Industry view: Mesuro on load-pull testing

-September 04, 2012

RF and microwave devices need to operate as part of a system. Because they are typically not 50-Ω devices, they need to be matched to the rest of the system. That requires characterization of performance parameters like output power, gain, and efficiency under a variety of loads to determine the best matching conditions. The default approach to this load-pull testing is to use a manual impedance tuner. Recently, the industry has been using active load-pull testing techniques to simplify and speed the process. We sat down with Simon Mathias, vice president of sales at Mesuro Limited, to learn more about their open-loop, active harmonic load-pull testing.

Kristin Lewotsky: Your open loop, active harmonic load-pull entry is a hardware and software solution based on technology developed at the University of Cardiff. Can you talk more about it?
Simon Mathias: We’re basically using signal sources to maneuver the load conditions around the Smith chart and present loads to the device under test. The measurement system comprises a number of key components, many of which can be re-used from existing laboratory equipment. The key hardware element to the WaveForm Engineering approach is the RFTS Multiplexer assembly. This unit comprises all of the components required to be able to provide the harmonic control necessary. The multiplexer and bias tee networks are capable of handling up to 150W CW, over a broad bandwidth.

Courtesy of Mesuro

K.L.: What is the primary benefit of the approach?
S.M.: The approach takes them closer to the theoretical understanding of the transistor they’re designing with. One of the main advantages is reduced design cycle times and processing times. It gives them the ability to take the amplifier information generated by the simulator back to the device to verify that it works before fabrication ever takes place. That is a major benefit in terms of time saving and changing the way they can design with a system and a transistor.

K.L.: How does it different from traditional passive or closed-loop systems?
S.M.: The loop is controlled by computer system so there's no chance that if the device oscillates, the system will react and take off with it. Our WaveForm Engineering software looks at the system in a time domain set-up. Given that waveform voltage and current waveforms are available, we can then use that not just to give the designers more information but also to allow them to engineer a particular waveform shape and class of operation, i.e. class F or class J, enabling them to achieve the desired performance.

Plotting the results of the 5W Si LDMOS device performing in the inverse class-F mode over a range of drive powers shows the device developing some higher currents at high voltages.

K.L.: How do the system capabilities improve testing?
S.M.: The system includes hardware that has broadband, high-power bias tee capability, enabling users to operate over a wide bandwidth and measure harmonics at high power. In order to control the harmonics presented and the impedance presented to the device, we use a multiplexing technique. If you look at the passive tuner world, when you set your desired impedance that is perfectly set, but in the tuner it creates additional random impedances that you don’t control. We try to ensure that those random events are closer to 50 Ω than they would potentially otherwise be and, therefore, the device doesn’t see anything that creates oscillations or instability.

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