A Switchable Active Filter Hybrid

A unique active bandpass filter is described, which is capable of selecting any of 12 different frequencies as defined by a digital input. This device utilizes both thick- and thin-film technologies to their best advantages. Filtering is performed by a state variable active filter circuit designed around a custom linear integrated circuit (lC) and precisely matched components. Frequency selection is accomplished by a custom MOS device. Functional laser trimming of both thinand thick-film resistors is employed to adjust circuit characteristics to extremely tight tolerances. The finished device must be packaged to maintain a high level of frequency stability over a wide range of environments. Two different packaging techniques are presently being successfully employed. The first is a standard chip and wire technique and the second relies on devices which are prepackaged in small ceramic carriers. This circuit, whose size, weight, and cost were previously impractical in discrete form, is presently being produced in production volumes.