Synthesis Design of Modified Wideband Balun Bandpass Filter With Compact Structure

A compact wideband balun bandpass filter (BPF) is presented. By adding a short-circuited microstrip stub at the unbalanced port, a wideband balun initially designed with three-pole quasi-Chebyshev response is modified to exhibit a four-pole fully-Chebyshev response. With its two-port equivalent circuit network, this proposed balun BPF is theoretically designed. To experimentally verify the design approach, a wideband balun BPF prototype centered at 2 GHz is simulated, fabricated and tested. An in-band amplitude imbalance smaller than 0.16 dB and phase imbalance less than 0.58 degrees between the two output ports are measured.