Variable Reflection-Type Attenuators Based on Varactor Diodes

In this paper, variable reflection-type attenuators based on varactor diodes are introduced and described in detail. Their main components are 3-dB couplers, varactor diodes, and lumped elements. Initially, a simple varactor-based reflective load is introduced and its advantages and disadvantages in the circuit of reflection-type attenuators are discussed. Based on this, improvements and refinements to the simple varactor-based reflection-type attenuator are then discussed and a novel circuit of the varactor-based reflection load is proposed. This circuit offers high attenuation and controllable insertion losses. It is also shown that the intermodulation (IM) performance of the novel varactor-based reflection-type attenuator is controllable and that the reduction of insertion losses leads to an improvement of IM performance. As an experimental verification, varactor-based reflection-type attenuators operating at a center frequency of 2.5 GHz are fabricated and measured. It is shown in one reflection-type attenuator realization that insertion losses as low as 1.1 dB with a minimum third-order intercept point of 25.2 dBm are achievable, while in the second realization a dynamic range of over 36 dB is shown to be practically achievable. The results are discussed.