ZigBee RF Physical Layer Test Challenges

Physical layer testing and measurement are important at many points in the development process to the finished product. This article is therefore relevant to many players in the ZigBee "food chain", regardless of the approach taken to the final hardware implementation. At the top of the food chain, ZigBee chip vendors have begun designing their next-generation system-on-chip (SoC) solutions. They must ensure:
? Compliance with specifications
? Effectively analyze the characteristics of their devices through automation
? Achieving tight tolerances through process monitoring
? Quality
? Interoperability
? Accurate datasheet information
? Providing the right tools to customers

Chipset manufacturers have begun to develop their own reference designs, and they must accurately analyze the characteristics of the chipset and define the datasheet specifications so that the design can be successfully replicated in the industry. This is a process of continuously optimizing quality control, accelerating research through automation, and releasing RF lab equipment. Manufacturers must also provide their customers with the right tools, including test and measurement expertise.

This article uses test solutions such as Agilent Technologies' N4010A stand-alone tester and 89601A vector signal analyzer (VSA) as examples. Manufacturers can easily share test expertise and self-developed test codes, which can also be used to handle other wireless technologies such as Bluetooth and WLAN. Test Challenges
Module designers play the role of integrators. They have the opportunity to modify the design and production of RF modules. For example, they can remove the need to obtain many components from outside to increase the value of their solutions, even SoC solutions. They can also choose to provide pre-certification solutions and complete almost all RF testing. But it is obvious that many manufacturers who invest in ZigBee technology are new to RF and implement new technologies in areas such as lighting, consumer white goods and security facilities. When the RF part of the finished product is almost complete, testing and measurement must be performed to ensure that the field installation can be successful. For example, after obtaining the module, creating a custom profile and modifying the antenna, the end device can be installed in an industrial environment and then monitor interference and power levels.

In all of the above situations, the right equipment must be used to complete development in the fastest time possible, and this equipment must be automated to provide traceable results throughout the life cycle, while also being flexible and economical to address the needs and operational pressures of widespread laboratory use. Take Agilent's N4010A Wireless Connectivity Tester solution as an example. It is a stand-alone tester that can be used to address the various RF challenges faced by many ZigBee developers and those at different points in this "food chain". Regardless of the approach taken to complete the finished product, the need for testing and measurement is equally important. In addition, throughout the development process, it is also worth considering future testing needs, such as applying automated test methods in the selected software environment. Agilent ZigBee Solutions

The 89601A Vector Signal Analysis software is a PC-based software package designed to measure the RF and modulation qualities of most digitally modulated signals. A wide range of demodulators, filters, displays, and analysis tools make this software ideal for evaluating and debugging modulated signals in the R&D lab. For 802.15.4 / ZigBee, the software provides demodulation presets for three different frequency bands. Figure 1: Agilent 89601A ZigBee 2.4 GHz demodulation screen
　

　
The ESA spectrum analyzers offer flexible mask testing, and the 89601A can be used to extend its capabilities. The PSA series spectrum analyzers also offer flexible mask testing, but with the flexible demodulation option (Option 241), they can also provide digital modulation analysis of ZigBee signals, including EVM, offset EVM, eye diagrams, and star diagrams. For signal generation, the ESG series vector signal generators offer the ability to generate arbitrary waveforms in real time for IEEE 802.15.4 / ZigBee signals.

For R&D, integration and verification of emerging communications and manufacturing, Agilent offers the N4010A Wireless Connectivity Tester, which provides traceable measurements for various environments from R&D to production. Using the N4010A with the 89601A vector signal analyzer software, a complete range of ZigBee transmitter tests can be performed, while the N4010A's internal flexible arbitrary waveform generator can be used to set, sequence, and play signals required for receiver testing. ZigBee signals can be generated or obtained in several ways: using programs such as Matlab, capturing actual signals from devices and downloading them to the N4010A memory, or with other Agilent instruments to generate and provide traceability. For example, the following is a comparison of the measurement results of the N4010A stand-alone tester with the 89601A VSA software and the PSA with the highest IFBW (10MHz). Table 1: Summary of demodulation results of the N4010A wireless connectivity tester with the 89601A VSA software vs. PSA. The results are highly consistent when averaging is not used and when the wideband PSA measurement signal sideband is considered. IEEE 802.15.4 / ZigBee Transmitter Measurements The 89601A VSA software allows test engineers to modify settings at will for experiments and tests, such as studying problems, improving designs, and understanding test parameters for transmitter testing. Based on experience, understanding the relevant measurement parameters will help improve test efficiency. Now that you understand the benefits of using settings and records, the next logical step is to use the macro capabilities to design based on these settings and records. The entire 89600 series uses VBScript as the macro programming language. VBScript is a popular scripting language based on a subset of the Visual Basic programming language. The 89601A provides all the tools you need to record, store, execute, call out, and edit macros. Editing can be performed in the program or using an external code editor. This automation in the 89600 VSA software environment provides an excellent basis for experimenting with different procedures and routines. It provides a visual way to illustrate how measurements are obtained. For example, this may be the first time that a test engineer has combined external software that automates a device (setting the device to the correct transmit state) with test results, and can also fine-tune the test automation process based on preliminary manual testing and evaluation.
　




　

The next stage in the automation process is to develop software that can more tightly integrate equipment control and test characterization within the scope of the test plan. The 89601A vector signal analyzer software provides an application programming interface (COM API) for its component object model. Measurement, calculation, and display objects can be used to control measurement settings, capture results, and integrate into the test plan. These objects can be used by any application software, programming tool, or language that implements automation, including Agilent VEE, LabVIEW, Visual Basic, and Visual C++.

If macros have been used before developing custom software, much of the code can be used. In some cases, it is faster to record macros and use the results directly than to use the COM API reference for each simple operation. In addition, programs can be used to execute macros that are already in use and use existing configuration files.

As discussed previously, the steps for developing custom tests in your chosen software environment have not changed much. For example, the following example program was developed using the Agilent Visual Engineering Environment (VEE) in Figure 1. This program implements a test plan to perform some important transmission measurements. Figure 2: Agilent VEE ZigBee transmitter automated measurement example
　

IEEE 802.15.4 / ZigBee receiver measurements
Receiver tests of ZigBee devices can be performed by controlling the N4010A arbitrary waveform generator. For example, in this test, the N4010A built-in arbitrary waveform generator can be used to transmit packets to the DUT and reduce the power level until the error standard is reached. The following charts are the results of using Agilent VEE to control the device and instrumentation to perform receiver sensitivity and link quality tests. Figure 3: Agilent VEE receiver measurement example