Integrating ZigBee into your wireless designs

In my last post, I highlighted the rise of the Internet of Things (IoT) and how it is contributing to the need to become “unplugged.” This time I’m shifting focus from how to power these unplugged devices to looking at how these devices are able to connect wirelessly to the network.

There are numerous wireless technologies and standards available today, but three standout from the pack for embedded applications: ZigBee, Bluetooth, and Wi-Fi.

The use of these technologies has been growing rapidly over the past few years with no end in sight. The Wi-Fi Alliance claims there are more than 4 billion Wi-Fi enabled devices ready to connect today. David Blaza from ARM highlighted the rapid growth of Bluetooth in his Predicting the future using CES blog post. ZigBee is in the process of emerging from a group incompatible or proprietary technologies to become the de facto worldwide standard for the smart home, and is expected to see broad adoption in the coming years.  

With all of the different wireless standards available to choose from today it can become a challenge just to figure out which will work best for your Kickstarter project design. Below is a table that compares these three wireless standards and will start to give you a better understanding of their typical applications.

 ZigBee, Bluetooth, and Wi-Fi each have their own strengths and weaknesses that can be used to evaluate which would be best-suited for a particular application or design. The standard with the highest data rate capabilities is Wi-Fi at up to 54 Mb/s. This makes Wi-Fi the optimal solution where high data throughput is important, for example streaming video or sharing files. The main downside to Wi-Fi is its large power consumption, a problem for IoT devices that run at very low power to extend battery life. Also Wi-Fi is limited to point-to-hub connections, meaning the devices cannot freely communicate with one another without first connecting to a central hub.

Bluetooth is in the middle compared to Wi-Fi and ZigBee when it comes to performance. It is low power and is able to connect to multiple devices directly without the need for a central node. This makes Bluetooth a great candidate for the use in peripheral devices such as wireless keyboards, mice, and headsets. Bluetooth is limited in range so it is best used when all the devices being connected are in the same room.

ZigBee is the lowest performance of the three standards, but can often be the ideal choice for IoT applications. A key factor is that ZigBee uses very low power compared to the other two. Where Wi-Fi and Bluetooth battery life might be measured in hours or days, ZigBee can be optimized to last for weeks or months. Another impressive feature is ZigBee’s ability to have up to 65,000 devices connected to one another in a mesh type network.

With so many new Kickstarter IoT projects using ZigBee, it is important that designers know how to test their devices properly. There are a few different form factors to consider when looking for a ZigBee module to add to your device. There is the radio-only IC that is the barebones of the wireless technology and provides the greatest flexibility in your designs. However you must provide all the amplifiers and microcontrollers needed to utilize a ZigBee IC. Next there is the radio module with microcontroller and amplifier already integrated. This setup is the easiest to use as all the components necessary to make the wireless connection possible are already configured and assembled. The downside is that this form factor is larger and less customizable.

The Cortex-M Series from ARM provides a powerful microcontroller for all of your wireless design needs whether you need a bare microcontroller so you can have full flexibility or you are looking to save time with an integrated development board. The Cortex-M family is optimized for cost and power sensitive MCU and mixed-signal devices for applications such as IoT, connectivity, smart metering, human interface devices, automotive and industrial control systems, domestic household appliances, consumer products and medical instrumentation.

The MDO4000 Series mixed domain oscilloscope from Tektronix is ideal for testing ZigBee and other wireless technologies because of its unique ability to analyze signals in both the time domain and frequency domain. Although you can use the MDO4000 Series simply as a mixed signal oscilloscope or as a spectrum analyzer, the real power comes from the integration of the two. For example, you can see how your ZigBee design performs in both the time and frequency domains on a single instrument. Powerful trigger, search and analysis tools help you zero in on analog and digital anomalies, ecode serial buses alongside waveforms, and view the RF spectrum at any point in time.

Once the approach to the radio implementation is selected, the appropriate printed circuit board laid out, and any necessary software written, there are a number of tests to be performed to assure good communication in your Zigbee-based design. For many applications, there will be serial communications between the radio system and other parts of the product. For example, the microcontroller IC and modules use a four wire SPI connection to control the radio IC and any related components such as a power amplifier.

To illustrate some of the tests that should be carried out to verify radio operation, an amplified radio module with a demonstration board was used. The screen shots were taken using a  Tektronix MDO4000 series mixed domain oscilloscope  which allows simultaneous time correlated viewing of RF, analog, and digital signals. Setup and data commands are sent from a PC to allow manual control. Figure 1 shows the test setup. Note that a direct connection to the radio is used to facilitate power and other measurements. A calibrated antenna could equally have been used to take the RF measurements.

Figure 1. Test connections between microcontroller, demo board, ZigBee module, and MDO4000 series mixed domain oscilloscope

ZigBee RF compliance testing – The MDO4000 is unique in that it allows simultaneous viewing of the radio spectrum and the power supply as shown in Figure 2. The channel spacing for ZigBee is 5 MHz. The 20 dB channel bandwidth should be significantly less than the channel spacing. The measured occupied bandwidth of 2.3 MHz shown in Figure 2 is well within the specification. The output power is expected to be in the range of 20 dBm. The screen shows the output spectrum in the lower part of the screen and direct measurements of bandwidth and power. The test cable drop is about 2 dB in this frequency range, so the power measurement is in the range of what is expected. The orange bar at the bottom of the top half of the screen indicates the time period in which the spectrum trace is displayed. The spectrum time is defined as the window shaping factor divided by the resolution bandwidth (RBW). In this example, using the default Kaiser FFT function (shaping factor 2.23) and the RBW of 11 kHz, the spectrum time is approximately 200 us. Moving the spectrum bar across the time domain window allows the spectrum and measurements to be taken at any time during the packet transmission. This acquisition correlates just after turn-on of a radio packet transmission.

Simulated power supply failure. In Figure 3, a 1.5 ohm resistor is placed in series with the module to simulate the effect of a depleted battery. The current drawn by the module is only a few milliamps lower, but the voltage drop is about 230 millivolts. Note that the output power is reduced by 1 dB as measured by the RF power measurement and there is a slight increase in the adjacent channel noise can be seen in the spectrum display. The lower output can also be seen on the amplitude versus time (Trace A). It is often necessary to understand the performance of radio transmitters during low battery conditions or conditions when the power supply becomes current limited to understand the margins of radio compliant performance.

Figure 3 Spectrum and measurements with resistance in series with the module power source to study low power performance behavior.

Digital Commands – Radio ICs and modules need to be set up to meet the operating requirements of the specific application and any protocol specific setups. The MDO allows decode of the SPI commands to the ZigBee module. The unique architecture of the MDO4000 allows simplified measurements between SPI command triggering and correlated RF events. In Figure 4, the trigger event is now the SPI command, the radio Transmit trigger command. Markers on the Time Domain display show the SPI command to current draw (at the beginning of the RF Tx turn-on) is 1.768 ms. This allows you to quickly determine the delay between when the command is given to the ZigBee device and when the ZigBee begins to transmit.

Figure 4 Subsequent acquisition triggered on SPI command shows delay between command and radio turn-on.

These are only a few of the tests that would be needed to test and troubleshoot your wireless designs. For a more in depth testing guide for ZigBee designs see this instructional video from David Saar, Electrical Engineer and President of SAAR Associates. In the video you will learn how to make sure your device is regulatory compliant and ensure your device is working properly. If you have any questions or comments, please add your thoughts below.

Since publishing the above blog, Tektronix is currently offering a free upgrade to 3/6 GHz spectrum analyzers and a free bundle of software applications on mixed-domain oscilloscopes. Check out the details here and save away!