Design of PXI and LXI to realize communication test automation

Main points

In the 1980s, the concept of virtual measurement instrument (VI) proposed by National Instruments (NI) (NaTIonal Instruments) in the 1980s triggered a major change in the field of traditional instruments, allowing computer and network technology to be driven into the instrument field for a long time Pioneered "the software is the instrument". Virtual instrument is actually a computer-based automated test instrument system, that is, it uses a computer open architecture to replace the traditional stand-alone measuring instrument, and performs computer processing, display and storage of various data. The outstanding advantage of virtual instruments is that they can be combined with computer technology, thereby opening up more functions, having great flexibility, and greatly improving the processing capacity of the test system. At the same time, people have shown unprecedented attention to the impact of the connection of hardware and computing in the virtual instrument system on the overall performance of the test system.

The instrument can already communicate with the controller and communicate with each other to serve such applications. This ability can be traced back to the late 1960s, when HP invented the HP interface bus (HP-IB), which was formulated as the IEEE-488 standard, and had a name irrelevant to the manufacturer-the general interface bus (GPIB ). Many instruments have long been equipped with computer-centric universal interfaces, such as RS-232, USB, etc. However, these interfaces have limitations. GPIB cables are bulky and expensive, and have limited data rates. USB cables are ubiquitous and cheap, but the interface does not have instrument-specific features, and communication is limited to several instruments near a single computer.

The LXI standard and the PXI standard are overcoming these limitations. The supporters of these standards are represented by the LXI Alliance and the PXI System Alliance respectively. During the summer and autumn industry events, they vigorously promoted the characteristics and benefits of their respective standards. People can use instruments or hybrid systems that meet one of these standards to bring automated testing capabilities to their laboratories. Several vendors stated that there is no single instrument system architecture for all applications. They are supporting two standards at the same time, or at least studying these standards (highlighted in Table 1).

The so-called LXI is a new instrument platform composed of small and medium bus modules based on industrial standards such as Ethernet technology. The LXI instrument is a new instrument strictly based on IEEE 802.3, TCP / IP, network bus, web browser, IVI-C0M driver, clock synchronization protocol (IEEE1588) and standard module size. Unlike the modular card frame with expensive power supplies, backplanes, controllers, MXI cards, and cables, the LXI module itself already has its own processor, LAN connection, power supply, and trigger input. The height of the LXI module is one or two rack units, and the width is full width or half width, so it is easy to mix modules with various functions. The signal input and output are in front of the LXI module, and the LAN and grid inputs are behind the module. The LXI module is controlled by a computer, so there is no need for the display, buttons, and knobs of traditional desktop instruments. At the same time, the LXI system composed of LXI modules does not require a slot 0 controller and system chassis as in VXI or PXI systems. Under normal circumstances, the LXI module is controlled and operated by a host computer or network connector during the test. After the test is completed, it transmits the test results to the host computer for display. The LXI module uses the standard web browser for error browsing and relies on the IVI-COM driver to communicate, which facilitates system integration.

LXI Alliance and PXI System Alliance

If people are working in the microwave field, they will need LXI systems or hybrid systems equipped with LXI microwave instruments. In terms of PXI specifications, Phase Matrix provides a 26.5GHz PXI-1420 downconverter, while Pickering Interfaces provides PXI microwave switches. Aeroflex and NaTIonal Instruments ’general-purpose PXI RF signal sources and receivers are up to 6GHz and 6.6GHz, respectively. PXI RF module manufacturers do not intend to increase bandwidth, but aim to support emerging technologies. For example, Aeroflex recently announced a new LTE measurement capability for its PXI system. In contrast, almost any type of bench-top instrument that people can buy offers an LXI-compliant version. For example, Rohde & Schwarz has just launched its R & S ZVA67, which is a 10MHz to 67GHz vector network analyzer. The instrument complies with LXI Class C.

In addition, if people need some form of remote access, or must cover long distances, then facts can prove that LXI is the technology of choice. For example, if people need to test the working distance of the radar, where the signal source and receiver are hundreds of meters apart, then the LXI system can easily complete unrealistic measurements in other ways (Reference 1).

On the other hand, PXI provides a way to easily configure an instrument system without having to deal with LAN problems and without the involvement of the IT department. PXI inherently provides clock synchronization between instruments on the backplane. LXI Class B and C instruments have this capability, but the more common Class C version does not have this capability, especially in the RF and microwave range. Moreover, the PXI system can transfer data at a very high rate and perform storage and analysis outside the instrument.

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