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An A.T.E. Solutions, Inc. Internet Publication
Volume 9 Number 18 October 1, 2005


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Which single criterion is most important to you in selecting an Arbitrary Waveform Gernerator?
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X - None of the above



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  This Issue's Feature Articles

Selecting your Next AWG

 By: 

Joan Mercade, Application Engineer at Tektronix

Memory Management in Arbitrary Waveform Generators

 By: 

Arlene Meadows, Sales Manager, Tabor Electronics Ltd.

Sophisticated Arbitrary Waveform Generation Requires Sophisticated Hardware

 By: 

Angsuman Rudra, Director of Systems, Interactive Systems and Circuits (ICS) Ltd. (Part of Radstone Embedded Computing)
Product/Service Focus

This issue's focus is Stimulus Generating Equipment/Arbitrary Waveform Generators. You can view and add to our existing list of Test Products/Services, Test Literature, Test Definitions, Test Vendors

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What's New in Test
Announcements
  9/30/2005 Backplane Testing in India
  9/29/2005 LTX Gets New Chief
  9/23/2005 Teradyne UltraFLEX checks Micronas video graphics device
  9/22/2005 Sony Shakes Things Up, Axes 10,000 Jobs
  9/22/2005 Viking Test Services announces sale of Microcraft probe tester
  9/21/2005 Amkor selects Agilent Technologies 93000 RF System for Singapore IC test site
  9/21/2005 TTI Donates PCB Electrical Test System to Community College
  9/20/2005 Amkor Selects Agilent Technologies 93000 RF System for Singapore IC Test Site
  9/20/2005 Ardence And National Instruments Collaborate On Software Development Environment
  9/20/2005 Fujitsu Siemens to use Digitaltest testing systems
  9/19/2005 Geotest Announces GSA Schedule Agreement with TestMart
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  9/16/2005 Extensions to the IEEE 1149.1 boundary-scan standard
  9/16/2005 Fast testers now tackle wireless handsets
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  9/30/2005 AMREL’s ePOWER Line Continues to Expand with the Release of New Programmable Switching Power Supply with Embedded Ethernet at Autotestcon 2005
  9/27/2005 Teradyne Releases Family of Digital Test Instruments
  9/26/2005 CheckSum provides low-cost in-circuit with boundary-scan test
  9/26/2005 New HVAC test tool from Fluke
  9/23/2005 EX1629 / Boeing 787 Dreamliner
  9/22/2005 Function Generator Adds Power Supply, Counter
  9/22/2005 New breakthrough in signal generation
  9/21/2005 Automated test suite serves second-gen PCI Express
  9/20/2005 Geotest Announces MTS-207 Ultra-rugged Portable PXI Test Set
  9/20/2005 Legerity Introduces VeriVoice Test Suite Software For VoIP Line Testing
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  9/20/2005 PH120 Manual ProbeHead
  9/19/2005 Agilent Technologies introduces industry's highest-performance 100 MHz oscilloscopes
  9/19/2005 Anritsu Launches MMS Analysis Signaling Tester
  9/19/2005 Gage Applied Technologies Announces New Family of PCI Arbitrary Waveform Generators
  9/19/2005 Geotest Unveils New 20 Slot, 3U PXI Chassis
  9/19/2005 Industry's Highest Resolution Digitizer
  9/19/2005 PCI-Based ARBs Offer Three Performance Levels
Web Postings
  9/16/2005 AutoTestCon is Not Just for Military Testing Any More
 
Selecting Your Next AWG

Joan Mercade, Application Engineer at Tektronix

Introduction

Arbitrary Waveform Generators (AWG or arbs) are among the richest and most flexible instrument categories in the instrumentation world. In theory, they can make create any signal in the mind of the most imaginative test engineer. Reality is, however, not so rosy. Selecting the right instrument for a given job may be a challenging task as many of the features and specifications required may be difficult to evaluate.

The Form Factor factor

AWGs come in all shapes and sizes. From card-based architectures to full-featured self-sufficient bench top instruments, there are a variety of form factors to fit any application environment. Arbitrary generators are a very flexible stimulus solution for ATE systems. After all, their flexibility and versatility match one of the desired characteristics of any instrument oriented to be used in any automatic test system: universality. The same device can generate, say, a serial digital system to emulate a communication interface while testing a telecom card and a video signal.  While testing a video processing card, an AWG can be downloading the right sample sequences and changing the signal generation set-up. All the AWGs in the market are potentially usable in ATE systems as external computers can control them and signals can be downloaded to them depending on the actual needs of the test procedure. In any case, there are different form factors that can make integration simpler and cheaper. Card-based architectures such as PXI or VXI are typically seen as more appropriate for automatic measurement systems. Although transfer speed and multiple device integration, through specialized built-in timing and trigger buses, may simplify system integration and speed, the lack of powerful user interfaces, internal mass storage (such as hard disks) can result in lower than expected performance levels, specially when compared to the most powerful, multi-channel bench top solutions available. As in any measurement situation, the most important factor should be the instrument performance and accuracy.  

Reading the Specs

A very useful way to understand an AWG is by thinking of an inverted digital storage oscilloscope (DSO). See Figure 1. The block diagram is composed of a waveform memory, a converter (a DAC instead of an ADC), and an amplifier/conditioning block at the output. Just like any DSO, the main specs are the maximum sampling rate, the vertical resolution, and the waveform memory length.  Generally speaking, the faster the sampling rate, the higher the vertical resolution (expressed as the number of bits for the DAC), and the larger the waveform memory, the better. There are, however, many additional issues connected to these basic specs.

Maximum sampling rate defines the frequency band that a given generator can re-create. According to the sampling theorem, the maximum frequency band that could be covered is limited to half the sampling rate. It is important, though, to look at the analog bandwidth spec as this could be the limiting factor for signal bandwidth. Does this mean that any extra sampling rate is a waste of capacity? Not at all. Oversampling can be used to improve the signal to quantizing noise ratio (as it is spread over a larger band) and the timing granularity of the resulting waveform. In other words, sampling faster is like increasing the number of bits of the DAC. In some application, such as wireless test, it is even possible to reach higher than expected carrier frequencies by using one of the signal images of the DAC.  This is possible in those cases where analog bandwidth allows us to do so. It is important to understand, though, that edges can be positioned with a much higher resolution and accuracy than the sampling period for any bandwidth-limited signal so, generally speaking, sampling rate is not the most relevant spec to properly define timing in a given signal for applications such as jitter tolerance testing.

Vertical resolution gives a measure of the amplitude detail of the resulting signal. This spec is typically connected to the sampling rate, so in general, the faster the sampling rate is, the lower the vertical resolution we can get. Current state-of-the-art DACs can offer up to 16 bit at several hundred MS/s and up to 8 bits at several GS/s. The latest generation of low-cost high-performance instruments are even offering 14 bits @ 2 GS/s. Again, this spec does not tell us everything. Theoretically, a higher resolution makes it possible to create finer details on the final signal. However, there are other factors that may limit this capability. Analog noise is one of them, especially for low level signals. The circuits at the output can also influence the actual quality of the signal. The availability of different amplitude ranges, switchable low-pass filters, and offset controls may help to greatly reduce the noise and adjust the available dynamic range to the requirements of the signal. Some applications, such as ADC or wireless test, may require some quality level in the frequency domain. For these cases, linearity (both from the DAC and the output circuits) may be the limiting factor. Specs such as the SFDR ( Spurious Free Dynamic Range ) or Spectral Purity (typically for sine waves) may be more descriptive than just the number of bits of the DAC.

Although the block diagram of an AWG is similar to a DSO, there is a major difference between both types of instruments: in DSOs signals are not a problem as they come from the real world while in AWGs every sample in the waveform memory is the result of a synthesis process. The capability to replicate a real-world signal is limited by the available record length. The longest available time window can be calculated by dividing the record length by the sampling rate. The only way to create continuous signals is by repeating the available time window over and over. This issue is important in applications such as serial data or wireless signals emulation where a perfect match between both ends of the signal is necessary to avoid the classic wrap-around artefacts that can destroy the signal integrity characteristics. High-performance arbs may implement an elegant solution to simulate extremely long time-windows based in real-time signal sequencing. Using this scheme, the available generation memory can hold different signal segments that are sequenced through a sequence list. The most advanced instruments can even use external signals to control branching at the sequence so real-world intelligent behaviours may be emulated.

Usability Issues

As in any instrument, the availability of the raw performance does not guarantee success. This is especially true in AWGs where creating the signals is an important part of the process. Signals can come from a variety of sources. Some may be external to the instrument environment such as simulation tools or digital oscilloscopes. In this case the capability of importing these signals is crucial. In other cases signals must be synthesized from scratch. Almost any AWG manufacturer offers general-purpose software packages running on external computers that allow creating, editing, and processing signals in different ways, ranging from drawing them with a mouse up to defining them by writing complex formulae. Some advanced instruments offer the same capabilities implemented in their user interface. Although these tools may be enough for a big portion of users, others will require specialized packages for vertical applications. A good example of this may be the creation of wireless signals. Analysing the available tools, the import capabilities, and the interfacing options for any AWG may be as critical for the final success as the raw specs.

Many applications may require more than one simultaneous stimulus channel. Although most AWGs in the market can be externally synchronized, multiple channel arbs make life much easier for the users as they guarantee signal alignment and greatly simplifies control and integration. Some instruments even offer additional digital signals that may be used for DUT synchronization. In some cases the waveform memory is connected to digital outputs so it is possible to use these devices as digital pattern generators or in mixed-signal application.

Many low-range arbs include function generator functionality. These instruments are known as Arbitrary Function Generators (AFG). The function generation block may be implemented as an independent internal device but most modern devices use a DDS (Direct Digital Synthesis) architecture, which reuses most of the AWG circuits such as the DAC. The continuous improvement of AFGs is greatly extending the number of potential users.

The AFG3000 series from Tektronix is a good example of the latest generation AWG/AFG, showing very high performance at an affordable price.  See Figure 2.

Conclusion

As digital scopes replaced analog scopes in the past, AWGs will replace many of the generators in laboratories and production lines. Probably, selecting your next generator means selecting your next AWG.

References

1. “XYZ of Signal Sources”, Tektronix application note 76W-16672-3, available at http://www.tektronix.com

2. “Replicating Real World Signals with an Arbitrary/Function Generator”, Tektronix application note 76W-18661-0, available at http://www.tektronix.com

3. “Waveform Sequencing with the AWG2000 Series”, Tektronix application note 76W-10373-0, available at http://www.tektronix.com

Memory Management in Arbitrary Waveform Generators

Arlene Meadows, Director of Sales, Tabor Electronics Ltd.

True Arbitrary Waveform Generators offer the ability to simulate an infinite variety of electronic and mechanical signals. In order to properly exercise many Devices Under Test, the test waveform may need to go on for long periods of time and to contain many different amplitudes, frequencies, shapes, timing relationships, or combinations of these parameters. To facilitate the creation and output of these signals, from simple sine waves to complex pulse trains and modulated waveforms, the generating instrument must have the ability to store a large amount of data.

To create complex waveforms with precise timing characteristics, true Arbitrary Waveform Generators have increased their sample rates so that 100MS/s and greater generators are readily available. True AWGs with sample rates over 1GS/s are also being produced, such as the Tabor Model 1281. The need to control the programming of each of these high-speed points increases the requirements for large memories in true AWGs.

But memory chips are expensive and can drive up the cost of a true AWG, making it too expensive for many users who really need their flexibility, requiring them to settle for less-expensive, but much less capable, Arbitrary Function Generators. To overcome this limitation, true AWG designers have developed a number of techniques to maximize control of all the points without requiring massive amounts of expensive memory components. These techniques, including segmenting, sequencing and triggering also facilitate the ease of creation and implementation of complex waveforms.

Sample Rate Selection

In selecting an arbitrary waveform generator, the first criterion will typically be the sample speed required to create the users’ waveforms. Sample speed in true AWGs is also a considerable cost factor, so the ability to make use of every point in designing a waveform is a key budgetary consideration. The instrument and its control tools should allow precise control over each point. By cleverly controlling how each point can be defined, and then concatenated to form segments that can be sequenced and stored, will make it possible to select a slower speed generator yet retain control over the necessary parameters, and ultimately save money.

Memory Management

Once a sample rate to define a user’s complex signal has been determinied it is necessary to determine how to control and store the points, at an economical price. Try answering the following questions, which will help in figuring out how much memory will really be needed:

  • What is the smallest common denominator of your waveform that you can draw?

  • Does it repeat itself? How many times?

  • How will it know when to advance to the next signal that is defined as its smallest common denominator?

Determining the answers to these queries helps to give us clues for selecting the best combination of sample rate and memory size. A true AWG should have the ability to segment its memory, then cycle the segments, sequence them and offer flexible methods of advancing through the sequence. This segmenting and sequencing is the key to good and economical memory management.

For example, the Tabor Wonder Wave series, of true AWGs offers standard memory sizes from 512kB to 8MB per channel, depending on the sample rate, with options for each model to double that level. In order to make these levels of memory work for sample rates starting at 50MS/s to 1.2GS/s, they offer repetitions of up to 1 million loops, with storage of from 2048 to 16k segments.

Flexible triggering means that the user can choose from many different ways to move from one defined segment to another. For example, a simple clock waveform can be programmed to run indefinitely until it receives a software or hardware trigger caused by some other event in the test system such as the cessation of a motor or an instruction from a complex communication bit stream. The clock signal then changes to the waveform defined in the sequence table, perhaps a modulating signal that is telling the Device Under Test to change speed, update an axis, or output a new audio or video pattern.

Alternatively, some instruments allow very complex waveforms to be stored, and then sequenced by an external signal to simulate a frequency-hopping radio. In all of these cases, the memory requirements are kept to a minimum. If the Arbitrary Waveform Generator did not have the segment memory, it would have had to store a complete waveform with a finite length and would not have been able to output the signal indefinitely. The radio hopping sequence would have to be completely defined before the test was initiated, meaning that the sequence of hops would be known in advance and not allowed to be random, or changed on the fly.

Summary

The true Arbitrary Waveform Generator that offers separate segment memories, large number of repetitions, various methods of sequence storage and flexible triggering modes for advancement maximizes the generator’s sample speed. This allows the true AWG to create and output long, complex waveforms, yet stay within the lean test equipment budgets that management demands today from R&D labs, production floors, quality departments and service.

Sophisticated Arbitrary Waveform Generation 
Requires Sophisticated Hardware

Angsuman Rudra, Director of Systems, Interactive Systems and Circuits (ICS) Ltd. 
(Part of Radstone Embedded Computing)

 

 

Introduction

Arbitrary waveform generation is becoming increasingly important for test and measurement applications. Current advances have made multi-channel wideband arbitrary waveform generation very easy and cost-effective. This promises a paradigm shift for test and measurement applications and also opens up unparalleled opportunity for scenario generation and simulation and stimulators. The main driving force behind this has been high speed digital to analog data converters (DAC), fast hardware implementation of digital up converters (DUC), very fast data buses and fast disks.

Due to formatting issues, the remainder of the article is only available in pdf format here.

Next Issue's Product/Service Focus
In our next issue of Product/Service Focus we will cover Calibration/Calibration Services. You can add or upgrade a listing before the next issue comes out.

If you would like to include an exclusive article on how to best select Calibration/Calibration Services, please contact LouisUngar@ieee.org.
 

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Events/Announcements
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10/19
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   IEEE International GHz/Gbps Test Workshop (GTW’05)
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11/17
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3/6 - 3/10
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4/3 - 4/6
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4/4 - 4/7
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5/7
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