Phase Optimization for Transmission-Type Varactor-Based Tunable Attenuators

A transmission-type varactor-based tunable attenuator with improved phase variation characteristics is presented. The attenuator is based on a standard 50 Ω microstrip transmission line with 9 identical varactor-loaded stubs. These stubs are equally grouped into three blocks, each with its independent DC controller. By contrast, the original attenuator has the same structure however with just one bias voltage control applied to the 9 stubs. Both attenuators can achieve a continuously tunable attenuation level up to 30 dB. By optimizing the DC bias for each subgroup of the proposed 3-block design, the phase variation across the attenuation range is predicted to be significantly reduced to 28.3°, from 63.2° of the original design. The measured results show a reasonable agreement with simulations, which demonstrate a compact transmission-type attenuator with large dynamic attenuation level, low reflection and small phase variation. The proposed tunable attenuator is suitable for seamless planar microwave circuit integration.