Series- and Shunt-Tuned Lumped-Element Microwave Circulators

Commercially available lumped-element circulators (LECS) are almost exclusively shunt tuned. This is probably a result of the inherently smaller junction diameter for shunt tuning. The introduction of a capacitor common to all three arms1 sheds a different light on the approaches. In the process of designing three LECS (500 MHz, 1,000 MHz, 1,400 MHz) using identical housings, these schemes were re-evaluated, together with the high- and low-field mode of operation. S-parameter measurements on the Hewlett-Packard automatic network analyzer showed some very interesting aspects of series tuning, which add a large degree of flexibility to the design.