long as a carrier signal reaches T9, rectifiers X11, X12 maintain a charge on C9.