Folded planar Marchand balun with improved isolation for radio frequency Automated Test Equipment applications

This paper describes a folded planar Marchand balun with an isolation circuit. The circuit is proposed for use in efficient, low-cost, high-volume Automated Test Equipment (ATE) solutions. 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 ATEs, however, only provide single-ended inputs and outputs; consequently a balun is necessary in the device interface board (DIB). 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 be simultaneously be used as an impedance transformer. Unfortunately, the Marchand balun suffers from poor isolation and impedance matching at the output port. To improve the performance of the Marchand balun, a compact isolation circuit was designed. The balun shows improved impedance match at the output port with a slight reduction in bandwidth.