GB1118525A - Improvements relating to the transmission of radio frequency coded pulses - Google Patents

Abstract

1,118,525. Radio signalling. SECURITON Ltd. July 4, 1966, No. 29982/66. Heading H4L. In an alarm system, a transmitter generates a R.F. signal modulated by a subcarrier which is in turn pulse coded and the receiver includes a tuned R.F. stage selecting the transmitted signal, a detector, a filter recovering the pulse coded subcarrier signal and means for decoding the received pulses to operate an alarm. Transmitter, Fig. 4. A multivibrator VT3, VT4 is controlled by a bi-stable device VT1, VT2, via transistors VT10, VT11 so that the pulse periods (i.e. each half-cycle) over two cycles of the multivibrator are of short duration (representing 0) or long duration (representing 1) the coded sequence being determined by the connection of points 1, 2 to two of the points A, B, C or D. Transistor VT3 of the multivibrator controls a further multivibrator VT5, VT6 via transistor VT5A, this multivibrator operating at the subcarrier frequency of 2À1 kc/s. A diode D3 provides compensation for temperature and supply voltage changes. The output of multivibrator VT5, VT6 is supplied via emitter-follower VT7 to a modulating transistor VT8 which controls R.F. oscillator VT12 so that pulses of energy are supplied to a radiating loop in accordance with the code. The transmitter is small enough to fit into a pocket and may be disguised as a wrist-watch. Receiver, Fig. 5. After detection in a conventional superheterodyne receiver the 2À1 kc/s subcarrier is supplied via transistor Q1 and filter L, C2 to a normally cut-off peak detector Q3 so that the signal supplied to trigger pair Q3, Q4 follows the code. By this means, if the input signal exceeds a predetermined amplitude the reconstituted code and its inverse are supplied via transistors Q4 and Q5 respectively to logic circuits detecting the four digits of the code, it being necessary to detect several sequential correct codes before the alarm is operated. Bi-stables B1, B2 are arranged to detect alternate half-cycles and monostable delay devices D1, D2 distinguish between long and short pulses. A third bi-stable B3 is controlled by B2 and registers the state of B2 during the period preceding its last change of state during a cycle. The coding sequence to which the receiver responds is determined by which of the outputs of B1, B2, B3 are connected with the output of Q4 or Q5 to a gate Q17. Gate Q17 triggers a monostable delay device D3 and after about ten cycles of operation of D3 a capacitor C9 is charged sufficiently to trigger the circuit Q8, Q9 and operate the alarm relay via transistor Q10.