A Design Approach for Asymmetric Coupled Line Fixed Attenuators and Their Application in Microwave Power Detecting Circuits

This article innovatively proposes a design approach for asymmetric coupled line fixed attenuators with arbitrary attenuation factors (AFs) and arbitrary terminal real impedances for the first time. The asymmetric coupled line attenuator (ACLA) reduces the series resistors in traditional T-type and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula>-type attenuation circuits and incorporates two bypass resistors, offering better power-handling capability and a natural dc and ripple blocking function. By using the decoupled branch-line model of asymmetric coupled lines, the relationships between terminal real impedances, image impedances, resistors, and the AF are established to simultaneously satisfy perfect port impedance match and attenuation requirements. For ease of design, the frequency responses of ACLAs under different parameter combinations are presented. As a commonly used security circuit, the power detecting circuit (PDC) is combined with the ACLA to form an ACLA-based PDC. The theoretical analysis and design methodology of the ACLA-based PDC, aimed at reducing the influence of the externally connected ACLA on port impedance match and power transmission performance of the main transmission path, are introduced in detail. For verification, three ACLAs with AFs of 7, 10, and 50 dB, as well as three ACLA-based PDCs with AFs of 40, 45, and 50 dB, were designed and fabricated. The great consistency between the simulated and measured results powerfully proves the rationality of the proposed analysis theory and design approach and strongly highlights the advantages of asymmetric coupled lines in realizing dc and ripple blocking, better power-handling capability, a broad attenuation range, impedance transformation, and higher design freedom for attenuators.