PXI switching capabilities continue to advance
If signals pass through a poor switch, measurements will be compromised.
By Walter Strickler, Giga-tronics -- Test & Measurement World, 11/1/2009 1:00:00 AM
Switching is probably the most overlooked part of a test-system design. Engineers are often familiar with test instruments used in the lab for testing products or debugging new designs, but they have little exposure to switching. It doesn’t matter how accurate the instruments are: If signals pass through a poor switch, measurements will be compromised.
PXI offers an excellent platform for switching because of its compact size and ease of instrument integration, but it’s a demanding environment in which to achieve high-performance switching. Nevertheless, PXI vendors have made great advances in switching capabilities.
A good automated-test-system design should include switching that emulates what the engineer expects when connecting instruments manually. That means the developer must view the entire switching system, including cables and connectors, as a high-performance transmission line. In transmission lines, insertion loss and impedance mismatches occur at every adapter and connector interface, including the switch contacts. Parasitic capacitance and inductance can limit the bandwidth and induce crosstalk between signals, as well as enable coupling of noise and interference. Grounding schemes and transmission-line stubs will influence performance as well.
| Read more articles from our November 2009 PXI Test Report. |
With over a decade of switching experience, leading PXI module and system providers have developed best practices for avoiding or eliminating these common problems. The best practices include the separation of grounds and ground planes and a focus on signal shielding and signal isolation. For example, some switching-module designs separate the grounds for the chassis, analog circuitry, and digital circuitry; isolate control, power, and signal lines by placing them on separate PCB (printed-circuit board) layers interwoven between ground planes; and carry the signal shield between instrumentation and the unit under test through the switch. These techniques can offer a 5X to 10X improvement in bandwidth compared to other designs, far greater signal isolation, and much lower crosstalk. Experienced providers of PXI switching solutions will use these best practices to help you build a system that meets performance requirements and does not compromise measurements.
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