Bbc Brown Boveri Ag

Urs Ammann

Georg Mastner

Martin Maschek

Thomas Dobler

Measurement value signals (S1), for example from a signal transmitter which is located at high-voltage potential, which are transmitted by a signal transmitter (1) with a transmit diode (6) on the high voltage side via an optical waveguide (8) to a signal receiver (9) with a photodiode (10) at ground potential, are subject to an optical signal attenuation along the way. To ensure automatic attenuation compensation both for alternating and for direct voltage measurement signals (S1), a reference direct current (I.sub.REF) is fed back to the transmit diode (6). The transmission system is calibrated before each measurement. During this process, the input of an amplifier (4) of the signal transmitter (1) is connected to reference potential (U0) via a switch (3) and the gain factor of the signal receiver (9) is controlled in such a manner that its output signal (S9) is equal to a predeterminable reference voltage (U.sub.REF). The output signal (S14) of a differential amplifier (14) is supplied via a lowpass filter (16) and a 1st post amplifier (17) to an instantaneous-value memory (18), the output signal (S18) of which controls the gain factor. After the calibrated state has been reached, the closed-loop control circuit is interrupted so that the gain factor remains constant until a new calibration is performed. The gain factor can be adjusted via electrically, electromechanically or optically controllable resistances or a multiplier or via the bias voltage of an avalanche photodiode (10).

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Wettingen, Switzerland

Urs Ammann