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Issue
Seven

Analogue Conversion

This may be the Digital Age but we live in an analogue world, and Scotland – like most other countries – has a shortage of people with analogue design skills. To address this problem, Dr Martin Reekie of the University of Edinburgh has developed a new teaching tool which turns students on to the wonders of analogue systems... …

Analogue Conversion

Dr Martin Reekie says that analogue design is  sometimes seen as one of the “Black Arts” of  modern technology. Digital systems may grab all  the headlines but almost every electronic device  needs analogue components in order to function,  and designing them requires more flair than  algorithmic competence. 

What makes analogue technology so challenging,  Reekie explains, is that it is not always an exact  science – experience or “inspiration” can often  play a critical role in design, and that is why it’s  hard to learn, as well as hard to teach.

 In fact, according to Reekie, who runs the analogue  design laboratory at the University of Edinburgh, it  can take up to seven years of practical experience  and further training for the typical graduate to  become truly productive in the commercial  analogue design environment. 

Reekie has spent his career in the analogue and  mixed-signal world, and until very recently, feared  that we were not producing a new generation of  analogue-trained engineers. Too many students  were being seduced by the ubiquitous nature of  digital technology, and being turned off analogue  because they perceived it as being old-fashioned,  too complex and full of uncertainties. As a  consequence, today’s analogue experts are all  getting older, and are not being replaced fast  enough as they retire. 

Industry is crying out for more analogue-trained  engineers, says Reekie. Digital solutions may be  very good at processing a lot of information very  quickly, but the interface with the real world is often  analogue. Analogue solutions often consume less  power, and as a consequence, mixed-signal  systems using both analogue and digital  techniques, are becoming increasingly common. 

Even in systems which appear purely digital, all  the very fast signals are essentially analogue,  and then we have the EMC and EMI problems,  both analogue in nature. When we add RF  communications, which are also purely analogue,  we see that analogue is posing ever greater  challenges to designers and, rather than fading  away, it is gaining in importance. 

Typically, electronics companies find they need  about one analogue designer for every 10 digital  designers. However, while we have a lot of digital  designers, we don’t have many analogue engineers.  It’s relatively easy to design a device that fits  millions of transistors onto a chip, following well-  tried and tested procedures, but even though the  analogue section may only account for 10 per cent  of the chip area, it can take up 90 per cent of the  total design time.  One factor that makes analogue different from  digital is that analogue design is so varied. 

 Analogue circuits tend not to have one of the big  advantages of digital – designs are not so easy to  reuse. With digital, you can easily reuse a system  designed for another project, but in analogue, even  small changes can mean a complete re-design.  As a consequence, while digital design automation  is absolutely routine, it is much more difficult in  analogue systems. 

Finally, says Reekie, digital simulations are reliable  – if you key in the right data, an accurate answer  pops up on the screen. As Reekie puts it, “digital  does what it says on the tin.” However, when  dealing with analogue systems, it is sometimes  not even clear what questions to ask the simulator,  and when the answers do come, they are only  approximate and have to be interpreted. 

When learning how to design analogue systems, it’s  important to learn from mistakes. And according to  Reekie, when designing analogue systems it is very  easy to make mistakes.

The Inside Story

In the past, students used discrete bipolar transistors to learn the ins and outs of analogue design, but most electronic devices today use more fragile MOS (Metal-Oxide-Semiconductor) transistors which don’t survive for long in the classroom.

To solve this problem, Dr Reekie developed a new integrated circuit, incorporating several small MOS sub?circuits, plus many individual MOS transistors, all connected to pins on the chip, which is mounted on a PCB (Printed Circuit Board) behind a layer of protection devices. This enables students to work with the MOS transistors and sub-circuits, without breaking them. Scottish Enterprise funding enabled Reekie to produce 70 chips, which Edinburgh students started using in 2006.

The prototype was highly successful, but Reekie has now developed a new PCB for the chip with more advanced new features such as a built-in microcontroller, display screen and signal generator, so students can work on the chip, while at the same time seeing how it interacts with digital circuitry in a fully mixed-signal environment. The new PCB was introduced into the course in September 2008. 

"When things don’t work, it’s very exciting,” says Reekie.

In an industrial environment, “exciting’” may mean that the company folds, or the designer is fired, but in the University environment it’s educational and fun. When a student tries something different and produces unexpected results, it may be a breakthrough, or it may mean that something bursts into flames, but there’s always something to learn.

Reekie says that more analogue “gurus” are needed in industry to develop new products, and in universities to promote the curriculum. And Reekie himself has come up with a novel solution which may address both of these issues at once.

With £100,000 funding from Scottish Enterprise’s Analogue Skills Initiative (ASI), plus support from companies such as Wolfson Microelectronics and Analog Devices, Reekie has developed a new integrated circuit (IC) that allows students to design small analogue circuits, make them, and compare the results of computer simulations with practical measurements.

One of the things the new IC demonstrates to students is that with analogue systems, what you get is not necessarily what you expect. When a new design is simulated, it may seem to function perfectly, but when it is actually made, the results can be rather different. This is particularly true if the designer failed to take into account “parasitic components”, as they can dramatically change the performance of a circuit.

Sometimes, says Reekie, the wrong question is asked at the start. “You simulate the circuit you think you have, not the circuit you have,” he explains. “Then you can’t expect simulation and reality to agree.” 

At the simplest level, Reekie says, “doing is  learning” for analogue students. The new IC allows  students to build small analogue systems, almost  as though they were designing, fabricating and  measuring a new IC every time. Over two years,  80 students and six demonstrators – teaching staff  and PhD students – have worked with the new IC  and Reekie says they’re learning all the time how  to make better use of its features. In addition,  adds Reekie, it is helping to develop students’  problem-solving skills, while “removing the fear”  from analogue studies.  The new IC converts what used to be a problem  into something that actually motivates students,  by providing a practical teaching tool that enables  students to see their ideas in action, and help  them appreciate the gap that often exists between  simple theory and practice. When the students see  that simulation and reality are different, they know  that they can’t change reality, so they begin to see  the problems associated with analogue simulation.  They then adjust their simulation, to make it better  model the true situation, and find that simulation  and reality begin to correspond. Throughout the  whole process, they gain the experience of  practical and theoretical work that is so vital to  analogue designers.

In most electronic devices, analogue signals are  converted to digital data and, over the last few  years, students have converted in a similar way.  However, if Reekie and his new IC continue to  succeed, perhaps the process will reverse and the  shortage of analogue design skills will soon be a thing of the past.

 

"Analogue Conversion". Science Scotland (Issue Seven)
Printed from http://www.sciencescotland.org/feature.php?id=27 on 24/04/17 07:59:40 PM

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