Reactive filters and their application to wave-form analysis

The main features of a number of different methods of waveform analysis are enumerated and briefly discussed and an account is given of the principles governing the design of an analyser incorporating reactive filters. The reactive filter consists of some conventional form of selective circuit in which the dissipation factor has been artificially reduced by the introduction of negative resistance or regeneration. General theory applicable to such filters is given. The reactive filter principle has been employed in the construction of apparatus covering the frequency range from 35~ to 2000 kc./sec. One of the models described has a range from 4 to 400 kc./sec.; another from 200~ to 80 kc./sec. The sensitivity and selectivity are such that harmonics 90 db below the fundamental can be measured to an accuracy of about 30%, or plusmn2 db, over most of the frequency range, although at 100~ the accuracy is reduced. A schematic diagram shows the application of the reactive filter method to the measurement of harmonics in an amplifier. With modifications, the circuit can be made suitable for other wave-form measurements.