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Compliance and interopability can only be guaranteed through testing

Wireless connectivity forms the heart of modern data exchange, whether that be between people or machines.

The Internet of Things, M2M or Home Automation – in various stages of their evolution – all rely on license free connectivity from protocols such as Bluetooth, Wi-Fi and ZigBee. LTE network providers also see an opportunity and are working with OEMs to create platforms that successfully combine wide area networking (WAN) with personal and home area networking (PAN/HAN) to deliver seamless access, control and monitoring.

However, license free, wireless connectivity isn't without its obligations and these extend to the design and operation of the radio front end, as well as the interoperability of any device certified under the relevant standard.

Because it is necessary to certify license free systems, it is important for design teams to evaluate rf based systems before embarking on expensive compliance testing. Access to test equipment that can combine mixed domains with standards based testing and protocol decoding can provide significant productivity benefits.

Standards are critical in creating market 'pull' for new technologies and wireless connectivity is a prime example of how this works. While it may be possible to put a wireless device into service without going through a certification process, its market 'value' lies in its compliance with a widely adopted standard. As such, compliance is strictly controlled.

The ZigBee Alliance, for example, operates the ZigBee Certified program, designed as a Type 1b certification program as defined in ISO/IEC Guide 67: 2004. This has four stages: testing; evaluation; the decision to grant/withhold certification; and licensing (extending to the use of logos).

It's important to appreciate the difference between testing and certification; while the Alliance alone can grant certification, many specialist companies offer testing services and a list of authorised test service providers is maintained on the Alliance's website.

The Alliance offers two levels of standard compliance – ZigBee Compliant Platforms and ZigBee Certified Products – while Manufacturer Specific Profile certification is available to products that do not use a public application profile (see fig 1).

However, fundamental compliance with the ZigBee standard can be evaluated without incurring test house fees by using standard test equipment and having an appreciation of the standard's key requirements. This kind of pre compliance testing can save time and money when developing ZigBee compliant devices.

Compliant Platforms must use MAC and PHY layers that comply with IEEE802.15.4, while Certified Products must, by necessity, be based on a Compliant Platform.

Today, many manufacturers offer ZigBee platforms that can either form the basis for a compliant platform (often as single integrated device) or as turnkey certified products (typically as a module). The choice of which route to adopt can be a commercial one as much as a technical decision.

Radio testing
Whether a modular or integrated solution is chosen, it can be advantageous to validate the radio front end for compliance purposes or to measure and optimise its performance.

Typically, a third party radio front end – such as the Microchip MRF24L40MB – will be controlled using a serial bus, such as spi. This mixed domain approach to system design is becoming increasingly commonplace, particularly where standards such as ZigBee are involved. While its desired operation can be controlled using spi commands – for example, to set registers to determine frequency channel or output power levels – measuring the radio front end's performance can still be an important and necessary part of the design process. Correlating the expected and actual performance can be difficult when working with both digital and analogue/rf signals, but it is the kind of task Tektronix' range of Mixed Domain Oscilloscopes was developed to tackle. Figure 2 shows a typical system based on a Microchip radio module and the Explorer 16 ZigBee test board, which uses the MDO4000 to validate the radio.

Using this method, it is possible to measure the rf output power in relation to the power drawn by the power supply, whilst confirming the radio is operating within output power specifications.

The channel spacing for IEEE802.15.4 is 5MHz; the 20dB channel bandwidth should be significantly less than the channel spacing, so the ability to measure this and to correlate it against a trigger event becomes important.

Because the MDO4000 also acquires a time record of the measured signal, it is possible to perform I (real) and Q (imaginary) down conversion to ascertain the instantaneous deviation from the centre frequency, allowing analysis of rf amplitude versus time.

This makes it simpler to measure the amount of current drawn during packet transmission, allowing the engineering team to design a power supply that better meets the end product's requirements.

Real world requirements
This can be further extended to measure how the system will perform when the power supply starts to degrade; a typical scenario for testing battery powered products. By placing a low ohmic resistor in series with the supply, to simulate a depleted battery, it is possible to measure how the radio's output power is affected.

A drop of 250mV on the supply, for example, may correlate to a 1dB drop in rf power and cause an increase in the adjacent channel's noise level. This can be captured and verified using the MDO4000's spectrum display.

It is important to appreciate the performance of radio transmitters under varying supply conditions in order to ensure the design continues to operate in compliance with the standard.

The MDO4000 can decode SPI commands directly, allowing a trigger to be set that detects a specific command and correlates the circuit's activity to both spi commands and rf events. For example, when combined with a trace looking at the power supply, it can be shown how an spi command causes the radio to begin transmitting and the impact this has on the power drawn, both in the time and frequency domains.

A delay shows the ZigBee radio is complying to one of the PHY layer performance requirements of IEEE802.15.4, using a pseudorandom delay between the command being issued and the radio turning on, allowing the radio to listen for other ZigBee transmitters or other interference.

Outside interference
The MDO4000's spectrum analyser function allows rapid scanning of a wide range of frequencies to detect unwanted or spurious signals. While radio certification and compliance will require a higher frequency spectrum analyser, most problems can be found with the MDO4000.

Using an antenna, the MDO4000 can evaluate the local spectrum, looking for radio sources that may cause interference while the radio is being developed. Often, this takes the form of a local Wi-Fi network which covers a number of channels potentially available to a ZigBee device. If the signal is too strong, this could impair or even block the ZigBee radio, although the ZigBee protocol takes this into account with a scanning function, which looks for a clear channel before transmitting.

The use of ZigBee compliant devices, as part of the overall increase in license free wireless devices, is putting greater pressure on developers. While many engineers may have only basic rf skills, effective end products can be created using modular solutions.

Hailey Percival is Tektronix' technical marketing manager, EMEA, for bench and midrange products.

Hailey Percival

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