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VirtualBench is a computerdriven 2-channel 100MHz digital oscilloscope, 34-channel logic analyser, waveform generator, 3-output adjustable power supply and multimeter, all in one box. It can be driven wirelessly using an iPad or from a PC via USB. Importantly, it integrates with National Instruments’ LabView software for automated measurement and testing. VirtualBench 5-in-1 F ollowing the trend of integrating test equipment into one unit, VirtualBench is an instrument which can be used to test and debug many projects all by itself. As you can see from the photos, it’s “headless”, with no screen and virtually no controls. The interface is handled by a PC or tablet. This has some significant advantages. For one, you aren’t stuck with whatever size or resolution screen the manufacturer has decided to put on the device. We’ve seen many multi-thousanddollar test devices with screens that are inferior to today’s bargain basement tablets! So this “bring your own screen” philosophy can work quite well. On the other hand, if you forget to bring your tablet/laptop or its battery is flat, you’re out of luck; VirtualBench 24  Silicon Chip is effectively tethered to a computer. Physically, as the name should suggest, the VirtualBench is very convenient for bench-top use. At 255mm wide, 190mm deep and 73mm high it doesn’t take up a great deal of space. In fact, since it has a flat top, you can argue that it doesn’t take up any space at all. At the very least, you can stack similar-sized equipment on top. An iPad fits quite nicely. All the connectors you need frequent access to are on the front. The power button and Wi-Fi power indicator are at upper left, with the IDC connector for the logic ribbon cable in the middle and the two BNC scope inputs, calibration terminals, waveform generator BNC output and external trigger input at upper right. Along the bottom, from left-to-right are the digital I/O terminal block, DC power supply output terminal block and DMM insulated banana sockets. Features The digital I/O block provides a 3.3V power source (but only up to 20mA), ground connections and eight general purpose I/Os which can be used for controlling the equipment you are debugging or testing (more on that later). The power supply outputs are 0-6V at up to 1A, 0-25V at up to 0.5A and 0 to -25V at up to 0.5A. Each can be set independently in 1mV steps and a current limit can also be set for each output. So between these three adjustable outputs and the (fairly limited) fixed 3.3V, you can power a large range of devices from the VirtualBench without resorting to an external power supply. Interestingly, the ground terminal for the ±25V adjustable supply is insiliconchip.com.au “Hands on” review by Nicholas Vinen The front panel houses most of the input/ output sockets, including those for the oscilloscope and function generator at top and digital I/O, power supply and DMM underneath. The rear panel, by comparison, is spartan, with just mains and USB sockets, an earth point, a Kensington lock socket, WiFi antenna and ventilation. dependent of other grounds, although in most applications it would be connected to the main circuit ground. The DMM is a 5.5 digit type with relatively high accuracy. It has four input terminals and six modes, covering all the most common functions: DC or AC voltage (true RMS, up to 300V DC/265V RMS), DC or AC current (up to 10A), resistance (up to 100M), diode test (forward voltage up to 2V) and continuity test with audible tone, via iPad/PC speakers. When measuring DC voltages up to 10V, a high input impedance (>10G) option is available which can be quite handy when measuring sensitive circuits. DC current measurement resolution goes down to 0.1A while resistance measurement resolution is down to 1m; however there is no 4-wire test mode. Still, such resolu- tion is quite useful for finding shorted tracks or components. iPad interface Most of the VirtualBench’s features are usable from an iPad, over WiFi (see screenshot opposite). At the time of writing this review, there is no Android support but this is expected relatively soon; presumably, before the end of 2015. When controlling the VirtualBench from LabView, many options are available. Fortunately, the built-in help explains them in detail. This screen shot shows the details for setting up the waveform generator. siliconchip.com.au April 2015  25 The VirtualBench PC interface allows control over all the MSO, DMM, power supply and signal generator features. There are many digital channels so several serial buses can be monitored. The interface is not difficult to figure out. The scope traces are displayed in the middle of the screen and can be moved around by dragging. Vertical scaling and the timebase can similarly be adjusted using two-finger gestures. The other displays and controls are above and below the scope traces, including the DMM features, scope measurements, power supply controls and function generator. What we couldn’t find in the iPad interface are the serial bus decoding options (which are present on the equivalent PC software) or controls for the eight-pin digital I/O bus on the front panel of the VirtualBench. Presumably these more advanced features have been left out because they wanted to keep the iPad app simple. Like a lot of WiFi peripherals, rather than joining your network, the VirtualBench requires that you connect your iPad to its WiFi network (ie, it acts as an access point). The annoying aspect of this is that 26  Silicon Chip this means you lose Internet access while using it, so if you want to say download a data sheet during a debugging session, you will have to disconnect from the VirtualBench’s WiFi and then reconnect to it again later. It also means that each time you turn the VirtualBench on, you have to remember to re-join its WiFi network before running the app or it won’t work. That’s because the iPad will automatically re-connect to your normal WiFi network when the VirtualBench access point disappears. As we said, this is a pretty typical way to interface with a device using WiFi and it does avoid the need to program your SSID and WEP password into the device but in the long run that would probably be a more satisfying solution. PC interface You have two options for using the VirtualBench on a PC. First there is the dedicated interface software which works similarly to the iPad software but with the extra features mentioned that are missing in the iPad version, such as the serial protocol decoding (SPI/I2C/Parallel); see the screenshot above. We found this software particularly easy to use. For example, the menu for setting up serial decoding appears next to the button to turn the digital channels on and off when you move your mouse near it, and similarly other set-up menus appear next to related buttons. The layout is visually clean and you really don’t need a manual to figure the software out; most users will be up and running right away and will be able to figure all the functions out easily. In some ways using a scope this way is very convenient because we often find when debugging a project that we refer to schematics, PCB overlays and software on a computer when trying to figure out what’s going on. Thus we often end up swapping constantly between the PC and a scope when troubleshooting a project. siliconchip.com.au In this case, the PC is the scope interface, avoiding the need to constantly switch between two different screens and sets of buttons. This also makes it easy to do things like save screen grabs to the computer. All the basic functions work well. As a mixed signal scope, its performance is on a par with a typical, good quality 100MHz unit. Similarly, the DMM has reasonable accuracy if somewhat limited functions. The power supply is convenient but with a maximum of 1A on its 0-6V output, won’t necessarily cover all possible uses and so you may well require a separate power supply with a bit more grunt. LabView integration The other option for driving the VirtualBench from a PC is LabView and this unleashes the full power of the device. It allows you to create automated testing procedures and control them via a graphical programming interface. Automated testing is not only useful in a production environment, eg, for QA where you need to do a quick check that all the functions of a device are operating correctly before sending it out the door but also in a test, service and debug environment, such as the sort of development and testing work we do at SILICON CHIP. For example, say you have a device which has a glitch after power up, if a certain sequence of buttons are pressed in a particular order. You’re trying to eliminate this glitch by making changes to the software and/or hardware but each time you make a change, you need to check whether it has fixed the glitch or not. You can design a test procedure in LabView which uses the VirtualBench to power up your device, wait for it to be ready, simulate button presses via its configurable digital I/Os, then run whatever tests are necessary using the MSO and DMM to verify correct operation. The LabView software can then report whether the glitch is still present and if so, you can make further changes and try again. That makes such testing a lot easier and more repeatable and is especially handy if the glitch you’re looking for is short-lived. The image below shows the LabView software interfacing with our VirtualBench review unit. This is one of the example programs provided on the National Instruments website which uses the adjustable power supply and DMM features of the device to plot a This demo program steps the power supply output through a range of voltages and uses the DMM to measure current and produce the plot at the bottom of the screen. siliconchip.com.au April 2015  27 V/I curve for the device under test. This sort of test works well because the adjustable power supply has such fine-grained control over output voltage. The program itself is shown at top. This consists of a variety of blocks representing different parts of the VirtualBench device and different stages in the test, which are “wired up” together to determine a sequence of events. It is broken into five sections and the control sequence flows from left to right, with the five different steps labelled below. The first step is initialisation and this involves the software connecting to the VirtualBench device and preparing the sections which are to be used (ie, power supply and DMM). The configuration step sets the various test parameters such as what mode the DMM section will operate in (DC current in this case), the voltage range over which the power supply will be stepped and so on. Parameters such as the voltage range and number of steps are set by the user in the “Front Panel” interface in the middle of the screen. This makes it easy to adjust the parameters and run a new test without having to change the block diagram. The selected values are automatically fed by the software into the orange and blue rectangles in the block diagram which then feed into the control process. The grey box outlined in the middle, above “Perform Operation” is a “for loop” which performs a set of tasks a fixed number of times. In this case, it’s used to step the power supply through the test voltage range and read the current level from the DMM each time. The voltage and current figures are then fed to the X/Y plot in the bottom pane via the pink “Analog Data” item at right. The remainder of the items in the top pane deal with shutting the VirtualBench down once all the data has been acquired and telling the user whether there were any problems during the test (eg, if the multimeter input range was exceeded). At left of the display is the “palette” with some of the blocks that you can place in the block diagram at top in order to perform different functions. For this test, we connected a 3.9V zener diode across the power supply terminals and the plot at bottom shows 28  Silicon Chip its soft knee characteristic over the 0-10V test range and 0-500mA capability of the adjustable +25V power supply output. Ease of use LabView can seem daunting at first even for an experienced computer programmer, partly because of the large number of built-in functions but mostly because its graphical nature is quite different from the more common text-based programming systems. However with the aid of examples and a little experimentation, it doesn’t take long to figure out the basics. We managed to build a test from scratch and get it working in less than half an hour. The supplied examples help a lot. In addition to the one described above, others include creating Bode plots, frequency response plots and stimulus/response measurements using the signal generator and oscilloscope modules. The built-in context-sensitive help is excellent, once you’ve figured out how to get to it – you need to right-click on one of the block objects using the correct selection tool and then choose “Help” and you’ll get a clear explanation of how the block works – see the earlier screen grab. If you have programming experience, you should become comfortable with LabView after using it for a short time but it is a complex piece of software and will certainly take some time to master. The advantage of this complexity is that it’s very powerful once you get used to it. We decided to see just how practical the combination of LabView and VirtualBench is and to do this, we wanted to set it up to perform a function that wasn’t mentioned in the documentation and for which there are no examples. We succeeded in setting up a real time distortion analyser with spectrum display and this only took about 30 minutes to figure out. It would have been much quicker if we had more experience with the software. We set up the waveform generator to produce a sine wave and fed this to the scope input. Our distortion analysis software then reported 0.1% THD+N with the second harmonic at -65dB, third at -61dB and fourth at -72dB as sas read off the spectrum plot. While this set-up would be no match for our Audio Precision system in terms of performance, that certainly demonstrates the unit’s flexibility when teamed with the LabView software. From what we can see, there are a lot of other analysis tasks which would be possible to perform using this sort of set-up. Note though that to do this sort of advanced analysis, you need to buy the more expensive “Full” version of LabView rather than just the “Base” version. For the list of differences, see this web page: www.ni.com/labview/buy/ Signal processing functions available in the “Full” version include waveform and signal generation and conditioning (useful in combination with the Arbitrary Waveform Generator VirtualBench function), waveform measurements, windowing, filtering, spectral analysis, transformations and PID control. Conclusion If you’re a die-hard iPad user you may appreciate the WiFi connectivity functions of this unit but in our opinion, to get the full benefit, you really need to use LabView on a PC. While LabView is available for Mac OSX, the VirtualBench driver appears to be Windows-only for the moment. The way that the various functions of the VirtualBench are integrated, combined with the power of the LabView software is by far its best aspect. And note that LabView will also integrate with other National Instruments products, including their large range of data acquisition and signal generator devices. So if you like the idea of a PCcontrolled all-in-one test instrument and are interested in taking advantage of the automated testing capabilities available in conjunction with the LabView software, VirtualBench could be for you. Pricing & availability The VirtualBench is available direct from National Instruments, PO Box 382, North Ryde, NSW 2113. It retails for $2987, including GST. LabView is $550 (incl. GST) for the Base version and $1100 for the full version. For enquiries or to purchase, go to www.ni.com/virtualbench/buy/ or call them on 1800 300 800. SC siliconchip.com.au