Null networks are widely used in audio and instrumentation systems not only for eliminating undesired frequencies and for measuring transient harmonic distortion (THD) but also as central components of selective filters and oscillators in feedback arrangements. Of the various three-terminal notch filters and four-terminal selective bridge circuits in use, the RC twin-tee is perhaps the best known and most widely utilized. However, as certain relations have to be maintained between the various elements (minimum six) of these networks, they are not readily tunable and independent control of selectivity is also rather difficult. Thus, the twin-tee structure is essentially suited to fixed-frequency operation. The purpose of this study is to explore canonic second-order RC structures with four elements that are easily tunable and whose selectivity can also be varied independently and conveniently. A practical realization of the feedback and feedfoward arrangements proposed is given.
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