How to Increase the Power and Accuracy of a Modular Instrument Using VXI  4.0 

How to Increase the Power and Accuracy of a Modular Instrument Using VXI 4.0 By Charles Greenberg EADS North America Test and Services March, 2011 In 2010, the VXI Consortium released a new version of the VXIbus specification (VXI‐1) called VXI‐1 revision 4.0 (herein VXI 4.0) to improve the VXI specification, making it a better bus for current and future applications. Already, VXI 4.0 is being incorporated into a number of new and existing modular products. I will discuss one of the present applications of VXI 4.0 which is being used to enhance the power and accuracy of a VXI modular digital test platform. Why Modular Instrumentation Systems? Before delving straight into the details of applications for VXI 4.0, it would be worthwhile to take a moment to review the reasons why modular instrumentation platforms are needed and the role (past, present, future) of the VXIbus. It will be a brief review so that we can quickly get on to a description of the new VXI 4.0 application. The diagram below shows the progression of modular and non‐modular platforms and distinguishes them by the availability of communication schemes controlled by a common standard and the sharing or lack of sharing of the power and cooling resources. Modularity increases as communication, power, and cooling are shared. Support of open industry standards increases as more available standards are incorporated into or supported by the bus architecture. In the upper left, a standalone instrument is shown. On the bottom are AXIe and VXI 4.0 which can incorporate the LXI and PXI standards within their structures. Of the two, only VXI shares analog bias power among all modules, making it a more modular standard. Above and to the left of AXIe is an LXI standalone box but with plug‐in modules including embedded PXI modules. Increasing Support of Industry Standards
Increasing Modularity
Figure 1: Progression of Test Bus Standards Modular instrumentation platforms continue to be relevant parts of an automatic test system because they help solve three basic problems in test system package integration, those of a) core connectivity, b) scalability, and c) space efficiency. a) Core connectivity: If the modular system contains the bulk of the instrumentation of the test system and if it contains it within a relatively small volume, it generally should be located near the Test Interface Adapter (TIA) so that interface cabling is kept short. This high concentration of test equipment will have a large number of connections to the TIA; therefore, it is logical to place it right behind the TIA. b) Scalability: Modular systems like the PXI racks in the system shown below can be designed to have custom configurations of modules installed in the mainframes. System variants can be defined without having to come up with a whole new system design; instead, just select the number of cards and cable harnesses needed for the application. c) Space efficiency: Space efficiency is achieved in modular systems by using a shared power supply, cooling system and communications bus. Available slots in the mainframe are utilized to add or subtract system capabilities. In summary, an Automatic Test System (ATS) is constructed using a modular instrument platform at its core as the central focus that the system is built around, the way the nucleus is the central to the structure of an atom or a cell. The platform chosen depends on the speed, power, cooling, cost, and space requirements specific to the tester. Variable Voltage Parallel Digital Test Some military assemblies under test require multi‐channel digital I/O at various digital voltage and current levels. Pin drivers used to drive the output channels work like linear regulators sourcing currents of up to 80 mA per channel, creating voltage drops inside the Pin drivers, even when outputting DC levels. At high frequencies up to 50 MHz and at high slew rates, these linear driver channels can easily dissipate 5 Watts per channel. For high channel count applications, with typical channel counts in the 200‐600 channel range, internal power dissipation can extend to the 1‐3 kW range. For ease of scalability and packaging convenience, modular architectures tend to be used for such applications. Such applications can push or even exceed the limits of even the best modular instrumentation platforms. Because of the high input power requirements to power the channels as well as the extreme cooling required to cool so many dissipative channels, a robust modular platform is needed. 3U and 6U PXI offer a limited cooling air inlet due to the card depth of only 160 mm and slot pitch of 0.8” for an inlet area of 5 in2. VXI, with its 340 mm depth and 1.2” pitch, has a reasonably large 16 in2 air inlet, enough area to cool 300 Watts of internally dissipated power in a VXI module that is appropriately designed. Incidentally, an AXIe module, despite being able to power a module with 1000 Watts or more, has a smaller air inlet, only about 12.7 in2 for 37% less cooling than VXI. Perhaps this is why the AXIe Consortium is developing a liquid cooled version to address high power applications. If 300 Watts is being dissipated per slot, significantly more input power than this must be applied to the module to bias the linear devices and the logic circuits. Even “powerful” modular instrumentation platforms like the PXI‐1 and VXI‐1 standards traditionally guarantee 75 Watts and 250 Watts, respectively, of input power per slot. Massively parallel digital applications must sidestep these limits to avoid either a) exceeding the backplane connector rating or b) having to limit the number of channels that can be used simultaneously to conserve power. Up until now, these power input limits have been overcome by bringing in power outside the power limit of the bus either by having a power connector on the front panel of the module or by adding an extra power connector on the backplane. TheTalon InstrumentsTM T964 and SR192A digital products use the former method while the Geotest G5960 utilizes a special high power connector found on their custom 6U PXI backplane. Other digital products warn users not to exceed the power ratings of the backplane connector which effectively limits the customer to the use of fewer than half of the channels simultaneously in order to avoid damage to the chassis or module. Power Connector Figure 2: Geotest GX5960 Figure 3: Talon InstrumentsTM T964 VXI 4.0 to the Rescue The new VXI 4.0 specification solves the power limitation by providing additional power using traditional backplane methods. The new 5‐row, 160 pin connector which was originally adopted by the VME64x standard is now utilized by VXI 4.0 to replace the traditional VXI 3‐row, 96 pin DIN connector. The new connector provides each slot with 418 Watts of traditional rail power, almost double the power available to an original VXI module. In addition, user power is available that can easily provide an additional 1000 Watts to a VXI slot (note that this is probably more than what can be cooled using forced air cooling techniques). Thanks to careful mechanical and electrical design, the 5‐row connector is fully backwards and forwards compatible with all previous revisions of the VXI‐1 standard. 5‐row Backplane Connector 3‐row Module Connector 3‐row Backplane Connector 5‐row Module Connector Figure 4: Diagram of Plug Compatibility between 96 and 160 Pin VXI Connectors The new Talon InstrumentsTM T940 VXI Digital Resource Modules support the 5‐row standard, solving the power input dilemma that has historically been an issue for high power VXI applications. It enables the T940 digital pin drivers to receive the 300 Watts or more of input power needed to do high speed, high channel count, and high‐voltage digital all from the VXI backplane. Furthermore, it utilizes the CLK100 (100 MHz clock) and SYNC100 lines, which are also new with VXI 4.0, as the reference for de‐skew calibration of the digital channels. This allows an improvement to be made over the 2 ns typical results that were obtained using the traditional VXI CLK10 (10 MHz clock). Because the T940 utilizes the 5‐row VXI 4 connector, it paves the way for speed improvements in the communication bus of the digital subsystem. By utilizing the 2eSST protocol which is emerging for VXI 4.0 systems, 320 MB/s of data throughput speed is achievable. In summary, modular architectures are used to try to make scalable, high‐speed, and/or high‐power applications a reality. VXI 4.0 adds power, speed, and standards‐compatibility capabilities that will enable the next generation of test system integrators to solve some of the major hurdles that have limited modular architectures in the past. About the Author: Charles Greenberg is a Senior Product Marketing Manager at EADS North America Test and Services. Charles is currently the Technical Co‐chairman for the VXI Consortium and has a BS in Electrical Engineering with Computer Science Minor from Cal Poly, Pomona. Charles has worked in the past in design of power instrumentation and measurement products, test system design as well as product marketing.