Patent GB-821794-A (GB)
Title: Improvements in or relating to arrangements operable on pilot current failure in telecommunication transmission systems
Inventor: NORMAN PETER
IPC: H04B3/10
Filed: 1957-12-13
Abstract: 821,794. Automatic gain control. STANDARD TELEPHONES & CABLES Ltd. Dec. 13, 1957, No. 38842/57. Class 40 (4). [Also in Group XL (c)] In a system comprising a chain of several repeater stations, each comprising an amplifier whose gain is controlled by the amplitude of a received pilot current, in the event of failure by any amplifier to receive such a pilot current the outgoing line is disconnected from the output of the amplifier and connected instead to a local source of pilot current. By this means it is ensured that the gain of subsequent repeaters does not rise undulyso that they are ready for immediate operation when the incoming pilot signal is restored. Reconnection of the outgoing line is delayed, however, until the amplifier gain (which will have risen to its maximum) is restored to normal. The locally generated pilot may be given a small lowfrequency modulation to indicate the occurrence of the fault to the receiving terminal of the system. As shown, the incoming line 2 feeds an amplifier 1 from whose output the pilot frequency is extracted by the filter 4 and rectified at 5 to control the gain of the amplifier I which operates in such a manner as to keep the level of the output pilot frequency substantially constant. The rectified output also controls relays X and Y. Relay X closes its upper contacts if the level falls 6 db. below normal and its lower contacts if the level rises 4 db. above normal. Relay Y closes its contacts y1 if the level falls to more than 20 db. below normal (or to that level below which working of the system is impossible). Closure of y1 causes a control circuit 6 to operate C after a slight delay to allow for momentary interruptions. Relay C enables a local oscillator 7 and connects it to the outgoing line 3 which it also disconnects from the output of the amplifier 1. The latter meanwhile will have assumed its maximum gain in the absence of the incoming pilot signal. When the latter is restored it will take about one minute for the amplifier to reduce its gain to normal, but during this period relay X makes its lower contact which inhibits the release of C by the control circuit 6. Fig. 3 illustrates the control circuit in detail. Closure of contact y1 on abnormal fall in pilot level operates A which supplies current to a bridge comprising thermistors 11, 12 and resistors 13, 14, the unbalance current of which indirectly heats the thermistor 9. When the latter is sufficiently hot its resistance becomes low enough to allow relay B to operate. This cuts off the feed to the bridge, but the thermistor 9 is now heated by the increased relay current. Relay B operates relay C for the purpose described above. The delay in operation of relay B is controlled by resistance 14 and thermistors 11, 12 are made responsive to the ambient temperature and so dimensioned as to render the delay independent thereof. When the incoming pilot current is restored, relay X makes its high level contact (+4), to lock C despite the release of A and B. Relay C only releases when relay X assumes its mid-position consequent upon the amplifier gain becoming normal. Circuits are described for suitable local oscillators (see Group XL (c)) producing two pilot frequencies (in one case one pilot frequency is a harmonic of the other) and for modulating one of the pilot frequency with 50 c.p.s. A detector for the 50 c.p.s. modulation and suitable for use at the receive terminal is also described.