A novel microstrip balun bandpass filter (BPF) is designed by using open loop resonators having interdigital capacitors. The interdigital capacitors are employed to control the center frequency easily. Opposite phase difference between the balanced outputs can be provided according to the suitable coupling topologies based on parallel and anti-parallel coupled lines. By this way, minimized magnitude imbalances between the balanced ports can also be obtained. In order to obtain two poles inside the passband, two identical resonators are coupled to each other. The designed balun BPF was fabricated and measured to validate the proposed methodology. Phase and magnitude imbalances inside the passband were measured within 180±5˚ and 0.5 dB, respectively. The simulated and measured results are in good agreement.
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