subject: Lxi Vs. Pxi In Switching Applications Part 2 [print this page] PXI and LXI Case Histories PXI and LXI Case Histories
To better understand the similarities and differences in switching applications for PXI and LXI platforms, these recent case histories illustrate how some customers addressed the selection process.
RF Matrix
One customers test system required a 4x4 RF switch matrix with performance at frequencies higher than 6 GHz. In these circumstances, the solution generally is to use mechanical microwave switches, typically SP6T, to construct a matrix from eight switches and 16 coaxial cables. Solutions were available in both PXI and VXI platforms. The customer had been using the VXI solution but was concerned about cost and ongoing availability.
Microwave switches are not the easiest components to fit into a PXI chassis. They are bulky and take a lot of current from the 12-V supply. The PXI solution took up 10 slots of a PXI chassis, and had to use high-efficiency switches to avoid overloading the limited +12-V supply. It is difficult to support the switches in PXI, which adds to the cost in the mechanical design. There also is the overhead cost associated with an underused PXI chassis.
The customer selected an LXI device with an 8x4 matrix in 2U of rack height. The design, simpler to implement because of the increased space and power supply freedom, resulted in a substantial reduction in the cost of the matrix compared to a comparable PXI solution. In this example, the mechanical freedom of LXI allowed better use of the available height and depth of the case, reduced the rack space requirement, and permitted a matrix size twice the size that a PXI solution would support.
Video Testing
A customer who needed to test video products using a base band signal illustrates another case where LXI proved to be the most appropriate switching choice. This application required a matrix that allowed eight video base band signals to be directed to up to 48 products as part of a quality test. The eight video streams were switched between the different products while they were being life tested. Space was at a premium so a compact form factor was essential.
With PXI solutions, the normal approach would be to link smaller matrices together using coaxial cables and loop-through connections on the matrix.
However, there simply was not enough room to support a 48x8 matrix while maintaining the required crosstalk and bandwidth performance. In this application, PXI would have been both a cumbersome and expensive approach.
Fragmented approaches like this also are more complex to manage because the modules are individually programmed so they do not behave like a single matrix but rather like a collection of matrices stitched together.
Instead of using this fragmented approach, a video matrix was designed that had a 48x8 solution into 1U of rack space. The increased board area and PCB tracking allowed the elimination of virtually all the coaxial cables, replaced by PCB traces and simple board interconnect systems.
The module behaved exactly as the customer wanted as a 48x8 matrix without the need to control fragmented individual modules. In this application, it was the LXI devices capability to support larger PCBs that was crucial because it simplified control, reduced space requirements, and minimized costs.
Testing in an Airframe
Whether measured from nose to tail or from wing tip to wing tip, an airframe is a pretty big UUT. In this case, our customer needed to test cable runs installed in an airframe. In addition to performing insulation testing between cables, the customer had to check end-to-end continuity for high or open resistance paths.
The test was accomplished by installing a tester and switches at one end of the cable run and a switching system at the other end that could loop the connections back to the tester. Although PXI modules were available with sufficient voltage ratings, the customer wanted to run the switching system at the far end of the airframe from the same location at which the tester was installed.
Although that can be awkward with a PXI-based system, an Ethernet cable or a wireless interconnect makes such an arrangement straightforward. We offer a PXI-based chassis that supports our PXI modules in an LXI-compliant environment. It was more effective to use LXI because of its capability to control switching functions at a distance over a network cable.
The LXI chassis provided all the Web browser support that the LXI standard required down to the level of the PXI modules installed in the chassis. The customer was able to develop the system using PXI hardware and transfer it to LXI with almost no additional effort. In this case, the critical issue was control at a distance over a network cable.
LXI Isnt Always the Best Solution
When comparing LXI to PXI for specific applications, all the advantages arent always on LXIs side. The PXI platform has its own systems that use relatively diverse and compact switching and compact instrumentation from multiple vendors.
But there also are many circumstances where LXI offers a better switching solution than PXI. LXI often may be the best solution for systems that need large switching architectures or control at a distance or must include large components. In instrumentation applications as well, systems requiring the highest parametric performance may best be configured using LXI. In all likelihood, however, many test systems will continue to be hybrids of LXI, PXI, and other platforms.
