Miniature circulators

Recent work on moderate-bandwidth and broadband miniature circulators is reviewed. The lumped-element design approach is the basis of a discussion of the size-bandwidth tradeoff. It is pointed out that a tradeoff situation exists with respect to size and insertion loss because ohmic losses in the coupling structure become dominant as the size of the circulators is reduced. The problems of achieving circulator performance over very broad frequency bands in circulators of moderate size are discussed for coplanar and microstrip circulators. The lumped-element approach is primarily applicable to circulators operating at relatively low frequencies and for relatively small bandwidths (up to 50%, for instance). The distributed-field approach has broader validity since it is not restricted by any assumption about the size of the ferrite disk. The application of this approach to broadband miniature circulators operating over the 6- to 18-GHz band is discussed