'Shadow' filters - a new family of electronically tunable filters

Abstract

It is shown that the 'shadow' filter, introduced recently by Lakys and Fabre, is capable of achieving not only a constant bandwidth band-pass filter, but also several other interesting characteristics depending upon the choice of the desired output port and the port from which feedback signal is derived. Two such characteristics are illustrated in this paper, namely constant centre frequency, variable Q, and constant value of the product of centre frequency and Q. A problem is also posed for further investigation. Finally, a suggestion is made for the design of a simple but interesting experiment for undergraduate students, using only resistors, capacitors and op amps, which illustrates the basic concepts of filters and what the shadow technique can do to improve their quality.