Compact design of planar continuously tunable crossover with two-section coupled lines

This paper presents, for the first time, the design and implementation of a planar tunable crossover. This crossover is derived from the conventional two-section branch-line crossover by substituting its three vertical transmission lines with the two fixed capacitors and one tunable capacitor to realize the tunable passband. By designing its horizontal transmission lines as coupled lines and replacing its vertical transmission lines by three capacitors, the circuit size is greatly reduced. Closed-form equations for design parameters are derived to simplify design procedure of the proposed crossover. For verification, a crossover with tunable frequency of 1.2-2.4 GHz is designed based on the given design closed-form equations. Finally, a microstrip tunable crossover with one varactor diode 1sv277 is fabricated and measured. The measured results are in good agreement with the simulated ones. The results show that the passband can be tuned in a frequency range from 1.29 to 2.06 GHz, yielding 46% tuning range with better than 20-dB return loss and isolation, while the insertion loss varies from 0.59 to 1.22 dB. The crossover occupies a small size of 0.25λg × 0.04λg, which is only 8% of the area of conventional two-section branch-line crossover.