Optimization of a Marchand balun isolation circuit

A folded planar Marchand balun with a parametrized isolation circuit is presented. The circuit is proposed for use in efficient, low-cost, high-volume test applications. Many radio designs use radio-frequency integrated circuits (RFICs) with differential inputs or outputs to reduce the effects of common-mode noise or interference. Most RF testing instrumentation, however, only provide single-ended inputs and outputs; thus requiring a balun on the evaluation or test board. The Marchand balun has been a popular circuit since its introduction because it has larger bandwidth than other types of combiners and because it can simultaneously be used as an impedance transformer. The Marchand balun, however, suffers from poor impedance matching and isolation at the output ports. To improve the performance of the Marchand balun, a parametrized compact isolation circuit was designed. The balun shows greatly improved impedance match at the output ports and increased isolation between them, with only a slight increase in circuit complexity.