RF instruments challenge PXI versatility
By Richard A. Quinnell, Contributing Technical Editor -- Test & Measurement World, 11/1/2008 2:00:00 AM
 TIM CAREY PXI product manager Aeroflex |
One of the most dynamic applications for PXI is its use in the testing of complex, high-bandwidth RF applications. I recently spoke by phone with Tim Carey, PXI product manager at Aeroflex, who explained the difficulties of creating RF-capable instruments and who cautioned that multi-module RF instruments could limit PXI’s versatility.
Q: How is RF a challenge for PXI?
A: Physical space is one of the main limitations that we have to work at overcoming. For one thing, the spacing of the boards makes it impossible for us to use double-sided component mounting and still provide shielding in a single-slot board. As a result, many RF instrument modules need two or more slots. Heat dissipation is also a challenge in such tight quarters. Also, the available rail voltages and currents, which are carry-overs from digital computing, are not what we would ideally like for RF.
Q: How is PXI providing an advantage?
A: The modularity of PXI allows us to separate the transducers from the processing capability of an RF instrument. CPUs evolve much faster than RF transducer technology, and the separation gives engineers access to higher-performance processors as soon as they become available. The separation also reduces the cost of upgrading. If a new processor or advanced transducer stage becomes available, engineers only need to change the system element that has improved rather than buying an entirely new instrument each time.
Q: What is the state of the art in PXI for RF?
A: Aeroflex and other vendors now offer modules that can produce and analyze signals with frequencies as high as 6 GHz, but we’re beginning to see systems capable of operating at even higher frequencies. One signal analyzer now being demonstrated operates on signals to 26 GHz. Pretty good for a technology not intended for RF applications.
Q: What gaps do you see in the RF functionality available on PXI?
A: We haven’t seen any real-time signal-generation capability for applications such as channel fading and base-station simulations. What is being offered now is the use of “canned” waveforms, not those computed in real time. PXI Express allows us to stream data through an RF module for complex, high-bandwidth signal generation, but some standards are best tested by having a closed loop and computing the test signal in real time.
Q: Any concerns for the future?
A: If we’re not careful, we may lose the versatility of the PXI architecture in RF instrument design. You have to break an instrument up into a number of functional blocks to put it on PXI. A signal analyzer, for instance, needs downconverter, signal-conditioning, and digitization blocks. But it is highly unlikely that vendors will do their partitioning the same way and have the same interface signals between blocks.
This will prevent engineers from creating instruments by combining modules from different vendors. The test system may be multivendor in nature, but not the individual instruments within the system.
Q: How can the industry avoid that?
A: The panacea would be for the industry to agree on the definitions of standard functional blocks and the interfaces between. Then engineers could mix and match vendor boards when creating their instruments. At the very least, though, there can also be collaboration between vendors to develop compatible blocks that will allow engineers to form instruments for themselves.
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