I. Introduction
Microwave attenuators are widely used in a variety of application systems such as RF communication systems, active phased array radar and test instruments. In the phased array system, the attenuators are applied to adjust the gain consistency between different channels and compensate the channel gain [1], [2], [3]. Attenuators are divided into digital step attenuator and voltage variable attenuator. The attenuation of the digital step attenuators are a series of discrete numerical values, while the voltage variable attenuator changes continuously with the bias voltage [4]. GaAs MMIC variable attenuators are widely used in many modern advanced electronic systems and equipment:1) broadband signal source; 2) acting as amplitude modulating or adjusting in a broadband vector modulator; 3) combining with a broadband amplifier to compose a broadband AGC (Auto-Gain Control) amplifier [5].