A Wideband Continuously-Tunable Quasi-Reflectionless Filtering Attenuator

This letter presents a continuously-tunable quasi-reflectionless filtering attenuator (QRFA). The continuous attenuation control is achieved by adjusting the bias voltage of the p-i-n diodes. By using two $\lambda $ /4 transmission lines instead of traditional grounding vias, the conflict between dc power supply and RF signal absorption is resolved, thereby realizing both quasi-reflectionless characteristic and continuously-tunable attenuation. Through a theoretical analysis based on the odd- and even-mode method, a prototype is designed and fabricated to operate at 2 GHz, which obtains a broad fractional bandwidth of 80%. Measurement results show that a wide continuously-tunable attenuation range of 3.2–23.8 dB is achieved while maintaining quasi-reflectionless characteristics from 0 to 4 GHz.