An Active-Filter Device for Extreme Selective Measurements at Low Frequencies

This paper describes the systematic development of an active filter destined for the extreme selective balancing of a low-frequency ac bridge. The power voltage with the frequency of 50 Hz and its higher harmonics were to be suppressed, the measurement voltage with the frequency of 190 Hz was to be amplified. The filter device makes use of the twin-T RC network in connection with operational amplifiers in a manifold way. A theoretical consideration of the stability of an elementary filter circuit is given. The signal (190 Hz) to noise (50 Hz) ratio of the whole filter device is greater than 160 dB.