Built-In Test of Phased Arrays Using Code-Modulated Interferometry

This paper presents a built-in self-test technique for phased arrays that applies code modulation to each element within the array to allow parallel in situ measurements and a built-in distribution network to allow injection or extraction of test signals. The aggregated test response is downconverted from radio-frequency or millimeter-wave frequencies using a direct (power) detector, resulting in a baseband interference signal composed of code-modulated complex cross correlations between all elemental signals. Using orthogonal code products, each cross correlation can be extracted from the interference signal, and then the full set of cross correlations can be used to obtain amplitude and phase data of each element. A four-element 60-GHz phased-array receiver front end that includes this code-modulated embedded test (CoMET) infrastructure has been fabricated in SiGe BiCMOS technology. The BIST overhead is less than 2% of the total die area. Comparisons between our built-in test technique and measurements using a vector network analyzer show that CoMET can be used to extract amplitude with 1 dB accuracy and phase with four degree accuracy. Furthermore, measurements confirm that CoMET can be used to extract the phase-step response of each element in parallel across all settings as well as phase offset introduced by the built-in test network.