Transmission-Type Varactor-Based Tunable Attenuator

A transmission-type attenuator with varactor-based tunable attenuation is presented. The design adopts a standard 50-\Omega host microstrip line equipped with varactor-loaded stubs on its side, which also include a resistor and a capacitor in series with a short-circuit termination. The topology allows continuously tunable attenuation without interruption of the host transmission line and thus permits a compact, low-reflection, and low-loss structure for seamless planar microwave circuit integration. A generic and accurate equivalent circuit model for the proposed attenuator topology is developed for fast evolutionary optimization. To verify the circuit model, one design with three identical stubs and one with three different stubs are optimized for different performance priorities. Furthermore, two attenuators with six and nine stubs are also developed to investigate the achievable performance for different stub numbers. Prototypes of the two three-stub and the nine-stub attenuators are fabricated and tested, demonstrating a successful validation. A low reflection of less than -11.4 dB is achieved for all attenuators, with a measured insertion loss in the non-attenuating state of less than 1.0 dB for the three-stub attenuators and 2.1 dB for the nine-stub version, and a tunable attenuation up to 13.5 and 32.7 dB, respectively. The phase variations across the range of attenuation levels for the three-stub attenuators are less than 25.0° and reach 82.3° for the nine-stub design at 5 GHz. These results demonstrate a large range of tunable attenuation with a small insertion loss in the non-attenuating state, which is achieved with a compact structure suitable for seamless microwave circuit integration.