Design and Characterization of a Low-Phase-Noise X-Band Voltage-Controlled Attenuator Based on Directional Coupler

This article presents the design and characterization of a low-phase-noise X-band voltage-controlled attenuator based on a directional coupler. The attenuator consists of a 3dB directional coupler, a pair of PIN diodes, and a Howland current source. Experimental testing demonstrates that the attenuator achieves an attenuation range exceeding 15dB within a bandwidth of 9.7-10.3GHz, with voltage adjustment ranging from 0-3.2V. It exhibits exceptional in-band attenuation flatness below 1.5dB. Throughout the entire adjustment range, the input and output reflection coefficients remain below -10dB, while the phase drift is effectively limited within ±10°. Notably, at a specific operating condition of 10GHz, with an input signal power of 10dBm and an attenuation of 4.78dB, the measured phase noise at a 10kHz frequency offset is impressively low, measuring at −162.25dBc/Hz.