Greater Harmony Boosts Amplifier Efficiency
For many RF engineers, their primary goal is to boost amplifier efficiency. If they can succeed in this regard with a mobile phone power amplifier, they will have extended this handset’s battery life, and in turn increased the chances its owner has to get through the day without having to put their phone on charge. And if they are designing an amplifier for a base station, their gains in efficiency will lead to power savings, reduced cooling demands and lower running costs.
The tried and tested route for developing a new amplifier involves building a part, measuring its performance and using this data to uncover areas for improvement, before constructing a better device. Often this process has to be repeated two or three times before a device is good enough to go into production. This makes it an expensive and time-consuming exercise to bring a device to market, because it can take several months to fabricate and test a chip, and the cost of every iteration runs into six figure sums.
Worse still, the final device is unlikely to operate anywhere near to its true potential. That’s partly because traditional on-wafer measurement techniques are unable to deliver a true characterization of the device, because they are plagued by influences from the local environment, such as vibrations that shake the testing equipment, including the probes. But the biggest drawback of conventional measurements is that they fail to uncover the impact of higher harmonic signals, which can actually deliver a constructive contribution if they are made to work in-phase with primary signal. One company that is addressing both of those weaknesses – device measurements that are plagued by external influences and device designs that fail to account for higher harmonics – is Cardiff University spin-off Mesuro.
This start-up, founded in 2008 and located just a stone’s throw from Cardiff University’s Centre for High Frequency Engineering, has an incredibly strong track record in improving the performance of RF devices based on a variety of technologies including LDMOS, GaAs and GaN.
“When we’ve looked at other people’s devices, they often give us a device that they know very well. They want to know if we can improve on that, and the answer is that so far we have a 100 percent record in improving people’s devices," says Mesuro’s Chief Executive, Richard Emsley. The scale of this improvement varies from customer to customer, reflecting differences in the lack of device optimization. “In some cases you can make an unbelievable jump," says Emsley. “In fact, sometimes the customers don’t believe the jump, and you have to go to some lengths to prove the reality."
Although customers tend to want to keep the details of their improvements under wraps, Mesuro is able to share the results of its efforts at increasing the efficiency of a low-power 900 MHz amplifier made by TriQuint. This device, which was built with a GaAs technology that the US outfit knew very well, initially delivered an efficiency of around 65-70 percent. Following harmonic tuning by Mesuro, efficiency rocketed to over 90 percent.
“I think that’s still an industry record," says Emsley, who adds that the engineers at TriQuint were also pleased with the characteristics of the waveforms producing this far higher efficiency, and the mode of operation of the device.
The amplifier required modifications before it could be used in the end application, so the team at Mesuro, in partnership with researchers in Cardiff’s Centre for High Frequency Engineering, built a MMIC around this device. It took just one go to build that circuit, which delivered a highly creditable efficiency of 82 to 83 percent.
“Bear in mind that it was a student designing the MMIC, and most commercial customers will take two to three goes to get a design correct," says Simon Mathias, Mesuro’s Vice President of Sales. “This was a very simple design, but it proved a point: You can get device data, translate it to a usable circuit design, and achieve very good performance."
A different approach
The key to this fast, successful approach for understanding and optimising device performance is that it is the device that is the model. Engineers adopting the Mesuro method run the device under its standard operating conditions and then optimise its performance by adjusting current and voltage waveforms, taking in account the impact of harmonics. “You are not mathematically modelling," explains Emsley. “You are obtaining a complete emulation of the environment in which the device is going to be used."
A more technical description for the measurement technology employed by Mesuro is open-loop, activeharmonic load pull.
Dynmaic load line measuremnets with Mesro’s system
The term open loop refers to the architecture of the system: It defines how the load is presented to the device, and how this device is driven. A more stable testing environment results from using an open loop rather than a closed one, according to Emsley: “The device reacts, but only to the instructions that you put in; and because of the open loop architecture, the tendency for the device to oscillate is significantly reduced."
To realise active load pull, the device is driven by an amplifier so it overcomes the losses in the system. In Mesuro’s case the approach is also described as ‘harmonic’, because the first, second and sometimes the third harmonics are controlled in this type of measurement.
Benefits associated with measuring the device in this manner extend beyond uncovering pathways to higher efficiency, and include the possibility of improving device reliability. That’s because insights provided by the Mesuro’s technology can expose overdriving of the device, which can be to blame for its modest efficiency. There are occasions when devices driven at their peak voltages produce current when they should shut it off, and this destroys the device.
This understanding of the relationship between current waveforms, voltage waveforms and reliability comes from the research group led by Paul Tasker, head of the Institute of High Frequency and Communications Engineering at the University of Cardiff. Tasker founded this group when he took up a chair in the department in 1997, motivated by a belief that there was tremendous room for improvement in the characterisation of power amplifiers.
An exmaple of how Mesuro’s system can be used to map out the boundary conditions of a semiconductor process/ device – the kneee region and the pinchoff region – using multiple bias points
Working together with his first student, Johannes Benedikt – now CTO of Mesuro and a professor in Cardiff’s Centre for High Frequency Engineering – they spent several years developing and refining the measurement system until it reached a level high enough to attract the attention of industry.
The system that they perfected employed a pair of Tektronix instruments: An arbitrary waveform generator that provides very good control through two channels, and a high-quality sampling oscilloscope. In addition, it featured their own multiplexer system that involved hardware insertions between the Tektronix’s instruments and the device, plus homespun software.
Getting off the ground
When Tasker his co-workers went to various tradeshows, they tried to convince employees of Tektronix of the tremendous promise of their measurement technique. Their words initially fell on deaf ears, but they persisted and their efforts were rewarded in 2008 when they were invited to showcase their system on the Tektronix stand at the International Microwave Symposium (IMS) in Atlanta.
Their demonstration at this conference was a big hit, with delegates queuing three or four deep at the Tektronix stand to witness the benefits of this novel, sophisticated measurement technique. The tremendous level of interest convinced Tasker and his colleagues that their technology could have mass industry appeal, and when they returned home they made enquiries within the University on how to exploit the commercial potential of their measurement technique.
These engineers were directed to Fusion IP, which has rights over university-owned IP at Cardiff University. When Fusion looked into the matter, they felt that the prospects for this venture were worth investigating, so they started a due diligence campaign, calling on the services of Emsley.
To gauge support for the innovative measurement technology, Emsley headed to the next big conference in the RF calendar, European Microwave Week. There he talked to most of the big instrument vendors, plus AWR, one of the world’s leading providers of software for designing circuits. “AWR’s interest and partnership with us extends from that very first meeting. They said ‘We want to work with you. When is this going to be ready?’ People were pushing for delivery dates."
At that point Emsley realized that if Mesuro launched right away, it could make a splash at a very, very good time for the industry: “There was a lot of pressure within the industry to look at highly non-linear device technologies, and obtain much better results than had been obtained previously."
Shortly after this conference, Emsley switched from a due diligence role to running the company. He then boarded a plane to the US to try and tie up a deal with Tektronix. The trip was highly worthwhile, with the two companies signing a memorandum of understanding.
Following this key milestone, Fusion stumped up £150,000 to get the start-up off the ground. The cash enabled Mesuro to build its first measurement system in time for launch at the IMS meeting in Boston, June 2009. While preparing for this meeting, the company closed a second funding round that netted £1 million, with contributions coming from Fusion, Finance Wales and ERA foundation.
From then on the Cardiff start-up has been working with an increasing large number of customers. Often, relationships begin with what Mesuro describes as ‘measurement services’, which is essentially device characterization.
“From there, it will develop into demonstration evaluations – that’s us providing software and hardware solutions to them," explains Mathias. “We integrate that into the existing equipment that they have in the labs, or new equipment that they are buying in to support this activity." Although Mesuro developed its technology using Tektronix’s signal generators and analysers, its hardware and software can work with all the leading brands of instrumentation.
“Recently, we’ve become technical partners to [instrumentation manufacturers] Rohde and Schwarz, and that has been based on their experience of our engineering team," says Emsley. Market success is also behind this move, with sales of systems made by Mesuro employing Rohde and Schwarz instruments. One attraction of this pairing is that it is possible to build a relatively compact measurement system by selecting a particular Rohde and Schwarz vector network analyser, which also incorporates signal sources.
The benefits associated with Mesuro’s measurement techniques are by no means limited to gains in power amplifier efficiency: It can also help optimise the combination of power and efficiency in amplifiers used to make jammers in the defence industry; and it can also enable improvements in other non-linear devices, such as mixers and oscillators.
What’s more, Mesuro’s technique can aid the development of semiconductor devices, or play a role in quality control. “We can tell device engineers an awful lot about their device technology – how the currents and voltages and biasing points work together," explains Emsley. Armed with this information, it is possible expose trapping effects, uncover pinch-off effects and also understand how the device really operates.
In addition, this current and voltage data – which is inaccessible with traditional techniques – is a valuable aid to the modelling of semiconductor devices. “In general, a foundry will be providing models of their process for their customers to use in simulations," says Mathias. “If they can improve the accuracy of the information contained in those models, that’s valuable to them and their customers. That’s another area where our system is used a lot."
The Mesuro MB20 open loop acttive harmonic integrated test system. This features three harmonic load pull over the 0.8-6.0 GHz range and is capable of testing 20 W CW active devices
Emsley says that the customers that have bought a Mesuro measurement system are very enthusiastic about the technology: “[Those] that we had last year will all re-order or have re-ordered."
To spur further growth, Mesuro is going to devote more resources to sales, a move that will be financed through the closing of another funding round this year. Revenue should also increase with a streamlining of its measurement services. “We have a fruitful measurement services pipeline at the moment," explains Emsley, who reveals that many of these requests involve similar types of measurements at similar frequencies and powers. “So we need to establish dedicated systems to enable us to deliver that much more effectively and much more quickly."
With a growing band of loyal customers and a clear vision for the future, Mesuro looks well placed to get through the testing times ahead. And when it does, this Cardiff spin-off should win further plaudits for its key role in driving up the performance of non-linear devices.