Evaluation Engineering - December 2008 - (Page 18)

TEST SOFTWARE First, install NI-VISA or Agilent VISA. VISA is the input/output application-programming interface between your software and a speciﬁc backplane to an internal device or a speciﬁc bus to an external instrument. Install the VISA Shared Components prior to installing your IVI driver. Install Agilent Connection Expert (ACE) or NI Measurement and Automation Explorer (MAX). These tools enable you to conﬁgure your I/O. Install IVI Shared Components. These are common capabilities shared by IVI drivers and normally provided with any IVI drivers you download but also can be downloaded from the IVI Foundation’s website. The development environment you choose, such Visual Studio, MATLAB, LabVIEW, or VEE Pro, may impact which IVI driver you install. This also could affect your version of VISA. VISA has a standard visa32.h header file for use in C and C++ as well a VISA32.bas header file for Visual Basic. Agilent and some other vendors offer a VISA32.cs for C# and a VISA32.vb for VB .NET. Install an appropriate IVI-C or IVICOM driver from the instrument vendor. Conﬁgure an IVI driver session in your chosen development environment. Hook up your equipment, then write, compile, and run your program. This also will include a test executive sequencing your test scripts. Since we are using VR as an illustrative tool, we can conﬁgure interactions between hardware and software without hard-coding I/O to each instrument from the previous step. After expanding the appropriate folders, choose the key function nodes; for example, the waveform output and the channel 1 waveform data under the waveform generator conﬁguration. By dragging and dropping the nodes into a sourceand-sink dialog box, a data or logic interaction is set up for their methods and properties. Figure 3 shows that six overall sets of interactions and conﬁgurations are required: 1. A software waveform ﬁle to hardware waveform generator. the SI LXI waveform generator, the VR macro recorded it in terms TestStand understood: “Set Agilent N8241A, Channel1, Channel1OutputEnabled value to TRUE”. Figure 4 shows this node highlighted in the IVI-C driver for the SI waveform generator along with a directory view of the four SIs in blue, the three software algorithms in green, and the computing in red. Agilent’s driver includes the base functionality, the extension capabilities, and the additional functionality of the proposed SI IVI waveform generator class driver. Loading my square waveform and driving it through the waveform generator using its 1.25-GHz internal clock resulted in the signal coming out of the WG at 128 MHz. Choosing a square waveform makes the overall distortion obvious. THD is a measurement of harmonic distortion caused by a signal passing through a nonlinear device, which adds content as harmonics of the original. It usually is expressed in percent as distortion factor or in decibels as distortion attenuation. Accordingly, THD is the ratio of sum of the powers of all harmonic frequencies above the fundamental frequency to the power of the fundamental or Figure 4. A Virtual Rack Directory With Waveform Generator Channel Output Enabled 2. A hardware waveform generator to hardware up-converter. 3. A hardware up-converter to hardware down-converter. 4. A hardware down-converter to hardware digitizer. 5. A hardware digitizer to software spectrum measurement. 6. A software spectrum measurement to software THD calculation. Since many engineers use NI TestStand as their test executive, I configured the VR macro recorder to target its output to TestStand. As a result, VR turned every action and instrument conﬁguration into a TestStand test step and saved my whole set of steps as a TestStand sequence. For example, when I clicked on the node to enable the output of channel 1 on Consequently, the 128-MHz signal has a 3rd harmonic at 384 MHz, a 5th at 640 MHz, and a 7 th at 896 MHz. Therefore, Sure enough, the output from the C++ THD calculation coming from the spectrum measurement reads 0.2297. Rules of Thumb to Ensure Good Results Many engineering decisions are required to get everything working right. Here are some observations that may help. • Choose a software development environment that meets your long-term 1 8 • E E • December 2008 www.e v al u a ti o n e n g i n e e r i n g . c o m http://www.evaluationengineering.com

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Evaluation Engineering - December 2008 Contents Editorial Product Briefing Test Software C-V Measurements Nanoelectronics Test Product Guide Company Guide Machine Vision EMC Test Index of Advertisers

Evaluation Engineering - December 2008

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