Code-Modulated Embedded Test and Calibration of Phased-Array Transceivers

We present improved methods for built-in test and calibration of phased arrays in free-space using a code-modulated embedded test (CoMET). Our approach employs the Cartesian modulation of test signals within each element using existing phase shifters, the combination of these signals into a code-multiplexed response, creation of code-modulated element-to-element “interference products” using a built-in power detector, demodulation of correlations from the digitized interference response, and parallel in situ extraction of amplitude and phase per element using an equation solver. In this article, we review CoMET's methodology and then analyze the impact of noise within the system. To improve CoMET accuracy, a reference-element methodology is introduced, where all measurements are referred to as one element in the array whose phase is held constant. This is compared with another method in which the modulation axes are rotated to allow accurate extraction of phase near the original 0°/90°/180°/270° axes. Our techniques are demonstrated for both receive and transmit modes using an eight-element 8-16-GHz phased-array packaged and assembled together with patch antennas. Compared with network analyzer measurements, CoMET-extracted gain and phase using the reference-element method are accurate to within 0.4 dB and 2°-3° for free-space measurements, respectively. CoMET is then used within a calibration loop to equalize elemental gain and achieve a 7-bit phase resolution. In free space, the maximum gain and phase offsets between active antenna elements are reduced from 3.5 dB and 20°-90° to 1.1 dB and 0°, respectively. Calibrated beam patterns show significant improvement with peak-to-null ratios of >30 dB.