D4 Digital Channel Bank Family: Thin-film dual active filter for pulse code modulation systems

This article describes the design of a new thin-film dual active filter for pulse code modulation (PCM) systems. A synergistic combination of circuit, silicon, and thin-film innovations has reduced the size, power, and cost of the filter and has expanded its applications. In the latest design, both the band-limiting and reconstruction filters are fabricated on a single ceramic hybrid integrated circuit in a way which significantly reduces their size, power, and cost, along with improving their performance. The filters are fabricated using laser-patterned and laser-trimmed thin-film resistors, a custom-designed complementary bipolar quint operational amplifier (op-amp), and reduced-capacitance circuit design with an innovative interconnection technique. The techniques used to minimize crosstalk between the filter sections are also described and performance data is presented. The evolution of thin-film filters for Bell System PCM systems is also briefly reviewed.