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The evolution of the discrete crystal single-sideband selection filter in the bell system

The Bell System in its long-distance frequency division multiplex systems has traditionally used crystal single sideband selection filters in the basic 12-channel modulation process. This paper reviews the changes this type of channel filter has undergone over its 40 year history. Special emphasis is given to the physical and electrical design and the characteristics of the most recent version of this filter. The original channel filter of 1938 was an image parameter lattice design using inductors, capacitors, resistors, and four split-electrode crystal plates in open pressure mounts. The filter was large, expensive, and heavy. In contrast, the newest discrete resonator design, while using the same basic circuit and element types, has had the benefit of modern synthesis and analysis computer programs and a complete physical redesign. This has resulted in a smaller, lower cost, improved performance channel filter with a volume reduction of an order of magnitude over its most recent predecessor and two orders of magnitude over the original Bell System crystal channel filter.