PXI for RF and Wireless Test and Measurement-How to use vector signal generator and analyzer
PXI system is mainly designed for test and measurement and automation testing. In this article we will go through PXI system basics. We will go through its application with respect to wireless test and measurement targeted for testing Wireless devices as per WLAN, WiMAX, Zigbee, GSM,LTE and other IEEE/3GPP standards. This article will help RF and system engineers in how to test and measure as well as how to use vector signal generator and vector signal analyzer for testing devices under development and before pre-certification as per various standards.
PXI system basics
PXI stands for PCI eXtensions for Instrumentation. As a whole PXI combines PCI or PCI express bus features with compactPCI and adds timing and synchronization features.
On top of this PXI system applications are developed in various software languages as per various test and measurement and automation need. PXI is governed by PXI Systems Alliance to maintain PXI specification and promote PXI standard across the industry so that products from different companies can interoperate.
PXI system is composed mainly of three parts viz.chassis,controller and peripheral modules/devices.
The same is described below.
The chassis provides PCI and PCI express buses to controller and other modules (signal generator,signal analyzer,digitizer,waveform generator). In addition it provides power and required cooling to keep the temperature down. There is a provision to house many modules due to provision of slots in the chassis. The monitors can be interfaced to provide user friendliness.
PCI bus operates at 33 MHz and has width of 32 bit. This makes theoretical bandwidth of about 132 Mbps. Due to its shared bandwidth approach among multiple modules, it will be difficult for other modules to make use of the bus if one module consume more bandwidth. To overcome this situation PCI express has been developed. In PCI Express shared bus concept is replaced with shared switch approach. As per this concept each modules will have their own dedicated data path with their own bandwidth to be used for communication with other modules connected on the bus.PCI express 1.0 supports about 250Mbps per direction per lane. Latest PCI express 2.0 supports 250Mbps to about 500Mbps per direction and is backward compatible with PCI express 1.0
PXI system incoporates time and synchronization with the use of reference clock which is 10 MHz or 100 MHz. PXI express system uses 100 MHz reference clock. This clock is provided to all the modules provided in the chassis.
PXI system houses one slot for controller. Various controllers are designed based on application need. To avoid external PC embedded controllers are designed which has functionalities of a CPU,harddisk,RAM,ethernet,USB ports,video cards, mouse and keyboard connectivity options. National Instruments(NI) is the leader in providing PXI based systems since years. One such NI controller is NI PXIe-8135 which works upto 2.3 GHz.
Various modules as mentioned in the figure can be housed in the chassis. Figure shows National Instruments NI PXI-5652(RF vector signal generator ), 5601 (down converter),5622 (16 bit IF digitizer),5450(400 MSPS IQ signal generator), 5611(IQ vector modulator) and NI PXI-5660 (RF vector signal analyzer). Various modules are designed by companies across the world and all are interoperable due to evolution of PXI standard.
The figure depicts typical PXY system used for RF and Wireless test and measurement.
RF VSG and VSA
One of the applications of PXI system is for RF Vector Signal Generation/Generator (RF VSG) and RF Vector Signal Analysis/Analyser (RF VSA). This is made by incorporating two RF cards in the slots of the chassis, one as transmitter and the other as receiver. RF up conversion and arbitrary waveform generation (AWG) features are available in RF Signal Generator card. RF down conversion and digitizer functionalities are available in RF Signal Analyser card.
To generate complex modulated signals as per various IEEE standards viz. WLAN, WiMAX, Zigbee the IQ vector of the respective wireless standard is written into the AWG module which generated modulated IF signal at low frequency. The same low frequency modulated signal is given as input to the RF up conversion module to produce modulated RF signal. The same can be given to test wireless device's receiver functionalities. Various frames are generated as per MAC messages defined in the standard to test functionalities of the standard compliant device.
Similarly to analyse complex modulated signal generated by wireless device can be given as input to the RF VSA part. This in turn first converts RF signal to the IF signal using down conversion module. This down converted IF signal is digitized to obtain base band IQ signal. Various mathematical equations can be applied on this base band IQ signal to measure and plot various parameters such as EVM, channel frequency response, DC offset, frequency offset error, timing symbol error and more. Figure depicts the 11ac application software and 802.11ac compliant DUT interfaced with PXI system. Either DUT can be connected with RF cable or can be tested using dipole antenna based on the specification requirement.
Wireless Test and Measurement Application
The development of applications can be carried out mainly in Labview and LabWindows/CVI. Engineers can also use .NET, visual basic and C/C++ programming languages for the development. This is mainly for GUI based applications targeted for R&D and validation testing. In addition to this PXI controller can run applications developed using NI TestStand, test management software tool. For real time applications, Labview realtime and Labwindows/CVI realtime modules are available.
As mentioned above, PXI system has become vital for today's RF and wireless test and measurement industry for testing devices as per various conformance documents defined by IEEE/3GPP or respective forums. The popularity lies in use of PXI instrument for any wireless standards as the applications are developed separately on top of the basic platform.
Test and Measurement Related Links
RF Vector Signal Generation and Analysis
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