Synthesis of Narrow-Band Direct-Coupled Filters

This paper describes a general synthesis procedure for the design of narrow-band, direct-coupled filters, based on an approximate first-order equivalence between direct and quarter-wave coupled filters. Thus a quarter-wave coupled filter, whose bandwidth is a few per cent wider than required, serves as a prototype. The approximations underlying the synthesis procedure for quarter-wave coupled filters, given by Lawson and Fano and applied by Mumford to the "maximally fiat" case, are re-examined and justified. The transmission characteristics of a six-cavity filter of total Q of 37.6 is computed exactly with excellent agreement with the prototype performance. Experimental confirmation is given.