Characterization and Diagnostics of Active Phased Array Modules Using Non-Invasive Electro-Optic Field Probes with a CW Laser Source Kazem Sabet 1 , Richard Darragh 1 , Ali Sabet 1 , Kamal Sarabandi 2 , Khalid Jamil 3 , and Sami Alhumaidi 3 1 EMAG Technologies, Ann Arbor, MI 48108 USA 1 The University of Michigan, Ann Arbor, MI 48109 USA 2 King Saud University, Riyadh, Saudi Arabia Abstract—Electro-optic (EO) field probes can be used very ef- fectively for simultaneous near-field and far-field characterization of radiating apertures. Due to their very small footprint and ab- sence of any metallic parts at the signal pickup area, EO probes provide a non-invasive method for ultra-wideband measurement of aperture-level fields in RF circuits and antennas with very high spatial resolution. In this paper, we describe the use of EO field probes with a CW laser source to characterize a vertically inte- grated X-band active phased array tile and verify the measured results with simulation data and anechoic chamber measurements. Index Terms—Active phased array, radiation pattern, near- field scanning, electro-optic field probe. I. INTRODUCTION The recent advances in the state of the art of monolithic mi- crowave integrated circuit (MMIC) technology have inspired innovative packaging schemes for active electronically scanned array (AESA) antennas. AESA architectures integrate radiating elements and frequency conversion and amplification stages along with digital beamforming networks or other beam shap- ing/steering devices such as phase shifters and attenuators. Dur- ing the last two decades, different types of AESA architectures have been proposed including wafer-scale system-on-chip con- cepts [1] and those based on printed circuit board (PCB) manu- facturing processes [2]. Active phased array modules are typi- cally characterized using port-based measurements with a vec- tor network analyzer. The radiation patterns of active phased arrays are typically measured in an anechoic chamber. Near- field scanning systems provide a compact alternative to ane- choic chambers based on the near-to-far-field transformation theory. Conventional near-field scanning systems use one or more metallic radiators or probes to sample the fields of the de- vice under test (DUT). Such probes, however, cannot get very close to the DUT because the presence of an intruding external metallic structure inadvertently perturbs the fields of the DUT. As a result, the spatial resolution of these systems is often poor, and they cannot be used effectively for diagnostic purposes. The use of electro-optic field probes for near-field mapping of RF devices and antennas has been explored in the past [3]- [5]. The previous work utilized the optical beam from a phase- stabilized ultrafast pulsed laser system with an optical wave- length of 900nm and a pulse output of 100fs duration at an 80MHz pulse repetition rate. The RF signal supplied to the DUT by the microwave synthesizer was synchronized to the la- ser pulse train using the phase-locked-loop electronics of the stabilized laser system. The electrical signal from the output of the photodetector was measured in a harmonic mixing scheme at an intermediate frequency (IF) that was derived from the dif- ference between the input signal frequency and an integer har- monic of the 80MHz repetition rate. A far more streamlined EO field probe system has recently been developed based on a commercial 1550nm continuous wave (CW) laser diode source [6]. The new EO probe and the associated optical processing mainframe utilize polarization maintaining (PM) fiber throughout the optical chain to achieve a high degree of polarization stability; hence, better sensitivity and repeatability. Since the new EO probe system does not re- quire synchronization with a laser pulse train, it provides a su- perior and more stable phase measurement capability. Accurate and reproducible phase measurement is particularly critical for the application of the near-to-field transformation required for the computation of far-field radiation patterns. In this paper, we present the measurement results from EO mapping of a verti- cally integrated 16-channel X-band AESA tile and verify them using both simulation data and measurement data collected in an anechoic chamber. We will show how the near-field maps can be used for pattern characterization of the AESA at differ- ent beam steer angles as well as for the diagnostics of malfunc- tioning channels. II. VERTICALLY INTEGRATED X-BAND AESA MODULE In the vertically integrated packaging architecture considered in this paper, an AESA tile is made up of several interconnected stacked layers. The topmost layer stack contains the radiating elements and their feed network infrastructure. In the X-band AESA tile shown in Fig. 1, the radiators are microstrip-fed slot- coupled patch antennas printed on a 1.5mm-thick Roger RO 4003C substrate. Fig. 1. Pictures of the 16-element X-band AESA module: (Left) a complete tile with integrated thermal management system and rear fan, (Right) the fre- quency conversion board showing 16 active up-converters and 16 SMPM con- nectors feeding the upper antenna board.