Maximising performance of PXI-based ATE by using cable-free mass interconnects
01 May 2019
The concept of using modular instrumentation at the heart of functional ATE systems was quickly established following the introduction of the VXI platform in 1987. Today, the PXI platform is at the heart of many new functional ATE designs.
This article was originally featured in EPDT's 2019 PXI for T&M supplement, included in the May 2019 issue of EPDT magazine [read the digital issue version of this article]. Sign up to receive your own copy of EPDT each month.
PXI has evolved to offer a broad spectrum of measurement capability, coupled with a wide variety of switching options, all packaged in a small footprint. Having such a comprehensive selection of instrumentation in a small modular platform has transformed the flexibility available to test engineers. Here, Gary Clayton, Sales Director at automated test connectivity experts, Mac Panel discusses how to maximise the performance of PXI-based ATE by using cable-free mass interconnects.
There are many priorities that drive ATE designs, depending on specific application requirements. These typically include: outright measurement performance; repeatability of performance; ease of maintenance and calibration; system scalability and re-use; future proof design; and minimum initial and long-term cost.
Another important consideration when designing ATE is which electrical interface, generally referred to as the mass interconnect, should be used. In the days of VXI, this was a straightforward matter: simply choose your preferred mass interconnect and connect to it using cables from the instruments. Aside from the inevitable noise and crosstalk issues always associated with cables, this worked well for most of the era of VXI popularity.
However, the PXI platform has evolved to offer measurement capabilities many factors higher than the VXI platform, with significantly greater I/O density on a small footprint. This, combined with ever more demanding test requirements and speed of test, requires a fresh approach to mass interconnect design: one that can eliminate as many cables and wires from the ATE design as possible. Removing cables from the system is essential to meet the full performance potential of a PXI ATE system – and meet the design priorities previously listed
The benefits of replacing cables with PCB connections goes beyond the obvious performance advantages of superior signal integrity and a significant reduction in cable associated noise issues. Just as important is that the signal can be controlled, predicted and duplicated along the entire signal path to the system interface. As a result, multiple instruments and ATE systems have near identical performance characteristics; this is all but impossible to achieve using traditional cable connection methods.
The modular nature of a cable-free mass interconnect simplifies the system design and build process, with the PXI element of the ATE build measured in hours – yielding significant time and cost savings. This modularity is a particular advantage for maintenance or system reconfiguration, since instruments can be removed and replaced in minutes, without ever interfering with connections to other instruments – minimising system downtime and again, keeping costs to a minimum.
Eliminating cables for ATE systems has been a key objective for system developers for many years, but the system design is only one side of the challenge. Test fixtures are also a critical element of the test architecture, and are also susceptible to the negative influence of cabling – and the more complex the fixture, the greater the potential performance and stability issues. So how can cables be removed from test fixure design?
Cable-free interface concept
• A PCB connection assembly is installed on to the PXI instrument.
• In very high-density applications, a flex-circuit connection is utilised and, where signal type demands, short wires can be used.
• All connections are incorporated into an aluminium case, offering complete protection for the connection, as well as excellent electrical shielding properties.
• PXI instrument, with its connection, is simply installed into the PXI chassis through the mass interconnect receiver.
• This process takes only a few minutes; significantly reducing typical system build times.
Application example: update of legacy transportable ATE platform
The challenge: Multiple transportable/deployable ATE systems needed to utilise modern hardware and software platforms. Timescales and budget were critical considerations.
The solution: The updated system has been designed utilising PXI at the heart of the measurement system. Initially it was planned to use a ‘compatible’ wired interface, similar to that used on the original legacy design. But it was quickly realised that this interface concept could not support the performance capability of the redesigned system. An advanced mass interconnect, using PCB connectivty, was supplemented with great success. The end user is in the process of deploying multiple ATEs globally, with near identical performance characteristics.
However, perhaps the major success of this project was the ability to produce multiple fixtures (test program sets or TPSs) to a very high, and again, near identical performance. The images show an example of an original fixture design for this project. This example is quite typical of how a typical TPS is designed in the defence and aerospace markets.
The fixture design was a particular challenge in this application, since there were to be multiple duplications of several TPS types. A complex wired TPS can take weeks to build; multiply that by the quantity of fixtures required, and the project would take years to complete. Another challenge with this design was the wiring itself. With all these cables, it is near impossible to get repeatable performance characteristics from one TPS to the next.
The solution was to redesign the TPS, utilising PCB connectivity as much as possible. The images show the new TPS, with all the same connectivity, but with almost all the discrete wiring replaced with circuit card interconnect.
This redesign, using a PCB for most of the interconnections, provides a TPS with optimium and repeatable performance on all TPSs of the same type, in a greatly reduced size. TPS build times are measured in hours, not weeks, so rapid build and deployment becomes viable, even in the most challenging of applications.
The solution delivered to the end user is a transportable ATE, that can be built and supplied in days, thanks to its modular building block design. ATE performance is optimised and uniform across all systems and TPSs, providing the user maximum deployment flexibility, as well as confidence that any ATE/TPS combination will perform identically. The build time savings across the project run into years, compared to using wired system and TPSs.
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