Patent GB-879996-A (GB)
Title: Improvements in or relating to selective calling systems
IPC: H04W88/02
Filed: 1959-06-08
Abstract: 879,996. Radio signalling; transistor circuits. ALBISWERK ZURICH A.G. June 8, 1959 [June 6, 1958], No. 19522/59 Classes 40(5) and 40 (6)  A selective calling system includes means for transmitting a starting pulse followed by a group of selector pulses in which the spacing between the selector pulses is equal to the width of the selector pulses or is a multiple of said width. A selective receiver comprises an H.F. amplifier 1, Fig. 1, having an aerial 2 receiving a pulse modulated 30 kc./s. carrier. Pulses are applied from H.F. stage 1 to a delay stage 3 and through a demodulator 8 to "AND" circuits 6, 7. Pulses are applied to a second delay stage 4 and a tone generator and acoustic transducer 5 only when the output pulse of each delay stage coincides with an applied one of said selector pulses. The H.F. stage is directly connected to a current source. The remaining stages are powered through a switch 9 which is closed by the initial pulse after rectification thereof by a rectifier 10. The switch 9 is maintained closed by the succeeding pulses, and while the generator 5 is operating a control voltage maintains switch 9 closed, until the carrier ceases. The first and second pulse delay circuits 3, 4 are formed by two monostable transistor multivibrators in which resistors and capacitors determine the delay times and hence the responsiveness of the receiver to its appropriate call sign. In the circuit of a receiver, a transistor Tl, Fig. 2, forms part of a known tuned H.F. stage of low Q operating in class B with no signal conditions, and in class A on receipt of a signal. The amplified output signal is applied through a capacitor C8 and a diode D2 to a first delay stage. Each delay stage comprises a monostable multivibrator including respectively transistors T3, T4 and T5, T6. T4, T6 are normally conducting. Resistors R9 and R16 and capacitors C11, C14 determine the delay times of the circuits. Positive base bias to T3 and T5 is applied to prevent accidental operation of the flip flops. The delay times are also dependent on the leakage current of T4, T6, which is itself temperature dependent. To attain temperature compensation the base voltage applied through R9, R16 is itself made temperature dependent, by tapping off a voltage from a potentiometer R21 in the collector circuit of T7, whose characteristics are such that the time delays of the two circuits become independent of temperature. The tone generator comprises transistors T8, T9 in a monostable multivibrator circuit. Transistor T9, in whose emitter and collector circuits are inductors L6, L5 is a feedback and its oscillator normally cut off. The base bias is derived through R26 by rectifications and voltage doubling in diode D3, D4. The output of each delay flip-flop circuit is applied to the next through an AND circuit comprising respectively C12, D5 and C15, D6. The diodes are biased to cut off and conduct only when the output pulse and a selector pulse are simultaneously applied thereto. The selector pulses are interruptions in the HF signal, from which the bias is derived by rectification in circuit GR via resistors R12, R19. The electronic switch 9, Fig. 1 comprises a normally cut off transistor T2, the base voltage for which is obtained by rectification of the H.F. signal by diode Dl, smoothed by capacitor C4 to maintain the transistor conducting between spaces. At the end of the control cycle the switch is kept closed by rectifying the a.f. voltage derived from the emitter of T8 via a voltage doubler circuit D7, D8, and C16, C4.