Signal Switching Transparency in High Channel-Count Test Systems

Signal switching subsystems are arguably the heart of all automatic test systems. Instrumentation is typically selected through careful analysis of test requirements and the associated levels of accuracy needed to accomplish successful unit under test (UUT) evaluation and qualification. While this approach will provide a solid starting point, and may even be suitable as-is for small test system implementations, actual test results may prove disappointing in larger configurations where numerous sources of measurement error are compounded in the signal routing / measurement path.Understanding and quantifying all sources of error and loss in the measurement path, including cabling, connectors, mass interconnect interfaces, and ultimately the switching/relay subsystem, will provide a more comprehensive view of the overall system error budget and where opportunities for improvement may exist. This analysis must also take the types of stimulus / response signals into account, as the challenges associated with routing a DC-level bias voltage are quite straight forward when compared to routing a 25 MHz arbitrary waveform signal that is impacted by bandwidth, insertion loss and crosstalk.Many sources of measurement error are independent of the switching subsystem and can be broadly categorized as fixed, making this error budget contribution straight forward to quantify. Conversely, switching subsystem error contributions, for the purposes of this paper, will be referred to as variable based on the wide variety configurations and combinations commonly found throughout the industry. It is not unusual to see different topologies, such as multiplexers, matrices and general-purpose relay configurations, combined in a single switching subsystem with the goal of access all of the defined UUT Input/Output (I/O) points with common instrumentation.Large channel count test systems, often spanning from many hundreds to many thousands of channels, require complex switching subsystems that further exacerbate the afore mentioned challenges. Designing a scalable any-resource to any-pin topology, in this case intended to maximize test system instrumentation assets with hybrid and multiplexed pins, certainly provides exceptional flexibility but further complicates implementation.This paper will delve into the challenges associated with implementing signal transparency in complex, high channel-count switching subsystems. It will cover both fixed and variable sources of measurement error including bandwidth limitations, isolation, crosstalk, contact resistance and capacitance, along with the impact that these have on signal delivery to/from the UUT. Best practices and design methodologies to maximize performance will also be discussed.