23 October 2012

Test equipment helps embedded system designers debug their rf connectivity

It wasn't too long ago that wireless communication was only available using devices designed solely for that purpose; walkie-talkies and the like. But over the last few years, wireless communication has found its way into all manner of products. Today, providing the ability for a product to communicate via Wi-Fi, gsm or other rf technologies is almost expected. What might once have been pretty much a digital design is now more complex – and the tools needed to design and test these products have had to adapt to new demands.

Speaking at the launch of the MDO4000 range in 2011, Roy Siegel, general manager of Tektronix' scopes business, said 38% of embedded designs now include wireless elements. "This is almost an expectation, rather than a 'nice to have'," he noted. "The availability of cheap ZigBee controllers is an example of the pervasiveness of rf, but this means that rf measurement is now the number one design challenge."

Looking to the future, rf will only become more prevalent. According to Tektronix, 2billion wireless lan enabled products will ship in 2014, while the number of wireless sensor network chipsets shipping in 2015 will reach 645million. The other big growth area which the company sees is RFID, where 4bn tags will ship this year. Examples of where rf technology is being applied range from logistics to smart energy networks and from children's toys to retail.

Trevor Smith, Tektronix' technical marketing manager for oscilloscopes, EMEA, said: "There are many low level rf applications in development. Those designing embedded systems with wireless connectivity need to make measurements on analogue channels and they're looking at digital channels with parallel or serial data. But the number one challenge for them is rf measurement; whether it's rf signals or emissions. It's a difficult area."

Tektronix' solution is the MDO4000 range – where MDO stands for mixed domain. Launched last year, the approach brought to the market the concept of an oscilloscope and a spectrum analyser in the same box.

Smith said designers looking to include wireless connectivity into their products face three main challenges.

"The first is measurement at the system level," he noted. "Designers need to validate the power supply holds up." Designers need this kind of approach when dealing with wireless modules, am or fm communications and with dual band transceivers, where wideband analysis is required.

The second challenge is that of timing analysis. "When a command is issued telling the device to go to a particular frequency, designers need to know how long that process takes." Other challenges include capturing the characteristics of phase locked loops turning on and off and monitoring how the spectrum changes with time.

"The third challenge," Smith continued, "is tracking sources of interference and noise. While this can be done with a standalone spectrum analyser, it's helpful to relate that information to the time domain." Issues which need to be explored include radiated and coupled emissions, switch mode power supply effects and the sources of spurious noise.

He gave an example of a Polish company that saw a problem with its design, but which couldn't work out what was happening. "The problem turned out to be in sync with the inverter driving the product's lcd. When the designers saw this relationship, they were able to cure the problem."

Smith sees rfid as having strong potential for MDO4000 devices. "When you put a tag near the reader, there's a burst of rf energy which powers the tag. The tag then clocks the transmission from the reader through an amplitude modulated signal. You can see this activity with the MDO devices; you can see the frequency characteristics, measure the frequency amplitude and look at the commands being sent to and from the reader. By folding back the frequency domain information into the time domain, you can validate the operation of the system."

He gave another apparently low level example; a wirelessly controlled light switch. "Designers of these products want the smallest possible battery in the switch, but the device needs to operate correctly. How well does the power supply hold up when the switch is turned on? What's the regulation like on the power supply during the transmission pulse? How loong does it take for the oscillator to start oscillating? It's a simple example of system level design and designers can see time correlated events across the whole system."

When the MDO4000 range was launched in 2011, the four models unveiled addressed rf frequencies of up to 3GHz and 6GHz with analogue bandwidths of 500MHz and 1GHz. Now, building on what it says has been a very successful introduction, Tektronix has extended the range towards the entry level with the introduction of two additional devices, with analogue bandwidths of 100MHz and 350MHz, with an rf range from 50kHz to 3GHz. The new devices have 16 digital channels and one rf channel.

The 3GHz range is there for a reason, said Smith. "There are a lot of applications which use the ISM band at 2.4GHz."

Tektronix claims the MDO4000 models offer the functionality of an oscilloscope, a logic analyser, a spectrum analyser and a protocol analyser in the one device which, it continues, costs the same as an entry level spectrum analyser.

Further investigation can be conducted using SignalVu-PC software. "This is a user interface available for spectrum analysers and for high bandwidth, high performance scopes," Smith noted. "What it does is enable users to to I and Q analysis on modulated rf signals."

He said that customers would previously have had to buy a high end scope to get access to this software. "Now, they can do it at lower cost using the same software, but running it on a pc."

SignalVu-PC normally runs on a Windows based scope. As the new MDO4000 models are not Windows based, the software has to be hosted on a pc. "Engineers capture all the waveforms they need to, then save them as a .TIQ file, which can then be imported into SignalVu-PC. Analysis of the information can then be done offline," he said.

The package allows analysis of amplitude, frequency and phase modulated signals, as well as audio. Users can set time measurements and make general purpose modulation analyses. Also available are pulse measurement features, along with what Tektronix calls 'flexible' orthogonal frequency division multiplex analysis.

* Meanwhile, Tektronix has launched the TPA-N-PRE, a preamplifier for the MDO4000 range. This lowers the noise floor of the rf channel to allow low level signals to be observed without reducing the dynamic range.

"While the MDO range has a dynamic noise level of about 148dB," Smith noted, "we have had requests for more sensitivity from some customers." The preamp has a gain of around 12dB, which adds another 10dB to the noise level. Using the preamp will require a firmware upgrade, which can be downloaded from the web.

Author
Graham Pitcher

Supporting Information

Downloads
45684\P37-38.pdf

Websites
http://www.tek.com/

Companies
Tektronix UK

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