Analysis of four-port circulators using non-reciprocal phase shifters and directional couplers

Abstract

An analysis has been made of the factors affecting the performance of 4-port circulators based on non-reciprocal phase-shifter and directional-coupler networks. The effects on the adjacent-arm isolation of phase-shift error, coupling error, dissimilarities between the couplers, and non-reciprocal loss in the phase shifters are examined. The effects on the alternate-arm isolations and input matches of the isolations and matches of the couplers, and the matches of the phase shifters are then discussed. The power-handling capacity is not considered.The results of the analysis lead to the choice of a network using similar E-plane multi-branched waveguide couplers, two similar non-reciprocal phase shifters of ½π differential phase shift, and one reciprocal phase shifter of ½π phase lag relative to the unloaded line.Components meeting the limits derived by the analysis were developed, and used in the construction of two circulators in waveguide 14 working in the band 5.925–6.425 Gc/s. One of these maintained over an 8% bandwidth isolations greater than 30 dB, voltage standing-wave ratios greater than or equal to 0.95 and insertion losses less than or equal to 0.3 dB. The second maintained over a 4% bandwidth isolations greater than 33 dB, and similar v.s.w.r.s and insertion losses.The development work on the phase shifters and couplers is described, and the results obtained are given. The performance of a circulator is predicted by the theory from measurements on the components, and compared with measurements on the circulator.Finally, the factors limiting the performance of the present circulators are discussed, and suggestions are made for further improvement.