Series- and Shunt-Tuned Lumped-Element Microwave Circulators

Abstract

Practical lumped-element microwave circulators are generally shunt tuned. S-parameter eigenvalue measurements on the Hewlett-Packard automatic network analyzer are used to demonstrate, however, that series tuning adds considerably to design flexibility. This is especially true in the high-field mode of operation and in conjunction with a capacitor common to all three arms. The measured eigenvalue curves also provide a more basic understanding of the two tuning mechanisms. The power of the approach becomes obvious in cases where the device dimensions are restricted. The flexibility of this design approach has been demonstrated with the development of three lumped-element circulators having identical size housings and similar circuit patterns. Circulators at 1.4 GHz and 1.0 GHz utilized shunt tuning and operated in the low-field mode. The other unit operated at 500 MHz with series tuning and the high-field mode.