Broadband electronically-tuned microwave filters

It has been suggested that single crystals of yttrium iron garnet (YIG) can be used in the construction of low-loss microwave filters which can be tuned by means of an applied dc magnetic field. This paper will discuss practical broadband circuits in coaxial line and waveguide for both single-tuned and multiple-tuned band-pass and band-reject filters using such YIG crystals. One version of a band-pass filter has operated with electronic tuning from 2000 mc to 6500 mc. The insertion loss was 3 db at 2000 mc, 1 db at 4000 mc, and 0.7 db at 6500 mc, with bandwidths ranging from 18 mc to 40 mc. A packaged version of this filter will be described which utilizes a combination of permanent magnets and solenoids for tuning. The complete filter weighs less than 2-1/2 lb. and can be tuned mc from 3000 mc with a power consumption of 5 watts.