IEEE 802.15.4/ZigBee RF PHY Test Challenges

When developing a ZigBee solution it is essential to comprehensively test the physical (PHY) RF layer. This article presents some of the key test challenges confronted by design engineers, of which the need for automated measurements is a prime example. Agilent’s 802.15.4 ZigBee test solutions will also be highlighted.





Testing Challenges

Test and measurement of the PHY layer is critical in many areas of development moving towards an end product. This makes this article relevant to many parties in the ZigBee “food chain”, whichever route is taken to final hardware implementation.



At the top of this chain, ZigBee chip companies designing their next generation of system on chip (SoC) solutions must ensure:



• Conformation to specifications

• Automation for efficient characterization of their devices

• Tight tolerances through process monitoring

• Quality

• Interoperability

• Accurate datasheet information

• Provision of tools for customers.







Major chipset companies are producing their own reference designs. They must accurately characterize their chipsets and define datasheet specifications for successful proliferation of their design in the industry. This is a continual process of optimization for quality control, automating tasks to speed investigation and free up RF laboratory equipment. Providing adequate tools to their customers is also vitally important, including test and measurement expertise. With a test solution such as the N4010A Wireless Connectivity Test Set and the 89601A Vector Signal Analyzer (VSA), companies can easily share test expertise and custom developed test code. For example, sharing of test setups and chipset recordings can illustrate expected transmit results.



Arbitrary waveforms can be adopted by chipset customers to eliminate a golden radio approach to receiver testing and played back using sequence files. This facilitates easy comparison, faster problem solving and takes a similar approach to other wireless technologies such as Bluetooth and WLAN.



Customizing and optimizing a chipset solution is another potential way to progress. In this area the demands for test and measurement expertise is readily understood as a fast and logical way of improving design.



Module designers adopt the role of integrators, where they have the opportunity to modify designs and produce RF modules. For example, they can add value to their solution by taking away the need for sourcing many components (a requirement even for SoC solutions). They can also choose to provide a pre-qualified solution and have virtually all of the RF testing complete. However, it is evident that many vendors involved with ZigBee are entering the RF world for the first time, and are implementing new technology in areas such as lighting, consumer white goods, safety and security.



Agilent’s test and measurement approach and expertise in wireless communications can help demystify much of the detail of the physical layer test. An example is the reduced complexity of a onebox tester approach. Similar to the chip companies, automation of tests and characterization is essential to test for tolerances and spot problems both in design and production. The challenges of high volume manufacture are also apparent, especially when aiming to achieve lower costs and make mass production more viable.







In all these cases, development must be achieved in the quickest possible time using equipment which can be automated, provide traceable results through the lifecycle and which can be flexible and cost effective to meet the demands of wide laboratory use and business pressures. Agilent’s N4010A wireless connectivity test set solution is an example of a one-box tester which may be adopted to address the variety of RF challenges faced by many ZigBee developers, and who work in different parts of the food chain.



Whichever approach is taken to an end product, the demands for test and measurement are equally important. Moreover, throughout the development cycle it is worth considering future test needs, for example, moving to an automated test approach within a chosen software environment.









Agilent ZigBee Solutions

The 89601A vector signal analysis software is a PC-based package designed to measure the RF and modulation quality of most digitally modulated signals. Its large array of demodulators, filters, displays and analysis tools makes it ideally suited for evaluating and troubleshooting modulated signals in the R&D lab. For 802.15.4/ ZigBee, the package contains demodulation presets for each of the three frequency bands.



The ESA spectrum analyzer offers flexible mask testing and 89601A can also be used to extend its capabilities. The PSA series of spectrum analyzers also offers flexible mask testing. However, with flexible demodulation options it can offer digital modulation analysis of ZigBee signals, including EVM, offset EVM, eye and constellation diagrams. For signal generation the ESG series of vector signal generators provides real time and arbitrary waveform generation of IEEE 802.15.4/ ZigBee signals.



Agilent also offers the N4010A wireless connectivity test set for emerging communications in R&D, integration and verification, and manufacturing, providing traceable measurements from R&D to production environments. A full range of ZigBee transmitter tests are obtainable from the N4010A when used in association with the 89601A vector signal analyser software.



Further, the N4010A’s internal flexible arbitrary waveform generator is used to setup, sequence and playback signals required for receiver testing. ZigBee signals can be formed or acquired in a number of ways: using programs like Matlab, capturing practical signals from devices and downloading to the N4010A memory, or from use in conjunction with other Agilent instrumentation.



Agilent equipment can not only be integrated closely but can also offer traceability for whichever solution you choose.











Automated Physical Layer Testing Agilent has been working with major chipset manufacturers in order to test ZigBee devices and address the IEEE 802.15.4 specifications. A major challenge for ZigBee designers is the need to automate such tests. The N4010A test set and the 89601A VSA software allow designers to develop an early integration of a systematic automated test which could ultimately result in a faster development cycle.





IEEE 802.15.4 / ZigBee Transmitter Measurements

The 89601A VSA software allows the test engineer to experiment and test with a high degree of configurability, investigating problems, improving design and understanding test parameters for transmitter testing. With experience this leads to improved efficiency in test by acquiring an understanding of relevant measurement parameters. The benefits of using setups and recordings are also understood and the next logical step is to build on them using macro functionality.



Macros let you automate a series of manual operations into a single command. All 89600-series products use VBScript for their macro programming language. 89601A VSA software has all the tools required to record, save, run, recall and edit macros.



This kind of automation within the 89600 VSA software environment provides an excellent basis for trying out different procedures and routines. It provides a visual representation of how the measurements are acquired. For example, it could be the first time the test engineer combines external software automating the device (to put it into the appropriate transmit states) with test results. This then allows fine-tuning of test automation based on preliminary manual testing and evaluation.



The next stage in automation is to develop software which integrates device control with test characterization more closely and within the realms of a test plan. The 89601A vector signal analyzer provides an Application Programming Interface to its Component Object Model, or COM API. Measurement, computational and display objects may be accessed to control measurement setup, acquire results and integrate into test plans. Exposed objects are accessible to any application, programming tool, or language that implements automation. Examples are Agilent VEE, LabVIEW, Visual Basic and Visual C++.



If macros have been used prior to the development of custom software, much of the code may be leveraged. In some instances it is quicker to record a macro and utilize the result as opposed to using the COM API reference help for every simple task. Further, programs can be used to run macros already in use and make use of existing setup files.



As discussed above, the step to custom test development in a chosen software environment is not a great leap. As an example of the possibilities, the following example program was developed using Agilent’s graphical programming language Visual Engineering Environment (VEE) in Figure 2. This program implements a test plan to complete a number of key transmit measurements.





IEEE 802.15.4 / ZigBee Receiver Measurements

Control of the N4010A arbitrary waveform generator allows for receiver testing of ZigBee devices. Taking receiver sensitivity as an example for this tests, the N4010A’s internal arbitrary waveform generator can be used to send packets out to the DUT, dropping the power level until the error criteria is met. In Figure 3, Agilent VEE was used to control both the device and the instrument to carry out receiver sensitivity and link quality indication.





For information on Agilent ZigBee testing see http://www.agilent.com/find/zigbee.