Evaluation Engineering - December 2008 - (Page 17)

acquisition, initiating the waveform acquisition, and returning a waveform. An IVI digitizer class driver for this SI component will cover common attributes of the typical digitizer as well as typical extended functionality found in digitizers that are more complex. High-speed data streaming capability that exceeds the I/O bus capability is an example of extended functionality that might be required. In this case, your program would implement the IVI digitizer extended class. The SI down-converter translates a frequency or group of frequencies from a higher frequency, usually in the gigahertz range, to a lower or intermediate frequency (IF) where the signal may be more readily digitized and processed. It also provides amplitude control and band limiting. Very high frequency signals currently can’t be handled by a 100-MHz digitizer and a 500-MHz waveform generator.4 As a result, IF bandwidths are needed for the converters. Today’s fastest converters are about 1 to 2 GS/s. At a sample rate of 2.5:1, they can handle a signal up to 800 MHz. Anything above this frequency will require this additional complexity. Figure 2 shows that the SI up-converter translates a frequency or group of frequencies from a lower frequency, usually in the megahertz range, to a higher CW or modulated RF/microwave signal. This CW typically is up to 26.5 GHz for wireless networking, radar, and satellite links. Often, the up-converter incorporates signal conditioning within its functionality. This core capability will be in the IVI SI up-converter driver with potential extensions for FM, AM, or pulse modulation as well as I and Q input and a reference frequency. Waveform generation most likely includes inputs such as data, a reference frequency, and a synchronization signal. Outputs consist of the analog output, the reference frequency, and markers. Extensions probably will be functionality such as differential signals, waveform depth, specialty waveform creation, and advanced sequencing. Different Technology Approaches to SIs Remember that SIs are software and hardware components based on elemental measurements or finer granularity. In effect, it is breaking an integrated instrument box into its elemental measurement hardware and software components. For example, an integrated spectrum analyzer consists of an SI hardware down-converter, an SI hardware digitizer, and several SI software spectrum measurement and calibration components. There are different technical approaches to delivering SIs. For instance, National Instruments uses virtual instrumentation technology to create SIs. NI deﬁ nes virtual instrumentation as a software-deﬁned system where software based on user LXI comes in three classes. Class A provides a standardized LAN interface, IEEE 1588 triggering, and a physically wired trigger bus (WTB) interface. Class A, though a superset of PXI’s overall functionality, has physical triggering like PXI, which yields submicrosecond I/O latency. Class B features the standardized LAN interface and the IEEE 1588 triggering protocol. Class C has only the standardized LAN interface. THD Application Using IVI Drivers With SIs Let’s illustrate the interaction between SIs and IVI drivers with a simple total harmonic distortion (THD) application using both IVI-COM and IVI-C drivers. We will use the interactive software platform called Virtual Rack (VR). This will eliminate using a programming language for hard-coding the instrument to the I/O interface and allow us to follow the application logic. To s i m u l a t e the UUT source, we will use a waveform ﬁle in combination with an SI waveform Figure 3. THD Application Using SIs With Disaggregated Measurement Algorithms generator and an SI up-converter (Figure 3). Instead of requirements deﬁnes the functionala typical spectrum analyzer, we’ll use ity of generic measurement harda combination of an SI down-converter ware.5 The company delivers on this and an SI digitizer and some software combination by using PXI modules in C++ that represents our numeric in combination with its LabVIEW algorithms normally in the ﬁrmware graphical programming language. PXI of the spectrum analyzer. The two C++ adds mechanical speciﬁcations and programs we need are a spectrum meaintegrated timing and synchronizasurement program that converts data tion to cPCI. from time domain to frequency domain Agilent, on the other hand, uses and the actual THD calculation. LXI-compatible instruments for its For maximum interchangeability of SI offerings.6 LXI has three pieces: drivers, use a computer with one of the a standardized Ethernet interface for following operating systems: Windows both peer-to-peer and master-slave 2000 Service Pack 3, Windows XP, or operation, a triggering protocol that Windows Vista. These operating sys enables synchronization over the LAN, tems are compatible with both types of and a physically wired trigger system IVI drivers and versions of VISA. for tight synchronization. Continued on page 18 www. ev alua t ion e n gin e e rin g.com December 2008 • EE • 17 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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