Easily Generated Complex Waveforms Accommodate Many Applications
Arbitrary waveform generators (arbs) now are being used by more engineers and technicians than ever before. The cause of this newfound popularity—arbs have more capabilities than their predecessors, and more of today’s test applications demand their use.
The enhancements mainly are due to PC-related advancements, such as increased processing power, lower-cost memory, and user-friendly software. And more applications demand the use of arbs because, in addition to acting as multipurpose signal sources, they facilitate economical what-if analyses and real-world signal simulation and emulation. The latter are being carried out more frequently today to shorten product development time and improve quality.
“Arbs provide design engineers with the tools to define and generate test signals containing virtually any signal characteristic,” said Bruce Virell, marketing manager of signal sources in the Tektronix Measurement Business Division. “By changing variables, you can easily devise what-if tests that evaluate ICs, prototype circuits, and production assemblies under both nominal and worst-case operating conditions.”
An arb can produce any waveform that has been digitally defined and placed in its memory. To output the waveform, memory content is sequentially read out, applied to a digital-to-analog converter (DAC) and selectively filtered.
Function generators (FGs) using direct digital synthesis (DDS) also contain memory, DACs, and filters. By adding circuitry which helps manipulate data and place
arbitrary waveform-defining data into their memory, FGs can become arbs. Conversely, since arbs can generate any waveform, storing common function-defining coordinates (such as sine, triangular, or square wave data) in internal memory lets them function as FGs.
Many DDS-based FGs also contain analog or mixed-signal data-manipulation capabilities in addition to their digital function-generation circuitry. “With this combination, arbitrary waveforms can be used in the same manner as standard mathematical functions, taking full advantage of FG capabilities—including modulation,” said Cheryl Diller, product manager at the Electronic Measurement Division of Hewlett-Packard. “By treating some arbitrary waveforms as functions, frequency can be adjusted on the fly rather than by creating a new waveform with a different number of points.”
Not all arbs affect frequency changes through memory content or address alteration. Many other techniques are available, each with its pros and cons.1,2,3 But regardless of the architecture used, all require that the waveform be defined in a digital format. And to perform this task, again several methods are available.
Digital Definition of Desired Waveforms
Capture and Playback
The most expedient way to create a real-world signal with an arb is to use a record-playback technique. “You begin by storing a live signal in the memory of a digitizing oscilloscope, then transfer the record via GPIB or floppy disk into the arb for playback,” said Mr. Virell of Tektronix. “This technique is especially useful for duplicating conditions that are difficult or physically impossible to replicate in the lab.”
Equation and Graphical Entry
Waveforms may be defined by entering descriptive equations, editing previously acquired data, or graphing. For instance, Yokogawa offers a function/arbitrary waveform generator with a touch screen display to simplify the graphing task. “You enter points, within the scaled ranges for the X (time) and Y (magnitude) axes, and the waveform can be generated using linear, step, or spline interpolation between the points,” said Thomas Pope, manager of the test and measurement division at Yokogawa.
Some arbs include computational facilities and a display for signal generation and editing. “Signals can be created easily and existing waveshapes modified with fine detail using PC-like graphic editing features,” said Mr. Virell of Tektronix. “The graphic editing capabilities include cut, copy, paste, shift, insert, convolution, and mathematical manipulation. There even is an undo function so you can return to the previous waveshape if the result isn’t what you expected. The graphical displays allow you to view and modify the resulting signals on the built-in monitor before outputting it.”
PC-Based Entry
If arbitrary waveform definition and signal generation do not have to be carried out within a single instrument, creating waveforms with the help of a PC is advantageous. Of course, arbs on PC plug-in boards automatically take advantage of the many features that PCs offer.
Most PC plug-in board-based arbs come with Windows-compatible setup software, drivers, and virtual front panels. “Instead of LCDs, knobs, and buttons, the board’s functions are controlled via the computer’s mouse, keyboard, and monitor screen,” said Skip Cook, tactical marketing manager at Keithley. “This combination provides an intuitive, easy-to-use GUI control panel that looks and feels like the front panel of a benchtop GPIB instrument.”
Being an integral part of the PC, the plug-in board arb also can be integrated with and use other PC-resident software including spreadsheet programs, data bases, or math packages. “For instance, the math-oriented program MATLAB v 5.0 allows you to call a 32-bit DLL, such as a CompuGen 1100 Windows 95/NT DLL,” said Karen McCurry, assistant marketing manager at Gage Applied Sciences. “Any signal created in MATLAB then can be generated using the CompuGen 1100 PC-based arb.”
Benchtop and VXI-based arb manufacturers also provide PC-based arbitrary waveform generation and editing software to extend the functionality of their instruments. The selection includes WaveCAD™ from Racal, WaveWorks™ from Pragmatic, BenchComTM from PC Instruments, WaveWriter™ from Tektronix, and BenchLink Arb™ from Hewlett-Packard.
“Waveform creation is easy with PC-based software,” said Charles Greenberg, product marketing engineer at Racal Instruments. “By using equations and built-in drawing tools, you can prototype new waveforms quickly. You also can access waveforms from digital storage oscilloscopes, math programs, and spreadsheets.”
Waveform Editing
Most of this software provides extensive filtering, mathematical processing, and fast fourier transform (FFT) operations. FFTs help you to examine a signal in another dimension since you can transform a time-domain-defined waveform representation into a frequency-domain depiction of the same signal.
“The availability of FFT expands the range of waveform-creation options,” commented Henry Reinecke Jr., president of Pragmatic Instruments. “Not only does it allow viewing, but also direct manipulation of the waveform in the frequency domain. After performing modifications and applying an inverse FFT, a time-domain file is regenerated in a form totally compatible with the input requirements of the arb.”
Growing Need for Multichannel Arbs
While most applications today are served well by single-channel arbs, more and more users need synchronized multichannel arb outputs. Common situations that require several arb output channels include three-phase power disturbance simulation, sonar system analysis, and communications equipment test.
An example of how three-phase excitation signals were generated by a PC-board-based arb configuration was explained by Ms. McCurry of Gage. “We used three CompuGen 1100 cards operated and controlled through CompuGen for Windows software to provide disturbance-simulating signals with a phase resolution of 0.01° and 0.1% in amplitude. Although being a multicard system, a common clock and trigger ensured full synchronization between channels.”
The most prevalent demand for multichannel arbitrary waveform generation, however, stems from the need to generate the synchronized in-phase (I) and quadrature (Q) signals essential for testing modern communications equipment. “To satisfy this requirement, two simultaneous electrical signals must be provided,” said Mr. Reinecke of Pragmatic Instruments. “Either a dual-channel arb, capable of synchronizing both channels and operating from a common sample clock, or a pair of generators meeting this requirement is needed.”
“The generation of I and Q communications signals is one of the most common applications for multichannel arbs,” concurred Dr. Michael Lauterbach, director of product management at LeCroy. “The two test signals must be absolutely locked to the same time base, which is easily accomplished by a two-channel arb. Another useful feature is the control of relative phase of the two outputs with high resolution to let you simulate the effects of phase jitter.”
Through waveform editing, the arb also can add a variety of impairments to the pure I and Q signals to simulate real-world effects, added Mr. Virell of Tektronix. “These include jitter, bit shift or drop out, noise, signal reflections, and multipath effects.
“A two-channel arb also is ideally suited for generating complex modulation signals like p /4QPSK,” Mr. Virell continued. “But one of the most difficult tasks has been creating the I and Q data that goes into the arb. However, new software products, such as IQSIM, let you generate I and Q signals conforming to a variety of modulation standards.”
Trends
Most of today’s arbs are easy to operate, are offered with excellent built-in or PC-based waveform editing facilities, and include sequencing features that generate continuous analog output signals. A few also provide concurrent digital outputs, a feature that will be more in demand in the near future.
While modern digital communications test applications require at least two analog arb outputs, a digital output capability enhances the application potential of an arb. Testing of some wireless designs already requires up to 24 concurrent digital output channels.
“Most engineers easily grasp the conceptual value of an arb as an analog or even a serial data generation device,” said Mr. Virell of Tektronix. “However, modern arbs which have a built-in parallel logic-generation capability offer added value by virtue of their built-in parallel digital editing capability. Virtually all analog design and test applications involving arbs today interface with digital circuitry. An arb which includes digital capabilities may supplant the need for data generators which otherwise would be used in concert with the arb in many applications.”
There also will be greater demand for arbs for design-verification purposes and prototype check out. This trend is aided or, in some cases, prompted by new simulation software and enhanced PC prowess.
For instance, SystemView by Elanix, a dynamic system design, simulation, and analysis software package, offers extensive libraries for communications, RF/analog, and logic circuits and creates a complete digital system model. “As such, every node in the system may be characterized by digital files representing the waveforms at each point,” commented Mr. Reinecke of Pragmatic Instruments. “These may be downloaded to the arb for real circuit testing. This approach also helps compare the actual system performance and the simulation.”
“Nearly every engineer now models circuits in some kind of PC-based software package,” added Dr. Lauterbach of LeCroy. “While one engineer may have just completed circuit modeling, another engineer already may have a prototype for an adjacent part of the system. A digital file representing the signal at the output of the model can be downloaded to an arb which, in turn, can create a real signal to test the already existing adjacent prototype.” As more engineers recognize the power of combining modeling and an arb to expedite development and testing, greater demand for arbs will result.
References
1. Barker, D. J., “Choose Your Waveform,” EE-Evaluation Engineering, September 1990, pp. 98-105.
2. Sauer, J., “New Technologies Drive Signal Generator Design,” EE-Evaluation Engineering, March 1992, pp. 26-35.
3. Jacob, G., “Arbitrary—But Neither Uncontrolled Nor Aimless,” EE-Evaluation Engineering, October 1993, pp. 30-37.
Arbitrary Waveform Generators
Arb Samples at 300 MS/s
And Offers 12-bit Resolution
The Model 3161 Arbitrary Waveform Generator occupies a single, C-sized VXIbus slot and dissipates less than 60 W. It operates at a rate of up to 300 MS/s with 12-bit resolution. The features include frequency hopping and production of sequences with variable sample rates. Built-in modulation modes offer AM, PM, and FSK. The output is 5 Vpp with <2.5-ns transition time. The internal oscillator may be locked to an external frequency from 100 Hz to 18.75 MHz. Racal Instruments, (800) RACAL-ATE.
Digital Synthesis Delivers
Functionality
The HP E1441A Arbitrary Waveform Generator is a C-size, single-slot, message-based VXI module that produces 12-bit, 40 MS/s, 16k deep waveforms. It provides internal AM/FM/FSK/Burst modulation and linear and logarithmic sweeps. Standard built-in waveforms include sine, square, triangle, ramp, noise, sin (x)/x, and exponential rise and fall signals. A high-stability time base and an external phase-lock feature are optional. $2,995. Hewlett-Packard, (800) 452-4844.
Synthesizer Generates Signals
Independent of Host Computer
The WSB-100-20 Waveform Synthesizer generates signals independent of the host computer, allowing full processor power to be used for other tasks. The output signal is defined by a set of up to 32,768 points with a software-selectable data-point generation rate of 50 ns/point to 107 s/point. External clocks can be applied from DC to 20 MHz. Address assignments are programmable to store multiple waveforms on- board. DMA operation supports high-speed waveform definition. $995. Quatech, (800) 553-1170.
Easy-to-Use Arb
Offers On-Screen Editing
The AWG 2041 Programmable Arbitrary Waveform Generator lets you create and edit waveforms on screen. It features 8-bit vertical resolution, 1-Mword record length (4-Mword optional), and a built-in PC-compatible 3.5″ floppy disk drive. A 1.024-GS/s clock rate helps generate 500-MHz signals. You can automatically transfer waveforms captured on a DSO, modify waveforms using one of seven editing techniques, or employ mathematical-formula entry. An optional FFT editor modifies waveforms in the frequency domain. $19,995. Tektronix, (800) 426-2200.
Waveform Generator
Features 20-MS/s Rate
The 2714A Arbitrary Waveform Generator features a large waveform memory and a 20-MS/s rate. As a function generator, it furnishes 10 standard waveforms. As an arb, it offers 100 waveforms with WaveWorks Jr. software to create an array of functions and sequences. Parameter selection keys and a menu provide access to instrument features. Windows®-based graphic PC software lets you perform synthesis, analysis, and editing in time and frequency domains. RS-232-C and IEEE 488.2 and dedicated software programs are standard. $1,995. Pragmatic Instruments, (800) 772-4628.
Generators Feature
400-MHz Sample Clock
The LW410A (single-channel) and LW420A (dual-channel) WaveStation Arbitrary Waveform Generators offer 100-ps waveform feature placement, a 400-MHz maximum sample rate, up to 1 MB of waveform memory per channel, and internal floppy and hard drives. Waveforms can be selected from libraries, imported from external sources, or created using the WaveStation’s equation facilities. The clock rate, continuously variable from 6 kHz to 400 MHz with 1-Hz resolution, and filter bandwidth are selected automatically. This feature avoids aliasing and assures that timing relationships within waveforms are maintained. The switching time between preloaded waveforms is <5 ms. LW410A: $13,945; LW420A: $18,950. LeCroy, (800) 453-2769.C-Sized Card Contains up to
Three Function/Arb Channels
The VXI VM3650 Arbitrary Waveform Generator provides up to three function or custom waveform-generation channels per C-sized card. Sine waves are available up to 20 MHz and square waves to 25 MHz. User-defined waveforms with up to 12-bit vertical resolution and up to 128k horizontal points (512k optional) may be created. Up to 4,096 different waveforms may be looped and linked to simulate complex waveform sequences. The VM3650 is SCPI and VXIplug&play compatible. $2,900. VXI Technology, (714) 955-1894.
Synthesized Function Generator
Includes Arb Capabilities
The FG310 (single-channel) and the FG320 (dual-channel) Synthesized Function Generators offer arbitrary sweep, modulation, simple arbitrary waveform definition, and sequencing capabilities. Waveforms may be defined by entering and connecting points or loaded from disks via the built-in floppy drive. A large LCD and touch screen provide an intuitive interface. The frequency range extends from 1 µHz to 15 MHz for sine and square waves and to 200 kHz for triangle or pulse and arbitrary waveforms. FG310: $3,650; FG320: $4,450. Yokogawa Corp. of America, (404) 253-7000.
Waveform Generator Card
Converts at 80 MS/s
The CompuGen 1100 is a PC AT-compatible arbitrary waveform generator card that offers 12-bit resolution at D/A conversion rates of up to 80 MS/s. Since the peak conversion rate is faster than what the ISA bus can handle, D/A data is first downloaded to 512 kS (expandable to 16 MS) of on-board memory. Key features include an output bandwidth of up to 20 MHz, dual-port memory, memory looping, programmable output filters, and programmable output gain and offset. $4,995. Gage Applied Sciences, (800) 567-GAGE.
Generator Features
Advanced Sequencing
The Model 296 Arbitrary Waveform Generator contains up to four independent 50-MHz channels and offers sequencing linking up to 4,096 waveform segments. The clock frequency is variable and may be independently programmed for each segment in the sequence. The signals have a 2-ppm frequency accuracy and a magnitude extending to 15 Vpp. The arbitrary waveform memory is 128 kpoints (512k optional) per channel with 12-bit vertical resolution. The four channels can be phase locked, and their signals summed. $8,195. Wavetek, (800) 223-9885.
Generator Offers
12 Bits at 50 MS/s
The single-channel PCI-311 and the dual-channel PCI-312 Arbitrary Waveform Generators occupy one PC expansion slot and feature update rates to 50 MS/s, 32-kS memory per channel, and 12-bit DACs with an output of 12 Vpp into 50 W . The instruments provide memory segmentation with looping and linking, 0.01% frequency accuracy, five selectable output filters, and nine built-in waveforms, including 10-MHz sine waves and pulses with 12-ns rise times. Waveforms may be created with BenchComTM software, imported, or downloaded. PCI-311: $1,495; PCI-312: $2,195. PC Instruments, (216) 487-0220.
Function Generator Provides
Arbitrary Waveforms to 30 MHz
The DS345 Generator uses DDS to provide waveforms to 30 MHz with 1-µHz resolution. Complex arbitrary signals with up to 16,300 points and sampling times to 25 ns as well as sine, triangle, ramp, or square waves can be generated. Internally synthesized modulation capabilities include phase-continuous linear and logarithmic frequency sweeps, and amplitude, frequency, phase, and burst modulation. Spurious components are less than -55 dBc (below 1 MHz). IEEE 488 and RS-232 interfaces, including Waveform Composer software, are optional. $1,595. Stanford Research Systems, (408) 744-9040.
Function/Arbitrary Generator
Creates Custom Waveforms
The HP 33120A 15-MHz Function/Arbitrary Waveform Generator has a sampling rate of 40 MS/s, stores up to four 16-kpoints waveforms, and provides arbitrary waveforms with 12-bit resolution. Waveforms can be internally modulated with AM, FM, FSK, and bursts. Linear and log sweeps are provided. Optional BenchLink Software captures, composes, and modifies waveforms on a PC. With the Phaselock/TCXO Option 001, phase-offset signals can be generated and multiple generators synchronized. HP-IB and RS-232 interfaces are standard. $1,725. Hewlett-Packard, (800) 452-4844.
Dual Output Board
Features Windows VI Panel
The PC-420 Arbitrary Waveform Board uses two fast-settling 12-bit D/A channels sampling at 40 MHz with eight software-selectable output filters. Signals are generated via front-panel functions, a polynomial equation parser, or downloaded waveforms. Amplitude/offsets may be controlled from the virtual Windows front panel or by software using separate D/As. Waveforms are stored in two 32,768-sample memories and may be looped to achieve non-stop signal generation. Outputs are ±10 V into 50 W . $1,495. DATEL, (508) 339-3000.
Generator Employing DDS
Occupies Single ISA Slot
The SM-1000 family of function/arb/pulse generators uses DDS technology to produce stable, high-purity signals. Arbitrary waveforms are generated from ASCII data loaded into an 8-kpoint wave table. The sampling rate is adjustable from 6.25 kS/s to 10 MS/s. The pulse width extends from 100 ns to 100 s and the pulse repetition rate from 0.01 Hz to 5 MHz. Ten standard waveform functions and noise can be generated (0.01 to 3 MHz). The generators are compatible with third-party waveform-generation software. From $585. Signametrics, (206) 524-4074.
Copyright 1997 Nelson Publishing Inc.
August 1997