Active Electronically Scanned Array (AESA) technology will enable next generation radars achieve better jamming resistance capability and low probability of intercept by spreading their emissions over a wide frequency range. These radars systems consist of a large number of transmit/receive modules (TRMs) which are electronically scanned in a tight time-synchronized manner. This requires digital control to move closer to the radio front end on the antennas. Other emerging technologies, such as cognitive radars and MIMO radars, will continue to drive the need for complex timing, synchronization, and high mix RF and digital measurement requirements. To meet these challenges, radar engineers will need a platform based approach which delivers capabilities such as multi-channel phase aligned measurements over wide bandwidths and high-throughput streaming. This paper discusses the fundamentals of AESA radars and trends in radar systems. It analyzes the impact of these trends on test system architecture and explains how the advances in PXI modular instrumentation can meet these challenging requirements.