PRECISE PHASE-SENSITIVE DETECTOR WITH SWITCHED TWO-TERMINAL RC NETWORK
Andrzej Met, Krzysztof Musio, Tadeusz SKUBIS · 2009
Abstract- Application of a switched two-terminal RC networks for the construction of phase-sensitive detectors (PSDs) which are used in self-balancing transformer bridges is described. The switched two-terminal RC network makes possible effective reduction of noise level, increasing the CMRR, removing the input DC offset and providing the galvanic separation of the input and output circuits. Investigation of properties of the precise phase-sensitive detector used in a prototype self-balancing bridge for maintenance of inductance standard shows that the noise attenuation is bigger than 100 dB. Nowadays, the precise phase-sensitive detector described in the paper successfully operates in the automatic impedance bridge KWL 1 designed for high accurate inductance measurement at the Physikalisch-Technische Bundesanstalt in Braunschweig (PTB, Germany) [4]. circuit. A phase-sensitive detector, in turn, needs preamplification for unbalance signal level over ten milivolts. For lower signals accuracy of these detectors decrease rapidly. It is significant that noise and interference are amplified during the signal amplification. In case of high noise level to signal level ratio the amplifiers become saturated and then we can observe distortion of the original shape of the unbalance signal (Fig.1).