As we continue to find better ways to transmit, store or process human-generated or physically derived data, signals representing this information are getting ever more complex. Fortunately, today’s arbitrary waveform generators can not only mimic the idealized compositions of most of these complex signals, but also can superimpose signal aberrations potentially caused by equipment limitations or real-world disturbances.
This latter feature is particularly important to test engineers. It enables you to evaluate the performance capabilities of the devices or systems that propagate, respond or otherwise deal with these new complex signals.
Arbitrary waveform generators, commonly called AWGs or arbs, have existed for decades. In many instances, their multiwaveform generating capability could have replaced a bench full of single-purpose generators. However, their versatility was often overlooked since they were not always easy to use.
This problem has been overcome. In addition to providing functions and features that are just not available from any other type of signal source, most AWGs now provide easy-to-learn, intuitive user interfaces.
The ability to generate or accept and replay signals representing electrical and physical phenomena and, equally important, extensive waveform editing facilities are functions particularly unique to AWGs. Waveforms coming from different sources can be blended, combined and modified in amplitude, timing and frequency content. This enables simulation of complex electrical interactions and replication of real-world events in a laboratory environment.
A feature gaining substantial attention today is the ability to loop, sequence and combine incremental waveform segments. While the required programming effort may have been daunting in the past, most of today’s implementations make the process intuitive.
But many test professionals are not yet familiar with the way that looping and sequencing can be applied to simplify many test and evaluation tasks. These application examples, each solving a test problem in a different electronics system domain, can provide a better understanding–and stimulate your thinking–about how looping and sequencing may help you solve your most intricate test problems.
EKG Signals for Pacemaker Testing
A captured EKG signal representing a normal heartbeat is loaded into one section of the AWG’s memory and one or several abnormal heartbeat signals are loaded into another section of the memory. “Looping can then be used to produce a predefined number of ‘good’ heartbeats before sequencing to a predefined number of ‘bad’ heartbeats,” explained Jim Wookey, Product Marketing Manager at Wavetek.
“This allows the AWG to simulate a continuous output of normal and irregular heartbeats for use in designing and testing pacemakers and heart monitoring equipment,” Mr. Wookey continued. “One of the advantages of using looping and sequencing for such an application is that very little waveform memory is required.”
Programming this application is also very simple. “Wavetek AWGs provide table-like screens to make setting up looping and sequencing easy. And in some generators, clock rates may be set independently for individual waveform segments,” Mr. Wookey added.
An Ultrasound Application
It is similarly easy to set up loops and sequences and keep track of memory utilization with some PC-based AWG cards furnished with suitable software.
The waveform display area in Figure 1 shows a distorted sine wave that has been loaded into Segment 1. The arb-memory readout indicates that the memory contains five different segments and that 26,233 memory locations are left for use by additional segments. The arb-sequencer control indicates that there are five steps defined, each with an associated segment and a repeat count.
“The segment size and sequencer characteristics are easy to change,” said Stuart Streiff, President of PC Instruments. “You just click on an entry in one of the control panels and then click the edit button to enter the new characteristics. The arb-display control can be used to page through the AWG’s memory and look at the different waveshapes in each segment.
“The memory on our AWGs can be divided into 99 segments, each containing a different waveform,” Mr. Streiff continued. “Then, a waveform sequence can be created that will step from one waveform segment to another, and each particular segment can have its waveform repeated (i.e., looped) up to 32,767 times.”
Mr. Streiff described a typical application where looping and sequencing provided ease of programming while saving memory space. “One of our customers used the looping and linking feature of our AWGs to simulate the transmit and receive pulse of an ultrasound system. The transmit pulse was programmed into Segment 1 and the receive pulse into Segment 2. A ‘dead time’ was programmed into Segment 3. Then, by changing the repeat number for Segment 3, the engineer could alter the delay time between send and receive pulses.”
Generating TV Signals
“Video waveforms are long and, at the same time, have moderate bandwidth requiring a high clock rate,” stated Dr. Mike Lauterbach, Director, Product Management at LeCroy. “A typical video waveform might clock at >357 MHz with a 60-Hz field rate. Without waveform sequencing, this would require more than 21 GB of memory.
“Luckily, a video signal can be broken into many redundant segments, allowing many common test patterns to be generated with less than 250 kB of memory,” Dr. Lauterbach continued.
“Even though the segments are used repeatedly, there is only one copy of each stored in the AWG’s high-speed memory. And still, all transitions between waveforms are seamless and there are no discontinuities,” Dr. Lauterbach emphasized.
The built-in sequence editor of the LeCroy LW420 AWG makes not only generating but also modifying waveforms easy. It includes insert, delete and block commands for cut-and-paste editing. For instance, the colorbar test pattern can be changed into a multiburst resolution test pattern by simply editing one line in the pattern sequencer listing.
Generating a Spread-Spectrum Data Packet
Wireless communications using spread-spectrum techniques need to simulate both data packets and the chipping sequence used to spread the spectrum of the data. It is critical that these waveforms be time synchronous for correct operation.
“This type of waveform is easy to generate using repetitive segments and sequences,” said Dr. Lauterbach. “The data packet can be created with three distinct waveform segments: 0-V, 2.5-V and 5-V DC levels repeated as needed to match the data codes.
“The chipping sequence, generated using a waveform sequence on Channel 2 of the LW420, uses a single pseudorandom pattern which is repeated synchronously eight times for each data bit. Using a common clock and maintaining sequences of the same length guarantee that the waveforms remain synchronous.
“Figure 4 shows the output from the AWG as displayed on an oscilloscope. The top trace contains the data packet. The next trace shows the chipping sequence. The third trace is a horizontal expansion of the chipping sequence which allows the data pattern to be seen. The lower trace is a trigger signal,” Dr. Lauterbach concluded.
Again, the waveform sequence capability facilitates fast and easy modification of the chipping sequence as well as the data. This allows you to evaluate the processing gain of various combinations of two waveforms.
Easy Downloading and Editing
Entire waveforms, or portions of them, can be created by specifying function-defining equations, drawing waveforms with cursors or a mouse, composing them from a library of stored waveforms, or downloading waveforms previously captured by a DSO. These signals can subsequently be combined, edited and manipulated.
“For instance, with the 20 insertable standard stored waveforms provided by our AWGs, a variety of composite waveforms can be created and directly loaded into memory,” said Henry Reinecke Jr., President of Pragmatic Instruments. “The front-panel editing system provides full parameter control of all waveform aspects and a built-in MATH feature facilitates manipulations.
“Specific editing commands are point, line, vertex, cut, paste, move, scale, add, subtract, multiply, insert and sum. A comparable set of capabilities is available using the Windows-based PC software. In addition, this software provides FFT, IFFT, timing chart editing, digital filtering, and data file manipulation,” Mr. Reinecke concluded.
Downloading data from a DSO can be accomplished using AWG-resident facilities. For example, on the LeCroy LW400 Series scope, you press the import waveform key. Then you have choices such as from computer file, from floppy, from internal hard drive, and from scope. “When you select from scope, a list of popular scopes from LeCroy, Tek and HP is displayed,” said Dr. Lauterbach. “You simply choose the model and the channel you want to import. The AWG will get the data from the scope and bring it in (over the GPIB cable) without any computer or any programming.”
Similarly, PC-based software may be used for downloading. “BenchLink/Scope software combined with BenchLink/Arb software, both running on a PC, facilitate the capture and downloading process,” said Von Campbell, Signal Sources Business Team Manager at Hewlett-Packard. “These low-cost (<$300) software packages give you a familiar Windows-based interface that makes the capture, modification and creation of waveforms easy."
Further Advantages of a PC Tie-in
Interoperability with a PC, found in most of today’s self-contained AWGs and inherent with PC plug-in board AWGs, offers many advantages which are not immediately obvious. Letting replicas of captured or modified real-life waveforms interact with CAD packages within the PC can eliminate or reduce the cost of prototyping.
“Large air-conditioning units in commercial buildings can generate substantial noise on the power lines if they are not properly filtered. A scope is used to capture the noisy waveform and the AWG is used to replicate the waveform at the engineer’s workstation,” related Mr. Streiff. “Then, using a mathematics package, a filter is designed and simulated in software. A low-power prototype is then built and the AWG is used to test it, altering waveforms as needed for limit testing. Finally, the high-power filter is designed with confidence that it will perform properly.”
Limitations and Outlook
As the examples demonstrate, the application potential of AWGs is almost limitless. Today’s AWGs produce almost any audio, sonar and RF waveform. But they are not yet reaching–without up-conversion–the higher RF or microwave regions.
“Both 50-MHz and 100-MHz AWGs are currently available at reasonable prices,” said Mr. Reinecke. “At higher speeds, both the memory cost as well as the D/A converter and amplifier designs impose limitations or add complexity. However, with the constant improvement of ICs, the performance limits will continue to rise.”
1. “Waveform Creation Made Easy,” LeCroy 1996 Test and Measurement Products Handbook, pp. 115 to 140.
Arbitrary Waveform Generators
The Model 296 Arbitrary Waveform Generator contains up to four independent 50-MHz channels and offers advanced sequencing linking to as many as 4,096 waveform segments. The clock frequency is variable and may be independently programmed for each segment in the sequence. Signals have a 2-ppm frequency accuracy, the magnitude extends to 15 Vpp and 16-bit digital outputs are provided per channel. Arbitrary waveform memory is 128 kpoints (512k optional) per channel with 12-bit vertical resolution. Channels can be phase locked and signals summed. $7,245. Wavetek Corp., (800) 223-9885.
Has Dual Capability
The 100-MHz 2416A generates arbitrary waveforms at sampling rates up to 100 MHz. It has a memory length of 63k and stores up to 100 waveforms. Standard functions include sine and square to 50 MHz; triangle and ramp to 10 MHz; and pulse, Gaussian pulse, exponential and DC levels. Each function has coefficients that can be set to define attributes, providing a palette of waveforms. Complete waveform families can be accessed and modified by setting coefficients, exponents or percentages. $2,695. Pragmatic Instruments, Inc., (619) 271-6770.
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 Inc., (216) 487-0220.
Sub-Sample Waveshape Control
The LW410 (single-channel) and LW420 (dual-channel) WaveStation Arbitrary Waveform Generators have 100-ps waveform feature placement, a 400-MHz maximum clock rate, up to 1 MB of waveform memory and Sub-Sample Waveshape Control. A simple editing function can change a waveform or move a feature. The time base has 3-ppm accuracy; resolution is 1 Hz. SSB phase noise is <-120 dBc/Hz at 10 kHz from a 10-MHz carrier. Waveforms can be directly transferred from most DSOs. LW410: $13,945; LW420: $18,950. LeCroy Corp., (800) 453-2769.
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.
Synthesized Function Generator
Provides 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 or eight 8-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 facilitates capturing, composing and modifying waveforms on a PC. With the Phaselock/TCXO Option 001, precise phase-offset signals can be generated and multiple generators synchronized. HP-IB and RS-232 interfaces are standard. $1,725. Hewlett-Packard Co., (800) 452-4844.
AWG Includes Built-in
The PM 5150 Arbitrary Waveform Generator features a digitally synthesized clock, sample rates to 20 MS/s, 12-bit amplitude resolution and a selectable 7-MHz analog output filter. The 32-kpoint nonvolatile waveform memory can be partitioned into 100 segments. Custom waveforms are built by sketching, using the built-in library or multiplying signals to simulate modulation. AnyWave software, running on a PC, facilitates saving, retrieving, downloading and importing of waveforms. Repetitive test signals may be created using an optional sequencer. $3,455. Fluke Corp., (800) 44-FLUKE.
Single Board Combines
AWG and Pattern Generator
The CompuGen 840 is an analog and digital arbitrary waveform generator and an 8-bit digital pattern generator on a single board. It features D/A conversion speeds to 40 MS/s at 8-bit resolution with 16,384 points of data, or 20 MS/s at 12-bit resolution with 8,192 data points. Signals may be generated by equation entry, cut and paste, waveform editing or ASCII file input. The accompanying software allows loading/storing of waveforms and setups as well as function generation. $1,900. Gage Applied Sciences, Inc., (800) 567-GAGE.
Dual Output Board
Features Window 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 to achieve low output noise (typically -72 dB THD). Signals are generated via front-panel functions, a polynomial equation parser or by downloading waveforms. Amplitude/offsets may be controlled from the virtual Windows front panel or by software. 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, Inc., (508) 339-3000.
The ET-A&DWaveGen is an expandable, configurable arbitrary waveform generator. It supports up to 192 digital channels at 25 MS/s and 24 analog channels. The base configuration includes a 16-channel, 25-MS/s pattern generator with a 4k x 16-bit buffer plus two analog channels. Each channel has 8-bit resolution and four independent voltage ranges. DC offset can be added in 0.01-V steps. Data may be imported/exported in *.DAT, *.TXT and *.BIN formats, edited on screen and saved to disk. $1,395. Emulation Technology, Inc., (408) 982-0660.
Board Plus Software
Provides Familiar Interface
The PCIP-AWFG with VisualGENä for Windows is a full-function arbitrary waveform generator on a plug-in board for IBM PC compatibles. The board, available as a one-channel or two-channel model, provides continuous analog output with 12-bit resolution as well as four bits of digital output at rates up to 5 MHz. Data is stored in 32 kwords/channel of RAM. Continuous, burst, scan, triggered and gated modes are provided. A built-in function library and a waveform editor are included. From $850. Keithley MetraByte, (800) 348-0033.
Generator Employing DDS
Is Easy to Use
The SM-1030 Function/ARB/Pulse Generator uses DDS technology to produce stable, high-purity signals. Arbitrary waveforms are generated from ASCII data loaded into an 8-kpoint wave table. Sampling is adjustable from 6.25 kS/s to 10 MS/s and vertical resolution is 12 bits. 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 generator is compatible with third-party waveform generation software. $1,195. Signametrics Co., (206) 524-4074.
Versatile AWGs Provide
Analog, Digital Outputs
The AWG 2040 and AWG 2005 are the most recent additions to the 2000 Series of portable, programmable arbitrary waveform generators. The single-channel AWG 2040 features 1 GS/s and up to 4 MB of memory. The two-channel (optional four-channel) AWG 2005 provides 20 MS/s and 64 kpoints of memory per channel. Both units feature a graphical user interface. Graphical, FFT, equation and sequence editors expedite waveform composition. Direct waveform transfer facilities and a standard waveform library are provided. AWG 2040: $19,995; AWG 2005: $9,995. Tektronix, Inc., (800) 426-2200.
100-MS/s AWG Available
In C-Sized VXI Format
The Model 3151 Arbitrary Waveform Generator offers programmable output voltages, up to 1 ppm internal reference, external clock input, nine standard waveforms, multiple trigger modes and multiple unit synchronization. Waveform output ranges from 100 m Hz to 50 MHz with 12-bit vertical resolution. Sequencing and looping features enable linking up to 4,096 waveforms. The module complies with the latest revisions of both the VXIbus and VXIplug&play specifications. Custom arbitrary waveforms are created with WaveCADã software. Starting at $3,995. Racal Instruments, Inc., (800) RACAL-ATE.
Copyright 1995 Nelson Publishing Inc.