13 Jan 2014
Modular & Software Test Instruments Improve Efficiency
Editor Ian Poole looks at the new trends in modular test instrumentation and software defined instruments to see what advantages they bring.
Investment in test systems and test time can form a large part of the costs associated with product development and manufacture.
Also building and maintaining test systems for field repair can become exceedingly expensive, especially as these systems need to be maintained over time.
There are many instances where high value and relatively low volumes mean that costs associated with test become a major issue and there can be pressure to reduce them.
These issues can be overcome, at least in part by using modular test systems as well as employing a new genre of test instruments that can be configured by software to exactly fit the bill for the test required.
They can help address the issue of legacy test equipment by providing the capability to be reconfigured and thereby meeting the exact needs over time.
These new software defined instruments, or as we’ll discuss later, a new class of instruments that are “software designed”, provide an excellent method of reducing costs while still maintaining the effectiveness of the testing.
There are several issues with setting up a test system using traditional test instrumentation:
Cost: Using traditional test instrumentation, it can be very costly to set up a test system using stand-alone test instruments, even though they may be controlled via GPIB, Ethernet, etc. Many companies seek to reduce the total cost of ownership, and some legacy systems can cost significant amounts to maintain and run.
Obsolescence: When setting up a test system for an item that may be in use for several years, the obsolescence issues associated with the test equipment can become an issue. If one item fails, getting it repaired or even replaced can become a problem.
Lack of flexibility: Some large test systems can have little flexibility. They address a specific need or sometimes a specific item of equipment. Reconfiguring test systems can also be time consuming where it is possible to use the basic system for different test applications.
Space: Test systems built from boxed instruments can be very large, taking up valuable laboratory space.
Performance: Using traditional test equipment, it may not be possible to completely configure the equipment to the exact requirements of the testing to be undertaken. Improvements may also be achieved in other factors such as reliability and test data collection and processing.
These issues may be addressed in a number of ways. Modular test equipment, and the use of software configurable, or software designed instruments enable far greater levels of flexibility to be achieved.
However change can be difficult to manage, but with planning and forethoughts, a move to more modern flexible test instrumentation can provide significant cost benefits as well as general operational and performance improvements.
Modular test systems
The use of modular test systems allows a number of advantages to be brought to the test system.
There are a number of modular standards that can be adopted. The first was VXI, but this is not as widely used these days. The standard that has really taken off is PXI. Based on the computer backplane PCI standard, it has been adapted for test instrumentation.
PXI provides top line functionality, enabling accurate triggering and timing to be achieved. It also is the ideal platform for undertaking tasks that may require large amounts of data to be collected because the backplane allows huge amounts of data to be passed very quickly.
While some may have the view that PXI instruments cannot meet the standards of boxed instruments, there are some very high performance instruments available for operation at all frequencies. While this perception may have been true some years ago, this is not the case now.
RF Instruments using a Modular System
There are also many advantages to utilising PXI test systems.
Modularity: The modular approach enables systems to be easily built that exactly meet the requirements for any given requirement. Systems can be built up very easily from the constituent instruments using one or more racks to provide exactly what is needed. Having a common rack means that there is a single interface format.
Space savings: Instruments do not need to contain all the processing, power supplies, front panels and other space taking items. These are all handled by the PXI chassis and controller. Accordingly very significant space savings can be made as the test instrument only needs to contain the circuitry required for providing the test instrument itself without all the peripheral elements.
Cost savings: Again, by not having to provide a power supply, overall control processor, front panel and other items, significant cost savings can be made. As the chassis and controller are shared between the instruments, so too are the costs making this a very cost effective approach.
Easy reconfiguration: using a modular approach, reconfiguration of a test system is much easier. The chassis, controller and the relevant instruments can all be re-used. This considerably reduces time in reconfiguring, and costs.
Performance: The performance of test instruments used in modular systems like PXI is equally as good as those that are contained within traditional boxes. The cost benefits also mean that better instruments can be obtained within a given budget enabling much higher levels of testing to be undertaken. Also with the very high data transfer speeds that can be achieved across a PXI bus, much faster testing is possible. Test speed improvements of ten times or more can be made on occasions, thereby improving throughput or even making more comprehensive testing possible.
Software configurable test systems
While the concept of modularity and the use of systems like PXI bring very significant benefits, another development has been introduced that is bringing further benefits to users of test instrumentation.
The concept of the software defined radio, SDR, has been around for many years, although it is only in recent years that the technology to support the idea has become really viable.
The basic idea has also been taken up in the test instrument domain. The main idea is to allow the programmable device - an FPGA within the PXI test instrument to be configured to enable the instrument to exactly meet the test requirement. Although a standard configuration may be used for most applications, being able to have a specialised configuration enables the test instrument to exactly meet the needs of the test.
Commented Jeremy Twaits of National Instruments: “Instruments that incorporate an open FPGA have led to the term “software-designed”, since the end user can fully customise the functionality of the instrument. This could be through adding custom triggering, application-specific measurements or the capability to use new digital protocols onto the system. Essentially, the user is able to edit the firmware of the device to meet their needs – they aren’t just defining in software how the data is used, but designing new functions the instrument can perform.”
Using this approach significant further test speed improvements have been made. In some cases this has enabled manufacturers to characterise ICs, circuits or systems in ways they have ever been able before. This has allowed far better performance to be achieved from the final devices or systems.
FPGAs give major improvements
One of the reasons for the dramatic speed improvements is that the FPGA that can be reprogrammed is very close the actual testing. This enables the system, almost at the test interface to be set to the optimum configuration to enable the test to be performed in exactly the way that is required.
Commented Twaits: “Often, the bottleneck in a test system is not the instrument taking the measurement, but rather the bandwidth of the bus and the processing speed of the CPU. A high bandwidth bus like PXI Express can solve data transfer issues, whilst many processing limitations can be overcome using an FPGA. Performing actions like averaging and FFTs inline on an FPGA takes the burden off the CPU, by reducing the amount of data needing to be passed back to the host. Additionally, multiple channels or measurements can be processed in parallel, by dedicating elements of the FPGA to each channel.”
RF Instrument Using an FPGA
This can lead to speed improvements in excess of 100 fold when compared to more traditional test methods.
Doug Johnson of Qualcomm agreed: "Using the software-designed NI PXI vector signal transceiver and the NI WLAN Measurement Suite, we improved test speeds by more than 200 times compared to traditional rack-and-stack instruments while significantly improving test coverage."
Whilst modular instrumentation has existed for at least a couple of decades, the concept of software designed test instruments has only recently been introduced. With development apace, further strides will be seen in the years to come. In the meantime, the concept is providing many manufacturers with speed improvements and also the ability to change the instrument via software downloads to provide exactly the test that is needed. Again this can ensure that systems can be maintained for longer.
Change in any organisation can be difficult to implement. It requires processes and people to change as well as the instrumentation. This may require additional investment in the short term. With capital expenditure sometimes being difficult to have authorised within a company it can bring significant benefits in the longer term.
Some key pointers when looking at how to address some of the issues with test instrumentation can be summarised as follows:
Don’t fall into the trap of just replacing existing instruments
Take the opportunity to solve the underlying problem
Look for opportunity to take advantage of advances in technology
For example the move from legacy analogue instruments to digital-based processing often creates opportunities to combine measurements or increase test coverage without additional cost.
By adopting new technology and new ways of working, a more effective test scheme can be developed for less cost over the project.
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About the author
Ian Poole is the editor of Radio-Electronics.com. Having studied at University College London to gain his degree he went on to undertake a career in electronic development working for companies including Racal. He became the hardware development manager at Racal Instruments where he was in charge of the hardware development activities within the company. Later moving in to freelance work as a consultant he also developed Radio-Electronics.com to become one of the leading publications for professional electronics engineers. He is also a Fellow of the Institution of Engineering and Technology and is the author of over 20 books.
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