Patent GB-1437964-A (GB)
Title: Asynchronous pulse receiver
IPC: G01S7/28
Filed: 1973-06-28
Abstract: 1437964 Automatic gain control RCA CORPORATION 28 June 1973 [7 July 1972] 30804/73 Heading H3T In a receiver for asynchronous pulses comprising bursts of R.F. energy, the gain is controlled by the ambient noise level. The arrangement is more particularly intended for use in the Secant aircraft anti-collision system. The receiver comprises R.F. and mixer stages 102, Fig. 1, a broad-band limiting I.F. amplifier 104 followed by temperature compensation circuit 108 and signal channel detectors 110. The ambient noise level is detected by a detector circuit 106 operating in a frequency range in which the signal pulses are absent and applied to an A.G.C. circuit 112 controlling the gain of the I.F. amplifier 104, the gain of which is independently controlled by a circuit 114. The I.F. amplifier 104 comprises an input filter 300, Fig. 3, defining its bandwidth followed by terminating resistor 310 and three cascaded integrated circuit wideband amplifiers 302, 304, 306 of balanced type; the coupling capacitors 320, 322 and 330, 332 in conjunction with resistors 324, 334 respectively provide an additional H.F. cutoff. The noise A.G.C. voltage provides a gain control current for amplifier 302 by means of resistor 314 while the independent gain control voltage applies a control current to amplifier 306 by means of resistor 336. The noise A.G.C. circuit 112, Fig. 1, comprises an amplifier 402, Fig. 4, with feedback over resistors 410, 408 and capacitor 406 which minimize the amplification of low frequencies feeding amplifier 412 forming with capacitor 418 an integrator circuit, the output from which is amplified by amplifier 420 to constitute the noise A.G.C. voltage. Potentiometer 426 provides balance adjustment. The noise A.G.C. circuit 112 also includes a comparator 404 one input of which comprises the noise input on line 400 and the other a comparison potential derived from potentiometer 430. The output from comparator 404 forms the base bias for transistor 432 which when conductive clamps the output from amplifier 402 to the potential supplied from source 428. In the arrangement as so far described, when pulses of large amplitude are received, the amplifier 104 limits, thereby reducing the level of the noise in its output the output from amplifier 402 would thereby be reduced, increasing the gain of the receiver. In these circumstances, the comparator circuit 404 produces an output, and the clamp circuit comes into operation, clamping the A.G.C: potential to the value which corresponds to the average normal noise level. In a modification, the noise channel detector 106, Fig. 1, is omitted, the noise A.G.C. circuit 112 being coupled to derive noise and signal from one of the signal channel detectors 110. In this case, on the receipt of a pulse, a large input will arrive on the line 400: the comparator 404 is then arranged to operate so as to clamp the output from amplifier 402 at a level corresponding to the mean noise level. Temperature compensation circuit 108, Fig. 1 comprises an attenuator or amplifier whose transmission is determined by a temperaturesensitive element so as to compensate from the change in level of the noise generated in the input circuits of the receiver with temperature. In a modification the temperature compensation circuit also precedes the noise channel detector (Fig. 2, not shown).