PXI for RF and Wireless Test: Vector Signal Generators and Analyzers

PXI (PCI eXtensions for Instrumentation) systems are primarily designed for test and measurement and automation testing.

In this article, we’ll explore PXI system basics and its application in wireless test and measurement. We’ll focus on testing wireless devices compliant with WLAN, WiMAX, Zigbee, GSM, LTE, and other IEEE/3GPP standards.

This guide will assist RF and system engineers in understanding how to test and measure, as well as how to use vector signal generators and vector signal analyzers for testing devices under development, before pre-certification according to various standards.

PXI System Basics

PXI combines PCI or PCI Express bus features with CompactPCI, adding timing and synchronization capabilities. Applications are developed in various software languages to meet diverse test, measurement, and automation needs.

The PXI Systems Alliance governs the PXI specification, promoting the standard across the industry to ensure interoperability between different companies’ products.

A PXI system mainly comprises three parts:

• Chassis
• Controller
• Peripheral Modules/Devices

Let’s examine each of these:

Chassis

The chassis provides PCI and PCI Express buses to the controller and other modules (signal generator, signal analyzer, digitizer, waveform generator). It also provides power and cooling to maintain temperature. The chassis houses multiple modules thanks to its slot provision. Monitors can be interfaced for user-friendliness.

The PCI bus operates at 33 MHz with a 32-bit width, resulting in a theoretical bandwidth of about 132 Mbps. However, due to its shared bandwidth approach among multiple modules, performance can degrade if one module consumes excessive bandwidth.

PCI Express addresses this limitation by replacing the shared bus concept with a switched architecture. Each module has its dedicated data path and bandwidth for communication with other modules. PCI Express 1.0 supports approximately 250 Mbps per direction per lane, while the latest PCI Express 2.0 supports 250 Mbps to about 500 Mbps per direction and is backward compatible with PCI Express 1.0.

PXI systems incorporate time and synchronization using a reference clock of 10 MHz or 100 MHz. PXI Express systems use a 100 MHz reference clock, provided to all modules in the chassis.

Controller

The PXI system has one slot for the controller. Controllers are designed based on application needs. Embedded controllers, similar to CPUs with hard disks, RAM, Ethernet, USB ports, video cards, and mouse/keyboard connectivity, eliminate the need for external PCs.

National Instruments (NI) has been a leading provider of PXI-based systems for years. One example is the NI PXIe-8135 controller, which operates up to 2.3 GHz.

Peripheral Devices

Various modules can be housed in the chassis.

The 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).

Modules are designed by companies worldwide and are interoperable due to the evolution of the PXI standard.

The figure depicts a typical PXI system used for RF and Wireless test and measurement.

RF VSG and VSA

One key application of PXI systems is for RF Vector Signal Generation/Generator (RF VSG) and RF Vector Signal Analysis/Analyzer (RF VSA). This is achieved by incorporating two RF cards into the chassis slots: one as a transmitter and the other as a receiver.

The RF Signal Generator card provides RF up-conversion and arbitrary waveform generation (AWG) features. The RF Signal Analyzer card provides RF down-conversion and digitizer functionalities.

To generate complex modulated signals according to IEEE standards like WLAN, WiMAX, and Zigbee, the IQ vector of the respective wireless standard is written into the AWG module, generating a modulated IF signal at a low frequency. This low-frequency modulated signal is then input to the RF up-conversion module to produce the modulated RF signal. This signal can be used to test the wireless device’s receiver functionalities. Various frames are generated according to MAC messages defined in the standard to test the standard-compliant device’s functionalities.

Similarly, to analyze complex modulated signals generated by a wireless device, the signal can be input to the RF VSA part. This converts the RF signal to an IF signal using a down-conversion module. The down-converted IF signal is then digitized to obtain the baseband IQ signal.

Various mathematical equations can be applied to this baseband IQ signal to measure and plot parameters such as EVM, channel frequency response, DC offset, frequency offset error, and timing symbol error.

The figure depicts the 11ac application software and an 802.11ac compliant DUT (Device Under Test) interfaced with the PXI system. The DUT can be connected with an RF cable or tested using a dipole antenna, depending on the specification requirements.

Wireless Test and Measurement Application

Application development can be done mainly in LabVIEW and LabWindows/CVI. Engineers can also use .NET, Visual Basic, and C/C++ for development. This primarily targets GUI-based applications for R&D and validation testing. In addition, the PXI controller can run applications developed using NI TestStand, a test management software tool. For real-time applications, LabVIEW Real-Time and LabWindows/CVI Real-Time modules are available.

Conclusion

PXI systems have become vital for the modern RF and wireless test and measurement industry, enabling the testing of devices according to various conformance documents defined by IEEE/3GPP or respective forums.

The popularity stems from the use of PXI instruments across various wireless standards, as applications are developed separately on top of the basic platform.