The virtual instruments are standard SCPI programmable and are available as standard Dynamic Link Libraries (DLLs). This offers completely language independency, and an application program can be written in any language - LabVIEW, C++ or BASIC for that matter for interfacing to the SCPI-VIs.
Examples of InstruWARE functionality beyond the more basic HW functionality include support functions for multi-tone signals in an arbitrary waveform generator, or post-processing functions for a digitiser.
The post-processing functionality includes: Zoom, frequency transformation, histogram transformation, advanced limiter test, and several types of averaging techniques (envelope, min./max., scalar, complex, etc.). The frequency transformation is performed with a highly advanced arbitrary block length FFT. Several window functions are available to compensate for non-periodic or non-coherent input signals, thereby avoiding corrupting distortion of the signal.
Software Instruments
The InstruWARE virtual instruments empower the user with the ability to perform complex mixed-signal testing in an easy and intuitive manner. Another virtue of InstruWARE products is that it is a user-defined rather than a vendor-defined technology. In other words, the InstruWARE products can be modified for the user - and by the user - to perform specific tasks defined by the user.
The software instruments, and their graphical user interfaces, are typically controlling off-the-shelf industry-standard instrumentation like VXI-bus instruments or GPIB-bus instruments. The functionality of the hardware is largely controlled by the software instrument.
For the purpose of efficient mixed-signal testing, InstruWARE virtual instruments are tailored to meet the specific needs of the user applications in mixed-signal test. Advanced functionality is built into several InstruWARE products and may include DSP processing, limit testing, etc.
Similarly, the virtual instruments stimuli part may include DSP processing for multi-tone testing (arbitrary frequency and amplitude distribution), MLS (periodic pseudo random noise), noise, frequency and amplitude sweep, etc.
The various InstruWARE modules come with a variety of efficient software functionality for optimised testing, in terms of signal integrity, processing capability, test throughput, etc. This is achieved through front-end DSP processing, for example on-the-fly feedback to the measurement process, and data reductions, both taking place as early as possible in the process.
Typical applications of InstruWARE features could be:
- Distortion measurements of ADC or DAC converters (using multi-tone, MLS, noise or single-tone signals for excitation and windowing functions in case of non-coherent signals).
- Measurement of linearity using histogram analysis (using sine, ramp or triangle signals for excitation).
- Transient response measurements of high speed DACs (using template limit testing or pulse measurement techniques: Slew-rate, rise/fall time, glitch energy, settling time, etc.).
- Filter frequency response (using multi-tone test and FFT for fast measurements, or applying traditional sweep test).
Analogue Instrumentation Control Package
The software for controlling an instrument is a vital part of a test system. This controls and enhances the functionality of the test hardware and to a large extent it does also control the instrument related costs. An inefficient instrument driver may easily slow down the throughput of an entire test system and result in added costs, often many times that of the HW instrument itself. In view of this, the software for instrument control may need an optimisation to obtain improved performance, high throughput, and minimum overhead time. Instrument drivers available from the hardware vendor or from a public domain source often do not meet such constraints, but the driver may be available at no or little cost. A good part of such drivers exists as "C" drivers and newer drivers may even be VXI.plug&play-compliant drivers. Drivers of this nature may be used as a starting point for a production test or engineering test Instrument Sub-System, and in some cases, where less critical applications are in question, the driver from the HW-vendor may even suffice. In other situations, rework is required to obtain the performance needed for a modern production test or engineering test environment and it may also need nurturing for the actual type of application domain.
In addition to the basic instrument driver itself, a number of processing elements are needed to handle the necessary digital signal processing, DSP, required in modern testing. Such DSP elements may include multi-tone test generation and control for function generators, FFT analysis, time-frequency analysis using digitisers, video test generation/analysis, optimised test algorithms for dedicated communication protocols, etc. Even for simpler modules, the inclusion of optimised macros has in many situations been seen to yield significant throughput advantages.
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In multi-tone testing the signals may be split into separate excitation groups to maintain appropriate S/N ratio. Subsequent to the test, all measured responses are combined. |
Multi-Tone Testing
In multi-tone mode, the multi-tone excitation signal is used to optimise the execution speed of the steady state frequency response measurement. The multi-tone stimuli can be programmed to comprise a sequence of multi-tone signals, each such multi-tone signal containing a sequence of multi-tones. A given signal of multi-tones may contain an arbitrary number of tones, including the start and stop frequency, as well as the frequency step type (linear or logarithmic). Programming of user defined, arbitrarily distributed frequencies, is supported. High speed and high signal-to-noise ratio can be obtained simultaneously by dividing the multi-tone into separate excitation signals using built-in, advanced and efficient heuristics.
The split algorithm will result in a number of stimuli blocks, but since several tones are applied simultaneously, the number of stimuli blocks may be less than 10% of the number of individual tones. As a result, the tests can be performed in a fraction of time otherwise needed to run a complete sweep. The separate responses from the block stimuli are subsequently combined to obtain the final frequency response curve. The phase of each tone in a given signal is randomly selected, thereby lowering the crest factor of the signal and thus preventing phase shifts in the signal path to cause unexpected overloads.
The various Instrument Sub-Systems delivered by microLEX are normally referred to as InstruWARE and assigned a type number, e.g. InstruWARE HFA621 for the high frequency analyser using the TVS621 hardware from Tektronix, InstruWARE LFA 3005 for low frequency analyser using B&K3005 digitiser, InstruWARE AWG1445 for an advanced arbitrary waveform generator utilising HP1445 AWG, and InstruWARE Audio Analyser for the analyser using the B&K3005 digitiser and B&K3105 AWG, etc.
A number of features are available with the various modules. For an arbitrary function generator, features such as amplitude sweep, frequency sweep, multi-tone test, MLS testing (maximum length sequence), etc. are supported.
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| The look-and-feel of the InstruWARE graphical user interfaces is intuitive and easy to use. Above a digitiser for audio-applications (upper) looks almost as the one for 1 GSa/s (lower). | |
The approach using multi-tone testing yields significant savings in test time, typically improving the signal-to-noise ratio, and first of all providing more realistic testing (several tones simultaneously present as in real world applications). The generation approach provided by microLEX also guarantees that the software handles the complex issue of multi-tone test generation. This will provide even novice users with the skill of generating multi-tone signals. Similarly, a number of other features for test signal generation is made available to the user. An example of the efficiency of multi-tone testing is shown in the figure above. Here the performance of a given hardware instrument is shown in two situations, one using a classic SINC algorithm (sin(x)/x ) and one using a multi-tone stimuli. The performance of the latter is significantly better than that of the Sinc signal.
A similar situation exists for the measurements. The DSP functionality included in measurement functions of the InstruWARE drivers, e.g. for digitisers like HP E1428, TVS621, TVS641, HP E1437A, HP E1429, B&K3005 and many others, comprises an efficient and versatile approach to mixed-signal functional testing. The Instrument Subsystem for such modules includes, among others in accordance with the SCPI model, the following functions:
- INPut controls input conditions, routing, filtering of analogue signals)
- SENSe controls the conversion of analogue input signals to a digital representation
- CALCulate performs post-processing of acquired SENSe data
- TRIGger provides timing and synchronisation capabilities with external events
- The CALC subsystem is responsible for processing of measurement data acquired by the SENSe function. The system offers fast fourier transforms, FFT analysis of data, etc. The FFT is extremely versatile, being a multi-radix system, which offers true arbitrary length FFT. This means no restrictions on test frequencies in coherent testing. One can choose any number of samples (e.g. 987 or 1000), not just based on the powers of 2 (e.g. 512 or 1024).
A number of windowing functions (Rectangular, Hanning, Hamming, Kaiser-Bessel, LowSideLobe, Flat-top, etc) provides efficient support for non-coherent testing as well. Calculation of distortion, noise, signal-to-noise, etc. is handled using the spectrum analysis features. Histogram analysis, averaging (linear, exponential, envelope, minimum or maximum), pulse parameter estimation, etc., are other features supported in the Instrument Sub-Systems. It also offers advanced limit testing using envelope limits and window limits with pattern recognition, where limits can be allowed to float vertically and horizontally. As a result, time and frequency domain test results can be obtained fast and efficiently using a minimum of programming efforts, and the throughput is improved as well.
Several other features are available for testing of mixed-signals when using the InstruWARE drivers. For example, a number of plug-in functions can be acquired for focussed solutions, e.g. special filtering examples (see bandpass filtering in the figure below), generation and measurement of pulse density modulation (PDM) signals, FFT of digital patterns (digitised analogue), etc.
Altogether, the InstruWARE solution appears optimal for mixed-signal testing, providing the solutions needed by the design and test people. Apart from saving significantly on time compared to making own solutions for this, InstruWARE solutions also offer the less experienced people a safe approach to mixed-signal testing. However, a public domain driver could be used instead, although it is not offering a comparable performance and functionality, and the integration with the entire system would be poorer. For that reason we recommend the use of the InstruWARE tools.
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| A bandpass signal can be obtained using DSP processing. In the example above a multi-tone signal is captured based upon a minimum frequency of 12,5 MHz, and a maximum frequency of 25 MHz. The frequency resolution is 125 kHz and the output rate is 25 MHz, thus yielding a number of output waveform points of 2000. The left display window shows the multi-tone signal; the right window shows the resulting bandpass performance in the frequency domain. | |
Limiters
The LimiTEST software tool also features measurement capabilities, and a given measured value can be used as set-up condition in a subsequent test.
Several limiters can be active simultaneously, hence allowing a test to result in different binning criteria. A database interface (SQL/ODBC) eases logging of values to a database.
Floating Limiters
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A given limiter - whether it is a tunnel limiter or a window limiter - can be fixed or programmed to float in the X-direction, the Y-direction, or both X and Y directions . This feature is of utmost importance in many practical test situations. |
LimiTEST Eases Test Programming
The LimiTEST software tool serves the task of simplifying the test criteria in mixed-signal measurements. By offering facilities for test of a signal-fitting to a given predefined shape, or some other signal characteristics, the mixed signal test becomes easy to handle. LimiTEST extends the normal approach of using limiters, or templates, by adding a broad range of new functionalities.
Limiter functions such as tunnel limiters, point limiters, and window limiters, as well as combinations of above, are available. A limiter function may be programmed to yield a fixed limiter function or a floating limiter. The latter allows for test of signal shapes having tight tolerances, and at the same time allowing for production variations for example associated with the behaviour at frequency roll-of. The frequency itself may not be of critical importance and varies as a result of production tolerances - which may be as much as +/-20%. However, the requirement for the actual shape of the frequency fall-off response of the given signal is high with respect to slope, over-shoot/under-shoot, ringing phenomena, etc. This calls for a rather tight tolerance of the limiter envelope. Using the floating limiter option this can be obtained. A tight envelope may be programmed - and at the same time, the entire limiter can be set to float for example +/-20 % in the X-direction.
