The first, and most obvious, challenge that average test engineer faces is the need to support legacy Test Program Sets (TPSs). Commercial and military aerospace programs are extending well beyond their intended lifecycles, and support teams must carry these fleets forward into the next wave of technology lifecycles. When looking to upgrade a test system (or subsystem) for one of these programs, Test engineers cannot only consider the technology insertion, they must also consider the hundreds or thousands of TPSs that have been developed for the system and the ripple effect that technology insertion will inevitably have on the program as a whole.
The most motivating and technologically savvy approach is for the test engineer to develop a completely new test system with exciting new instruments, instrumentation test adapters (ITAs), and fixtures while rehosting as many legacy TPSs as possible. Unfortunately they ultimately have to answer to a budget and usually end up refurbishing existing test systems to replace the obsolete pieces through planned maintenance.;
Let’s take the example of refurbishing an existing system by replacing an obsolete oscilloscope with the objective of minimizing TPS migration costs. Sounds simple, right? On the surface, the test engineers job sounds relatively straightforward – find an oscilloscope that can perform as well (if not better) than the existing scope in the system. After all, most scopes in 2015 are going to pale in comparison to the dinosaurs that were designed into the system 10-, 15-, or 20-years ago.
The first bump in the road is form-factor. The new instrument needs to take up the same or less space in the 19” Rack so as not to warrant a reconfiguration of the rack layout. Because there is a significant amount of system-level documentation, changing the layout of the rack will introduce a massive amount of documentation changes (not to mention any possible signal integrity issues with changing cable lengths to the mass interconnect). This form-factor challenge is one of the many reasons that modular platforms like PXI (and formerly VXI) have dominated the Aerospace/Defense ATE market for the last 30-years. By following the strict guidelines of the PXI specification, a scope from vendor ‘A’ will be the same size and utilize the same backplane power as vendor ‘B’, giving test engineers an easier upgrade path for their systems.
The second hurdle in the road is hardware abstraction layer (HAL) integration. Any test system that is expected to last for 5-10+ years will inevitably have planned maintenance and operational costs. These are significantly reduced by abstracting vendor-specific hardware and drivers into a HAL or MAL (measurement abstraction layer). The test engineer is also tasked with evaluating the driver stack of the new instruments to ensure they plug into the HAL to mitigate the risk when migrating the thousands of TPSs still to come. Many HALs utilize the IVI driver class where possible and supplement with Plug-and-Play drivers. Since this example is an oscilloscope, we’ll make a blanket claim that the test engineer has it ‘easy’ and gets a pass on software because there is an existing IVI class specified for oscilloscopes.
Figure 1. Hardware Abstraction Layers (HALs) significantly mitigate the impact of hardware obsolescence, but are difficult to justify in the absence of a long-term support strategy.
A third and often hidden hurdle is the answer to the question: ‘Is Better Really Better?’ The specifications of this new oscilloscope are multiple generations of technology ahead of the obsolete equipment, so where’s the issue? The issue comes when, for example, you insert this new oscilloscope into the system and the rise time or settling time measurements change significantly because you’re sampling at 3-, 5-, or 10-times the rate of the previous instrument, which results in dozens of incompatible TPSs that previously provided great system utilization. Another issue arises when legacy TPSs require trigger functionality that instrument vendors obsoleted years or decades prior. In this situation, the test engineer is challenged with looking across the entire database to identify which TPSs will be broken by inserting a new instrument that does not support the legacy trigger functionality - a database which often doesn’t exist and requires weeks or months of manual effort to identify.
In order to minimize the unknown risks of TPS rehosting, many test engineers are taking advantage of software-designed instruments (SDIs) to give more flexibility in the rehosting process. Software-designed (also known as synthetic) instruments combine core analog and digital front-end technology with powerful, user-programmable FGPAs to provide the most flexible instruments on the market. If we apply the SDI approach to the oscilloscope challenges above, the test engineer (or TPS developer) can easily implement custom trigger functionality on the FPGA of the SDI to emulate legacy trigger technology. Some go further and use digital signal processing to emulate the analog performance of the legacy instrument’s analog-to-digital converter technology.
Figure 2. While difficult to accomplish, emulating legacy instrument capabilities greatly reduces the risk of TPS migration issues. Software-Designed, or Synthetic Instruments offer a unique approach to test equipment emulation.