GB1201617A - Improvements in or relating to code recognition systems - Google Patents

Abstract

1,201,617. Selective signalling. PLESSEY CO. Ltd. 30 Oct., 1967 [3 Nov., 1966; 7 Jan., 1967], Nos.49416/66 and 1061/67. Heading G4H. [Also in Division H4] A code recognition system comprises desensitizing means which passes a first pulse and suppresses for a pre-determined period all subsequent pulses whose amplitudes are not within pre-determined limits from that of the first pulse. A counter means associated with a stable frequency source is triggered by the said first pulse, and is interrogated, reset and restarted as a second pulse is passed on by the desensitizing means. Mode recognition means gives an output if the two pulses represent the particular code. As described, receiving equip. ment comprises a store 4, of analogue or digital type, which gives an output which is approximately equal in amplitude to that of the largest previous input pulse, a comparator 5 which gives an output when an input received from line equals or is greater than an input received from the store 4, a counter 8 supplied with timing pulses, and a mode recognition means 10. There is also a delay circuit 6 connected from the output of the -comparator 5 to the store 4, for cancellation of the contents thereof. An initial pulse 1 starts the counter 8. A succeeding interference pulse 2 of smaller amplitude has no effect, but a pulse 3 of equal or greater amplitude causes an output from the comparator 5. This is connected by a line 9 (line 11 not then being provided) to the mode recognition means 10, which then interrogates the counter 8 and gives an output if the time period stored corresponds to the pre-determined time period. The counter 8 is re-set to zero and operation recommences. In an alternative arrangement, an amplitude comparator circuit is provided within the comparator 5 the output of which is connected by a line 11 (line 9 not then being provided) to the mode interrogation means 10. The counter 8 is not interrogated unless the two inputs to the comparator 5 are within Π3dB of each other. An aircraft control transponder for I.F.F. may include a code recognition system of the kind described.