GB1445734A - Data transmission system pilot controlled devices - Google Patents

Abstract

1445734 Automatic gain control SIEMENS AG 29 June 1973 [18 July 1972] 31135/73 Heading H4R The level of a transmitted pilot wave is monitored at a receiver which is disconnected if pilot amplitude falls outside determined limits, by a controller whose output voltage level is compared with an auxiliary signal derived from controller output setting. In this way, criterion for shut off is not dependent wholly on output level but is also controlled by input level of controller circuit. The arrangement is stated to avoid a situation in which a slow drop in pilot signal level will delay an alarm until signal levels are too low, i.e. such that a fault will have been in existence for some time. Input signals at Pe pass through amplifier LV to a unit G which may be a hybrid coupler, with terminal Pa coupled to a utilization device. The pilot frequency is led to an amplifier/ rectifier PE coupled to one input of a differential amplifier DV1 also receiving a reference voltage U. Output of PE is also led to further differential amplifiers DV2-DV4. The output of DV1 is led to a capacitive store RS maintained at a voltage immediately prior to opening of a switch by a MOS-FET transistor; the output from this is fed to a linear converter U output from which is supplied to a control element, e.g. temperature dependent resistor in amplifier LV giving a non-linear characteristic such that input level = output level-instantaneous amplification. Output lead SA connects to a display or indicating instrument and or alarm. Amplifier DV2 also receives a voltage Us and if rectifier PE voltage is greater than this the switch in RS is closed but if rectified voltage becomes lower, the switch is opened and hence disconnects the control action; an alarm may also be energized. Alternatively the alarm may also be operated when the rectified pilot voltage is outside limits determined by reference voltages defining Œ4dB levels by coupling amplifiers DV3-DV4 through OR gate OG. In an alternative arrangement, Fig. 2 (not shown), the pilot frequency is filtered by hybrid coupler G with a transistor (T2) supplying the non-linear resistor and a transistor (T3) supplying a - 6dB bias. The non-linearity of the control resistor in LV may be compensated by a diode D biased by a voltage divider comprising resistors R7, R8 which ensure that voltages S is substantially proportion to Us. A differential amplifier DV2 is used as in Fig. 1.