360° Tunable Balanced Filtering Phase Shifter With Reflectionless and Wide Stopband Characteristics

A tunable balanced filtering phase shifter with reflectionless and wide stopband characteristics is presented in this paper. The proposed circuit consists of a six‐port balanced‐to‐unbalanced (BTU) quadrature coupler, two identical varactor‐tuned reflective loads, and two absorptive filtering networks. The BTU coupler provides quadrature excitation to the reflective loads while suppressing common‐mode (CM) noises. The absorptive filtering network is composed of the resistor‐terminated coupled lines and stub‐loaded resonators, which enable broadband absorption and introduce two transmission zeros near the passband for sharp selectivity. In addition, the BTU coupler yields odd‐harmonic transmission nulls through a round‐trip recombination process, which works together with the even‐harmonic rejection of the absorptive networks to extend the stopband range. A 2.45‐GHz microstrip prototype on an F4B substrate is fabricated and measured. The prototype exhibits a 360° phase shift range, simultaneous differential‐mode and CM reflectionless operation with 55.92% and 46.94% reflectionless bandwidths, 42 dB in‐band CM suppression, and stopband rejection better than 25 dB up to 20 GHz.