Miniaturized thick-film RC-active filters for PCM application

This paper presents thick-film RC-active filters developed for use in pulse-code modulation (PCM) systems. Filters for PCM telephone transmission systems are required to be small in size and low cost in production, and they must have good performance. The filters presented have a twelve-lead single in-line package configuration. Dimensions are 5-mm thick, 10-mm high, and 30-mm wide. They meet the D3-Channel Bank requirements. The transmitting filter is a seventh-order bandpass filter (BPF). The receiving filter is a fifth-order low-pass filter (LPF) with an input RC stage which acts as a lossy sample and hold circuit. To achieve miniaturization, thick-film resistors are printed on one side of alumina substrate. On the other side, one chip of quad op-amps and monolithic ceramic-chip capacitors are assembled. Considering economical production, these filters are designed with the following features: 1) only one type of equal-valued capacitors is used; 2) to get good yields, a multiple-feedback configuration having low sensitivity to circuit elements is adopted; 3) a fast deterministic trimming method is used with computer-controlled laser-trim equipment.