A 100-KC QUARTZ CRYSTAL COMB REJECTION FILTER
NAVAL RESEARCH LAB WASHINGTON DC
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A comb filter that provides a high attenuation at each of 63 notch frequencies was designed for a center frequency of 100 kc. Each tooth, or notch, is produced by a quartz crystal operating at series resonance in a bridge circuit, and the shape of the characteristic is typical of a null response. This characteristic is greatly improved by stagger tuning additional stages at each notch. The development model consists of three banks of 63 crystals each and provides 63 notches, each with a bandwidth of 1 c at -60 db and 12 c at -3 db. There is no insertion loss. Great flexibility in the bandwidth and the shape of the attenuation characteristic is made possible by the use of quartz crystals. The effective Q may be either raised or lowered appreciably from the values used in the present model. The skirt factor may be improved by increasing Q and adding more stagger-tuned stages. The temperature stability is good and permits the retention of original tuning and attenuation characteristics. Although the present model has 3 banks of 63 notches each, 2 additional banks are planned for the near future.
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