A Zero-IF Auto-Calibration System for Phased Array Antennas

A simple, compact, and low-cost implementation of an auto-calibration system for evaluating a Ka-band phased array active element is presented. The proposed technique estimates the amplitude/phase unbalance of each antenna element induced by feed circuit and characterizes phase shifter and variable gain amplifier (VGA) attached to each individual antenna element in an array configuration. This intelligent calibration system employs phased locked loop (PLL) oscillators to generate an RF test signal and a LO calibration signal. The former is used for down-converting the signal output from the antenna element under test for phase and amplitude measurement. The latter is used to compensate the quadrature mixer error in low-IF topology. The antenna signal amplitude and phase are extracted from the I/Q signals. This approach can compensate for most of the associated errors caused by nonlinearity of the quadrature modules and amplifiers using internal time-varying phase. A back-to-back measurement shows the proposed scheme can offer an accuracy of ±2 degrees in phase and ±0.3 dB in amplitude over a 30-dB dynamic range.