Patent GB-840884-A (GB)
Title: Improvements in or relating to measuring circuit arrangements
IPC: G01R19/18
Filed: 1956-05-01
Abstract: 840,884. Electric correspondence control. PHILIPS ELECTRICAL INDUSTRIES Ltd. May 1, 1956 [May 4, 1955], No. 13382/56. Class 40 (1). [Also in Groups XXXV and XXXVI] A measuring device in which an alternating voltage is modulated in accordance with a factor to be measured, the modulated voltage being amplified and then demodulated to obtain a voltage which is substantially proportional to the factor, is characterized by the alternating voltage having a marked asymmetrical waveform consisting of successive periods of short duration and high voltage and periods of longer duration and lower voltage; an amplifier 3 which amplifies the fundamental wave and at least some of the lower order harmonics without phase variation and a demodulator 32 comprising two parallel connected branches each comprising a capacitor and a rectifier in series 24, 26; 25, 27, the conduction direction of the two rectifiers being opposite, the arrangement being such that one capacitor acquires a voltage which is substantially proportional to the amplitude of the voltage during the high voltage periods and the other capacitor a voltage which is substantially proportional to the amplitude of the voltage during the low-voltage periods. As applied to an automatically balanced bridge, Fig. 9, for measuring a physical variable such as temperature or mechanical stress the bridge comprises a fixed resistor 8, a resistor 9 which for temperature measurement has a high temperature coefficient or which for measuring mechanical stress is a resistance strain gauge and a potentiometer 10, the slider of which is driven by a motor 12. The bridge is supplied with the asymmetrical voltage by a generator (not shown) connected to the terminals 5 and the output of the bridge is applied to the demodulator 32 through the amplifier 3. The output of the demodulator is applied to the motor 12 which moves the slider 11 to balance the bridge, the position of the slider indicating the value of the physical variable. The output of the demodulator which is taken from the junction of two resistors 28, 29 and the junction of two condensers 26, 27 is positive for positivegoing pulses and negative for negative-going pulses. This output is applied either directly to the motor 12 or through a polarized relay indicated at 30 to drive the motor in the appropriate direction for balancing the bridge. As applied to measuring a D.C. voltage, Fig. 10, the voltage is applied to the terminals 1 and is converted to an alternating voltage in a modulator 2, the input to the modulator at the terminals 5 being the asymmetrical pulse waveform as described above. The output of the modulator is therefore asymmetrical A.C., the amplitude of which varies in accordance with the voltage applied to the terminals 1 and which has positive-going pulses for a positive input and negative-going pulses for a negative input voltage. The modulated A.C. voltage is applied to the demodulator 32 through the amplifier 3 and the polarity of the demodulated output voltage which is indicated by a meter corresponds to that of the input voltage and its amplitude is substantially proportional to that of the input voltage. The output voltage may be applied through a resistor R across a series resistor r which is connected in series with the input of the modulator 2 to provide negative feed-back. The modulator 2 can be a normal balanced ring modulator or a rectifier bridge, one diagonal of which is supplied with the alternating voltage and the other of which is connected in series with one of the input connections, or it may be as shown in Fig. 6, in which four rectifiers 14-17 are used and resistors 31 and 32 make the rectifiers operate on an advantageous part of their characteristics.