You have probably come across this scenario before: Management or the marketing department approaches you asking you to add wireless functionality to an existing product, or to develop a new product that needs to be able to support a wireless link. Today, there are many wireless technologies and options to consider.
It is also quite possible that marketing has already made part of that decision for you.
The marketing requirement may stipulate that you use Wi-Fi, Zigbee, 6lowpan or Bluetooth low energy (BLE). Or, maybe marketing has no idea what is required, and just tells you to implement a wireless link!
So, after a number of meetings and conference calls, you decide to use a solution that is based upon 802.15.4. This could include Zigbee, 6lowpan, Wireless HART, ISA100.11a, Openwsn, Lwmesh, among many other wireless stack solutions that all require an 802.15.4 compliant transceiver.
At this point you would need to decide if your solution, or the protocol you’ve selected, will operate in the 2.4 GHz band or in a SubGhz band. There are times when you will need to do some experimentation or RF performance evaluations to determine which RF band to use in your particular situation.
When evaluating Atmel 802.15.4 wireless solutions, the first tool you should turn to is Wireless Composer. Provided as an extension to Atmel Studio 6.x, the Wireless Composer is a free tool. In order to make it simple, each of the current Atmel 802.15.4 evaluation kits/platforms comes with a Performance Analyzer firmware application pre programmed into the kit. Running on the evaluation kit, this Performance Analyzer firmware is designed to communicate with both the Atmel Studio and Wireless Composer extension.
Some of the capabilities of Wireless Composer include:
- Energy Scan: Scan selected or all available 802.15.4 channels
- PER (Packet Error Rate) Testing: Transmit and receive 1000’s of frames at a specific TX power level and RF channel and then review the results for errors (dropped bits/frames) while also evaluating throughput metrics.
- CW Test Modes: Place a device in a Continuous test mode to monitor emissions with a spectrum analyzer or other RF test equipment
- Antenna Evaluation: Provide a Large Digital Display to allow testing antenna radiation pattern’s at distances of up to around 3 meters from the device connected to the laptop PC.
- Range Testing: Gather and log range data generated from a wireless link set up between two nodes — this data includes RSSI (ED signal strength) and LQI (signal quality) from both sides of the RF link.
Here are a few additional example screen captures, available from within Wireless Composer.
Energy Detection Scan Mode:
Have you ever wanted to set up some RF tests and wanted to know if there were other transmissions already taking place on the channel you intended to test on ? Maybe your colleagues are performing tests in another section of the lab or building, or maybe at home you have Wi-Fi or Bluetooth or home automation devices operating in close proximity to where you want to run some experiments. The ED scan mode, as shown here, allows you to get a quick glimpse of what RF activity is happening around you. You can do a one time scan or you can configure the test to continuously scan one or all channels and repeat this until you stop the test.
A common RF test to perform on a packet based wireless communication system is a PER (Packet Error Rate) test.
This test mode allows you to configure operation on a particular channel, at a specific TX power level, using a selected antenna option. You are then provided the ability to set the number of bytes to send in a transmitted frame, and to set how many frames you are going to send during the test. All of these parameters are configured in the left hand Transceiver Properties Pane, as shown in the capture below. Once the test is performed, the right hand window provides data regarding the results of the test.
This can be useful for confirming RX sensitivity parameters, and data throughput characteristics under different conditions. Here is an example of sending 1000 frames and achieving zero errors using a frame length of 20 bytes.
Continuous Transmission Test Mode:
If you have attempted to develop a wireless RF product before, you know that a considerable amount of time will be spent performing regulatory pre – scan certification testing. This typically involves configuring your device to transmit a continuous wave RF emission on a particular RF channel using a specified amount of Transmit power. The RF emissions are monitored using a spectrum analyzer or other RF test equipment. To help save time and provide a useful tool, Wireless Composer provides a Continuous Transmission Tab that allows selection of a few different tests of this type.
In the example shown below, an unmodulated CW test transmission has been started on channel 16 using a TX power level of +4dBm. These are no results reported here, because all measurement results would come from observing the RF test equipment that monitors the RF emissions.
Antenna Evaluation Range Test Numerical Display:
For any wireless product, the antenna is one of the most important sections of the design. A great radio with a poor antenna results in poor product performance, while a mediocre radio with a great antenna can end up with very good performance. So, one of the tasks for any wireless product developer is to understand the characteristics and performance of his antenna design. This may be some type of on board antenna like a ceramic chip antenna, or a pcb trace antenna, or it just may be connecting an external antenna to an RF connector mounted on the product’s pcb. Many on board antenna designs are shortened quite a bit to reduce the footprint or space required by the antenna. This usually will affect the performance of the antenna in a negative way, or at a minimum create directivity to the antenna’s radiation pattern. A nice capability of Wireless Composer is the ability to allow you to place the device connected to the PC, running Wireless Composer, on a table or tripod at a specific height above the floor in an open indoor or outdoor area. Then, in the range test tab within Wireless Composer, select “Numerical “ as the display mode. This will then display a screen as shown below.
One would then take a battery operated mobile node about three meters away from the PC display and watch the values being displayed for ED/RSSI and LQI change as you rotate or change the orientation of the antenna with respect to the unit at the other end of the link. This display shows the LQI and ED/RSSI values at both ends of the link and can be used to examine any changes in antenna pattern, as the device orientation is changed. Knowing what orientation provides the best signal levels will later help you understand how to position the unit when mounting it at its final location. You will also acquire information on how to set up additional range tests where you could be up to one mile away, and all you have is a blinking led to indicate whether or not you still have communications with the unit under test.
Range Test Log With Multiple Markers (Push Button Marker Recording):
Wireless Composer also has a range test mode for logging signal level and quality to a PC display or to an Excel file, as shown in the screen capture below.
When two paired devices are configured in this range test mode, the unit connected to the PC will periodically (about every two seconds to conserve battery life) send a beacon type frame to the mobile unit, at which point the mobile unit will send back a reply to the logging device. This activity can also be seen in the screen capture below.
The LQI and ED (average RSSI) levels for each side of the wireless link are recorded with a time stamp to an Excel file.
Have you ever tried to do an RF range test by yourself? If you have, then you know that it sometimes can be difficult to set up a test, such that you can leave one node at a fixed location and take the other battery operated mobile unit to various locations where you want to gather signal level and link quality information.
This is especially true when your simple wireless device lacks any type of user interface, or display attached to it, as in the case of a wireless sensor, or an simple evaluation board. This becomes even more difficult if you are doing LOS (line of sight) measurements outdoors. The performance analyzer app only assumes you have access to two IO pins — one is typically an input for a push button or jumper, while the other is an output for an LED.
Outdoor LOS measurements may allow you to achieve distances of hundreds of meters, as well as one or more miles in the SubGhz RF bands.
To make this measurement task a lot easier, the performance analyzer app has the ability to enable you to press a button on the battery operated portable unit that you have in your hand, and have this RF device send an RF frame back to the unit connected to the PC that is doing the logging; as a result, that marker frame is recorded into the log, allowing you to place marker indicators for time and place in the log file. This will enable you to determine where you have been when you return to review the log data.
For instance, you could press the button once while at a specific location in room A, and then press it twice in for a location in room B. Or, if you are outdoors you could press the button and insert markers at various distances as you move away from the logging unit. Then, all you would have to write on your notepad while doing the test would be the name of your location (or the distance at which you were away from the logging unit) and the number of times you pressed the button at that location.
Upon your return to examine the recorded log, you’ll have all of the necessary information to understand the recorded results, including where in space and time the measurements were made.
See the example below:
For each of the supported wireless platforms, Atmel Studio contains complete example projects with source files for the performance analyzer application. When you are finished making measurements on an Atmel evaluation board that you used to help make device selection or RF band selection decisions, you can then use this same application with possibly some minor modifications to support your own final hardware design with regards to the IO assignments for a push button or led. This performance analyzer application along with Wireless Composer have proven to be very useful when performing tests on first prototype boards, and even for use in performing FCC or other governmental regulation pre-scan testing.