When the arbitrary waveform generator (Arb) was introduced by Wavetek in 1975, a few design and test engineers jumped up and down for joy, but most failed to see the potential of this new signal source. Attitudes have changed, however, and new applications are developed every day. If you can conceive a waveform, probably you can generate it in your Arb.
Today’s Arb designs cover a broad spectrum. You can find a high-speed, high-resolution instrument with lots of bells and whistles for a demanding application or one at the other end of the spectrum for a more mundane job. You can pay $36,000 for an Arb that generates 2.6 GB/s and has dozens of features or find one for $3,000 if you just need rates no greater than 100 kS/s. At least one Arb carries a price tag of less than $1,000.
What Can I Do With an Arb?
Arbs are used in virtually every phase of design and test. An Arb often uses a real-world signal, previously captured by a digital oscilloscope, as the starting point to create a single-cycle waveform. A math program can make the waveform worse by adding noise or improve it by filtering before downloading it to the Arb for a testing application.
“The versatile instrument can capture a heartbeat to test a pacemaker,” noted Cheryl Diller, product manager at Agilent Technologies. “In automobile testing, you can capture the signal that initiates airbag deployment and then use it for testing. Even the pressure that a member of a rowing crew applies to the oar can be stored and used for troubleshooting.”
A versatile Arb can simulate baseband CDMA signals to modulate an RF signal generator for testing low-noise amplifier chip sets for CDMA mobile phones. The Arb can generate noise signals to test the immunity of a PC motherboard to power supply noise or compile neurological signals to be sent to an implanted IC for controlling the delivery of pain medication.
“An Arb simulates vibration of the motor used by NASA to drive a cryogenic cooler,” Jim Wookey, regional manager at Fluke, added. “The vibration profile can help determine the suitability of the cooler for applications in space. Another Arb emulates sensor signals from an aircraft in flight, then replays the waveforms to test the flight computer in the laboratory. Another popular application is generation of analog composite signals and S-video inputs or outputs for testing DVD players, closed-circuit television (CCTV) systems, and cameras.”
Some Unique Applications
While the very mention of Arbs prompts interesting stories of what people are doing with this workhorse, we found some applications that will tweak your imagination. They show how the Arb simulates radar signals, emulates tones from a communications satellite, drives a tiny MEMS, and tests a disk control circuit.
Radar Signal Simulation
“Our 3153 Arb provides a special capability in radar testing,” Charles Greenberg, product marketing engineer at Racal Instruments, noted. “Its three outputs drive the AM, FM, and pulse inputs of a microwave signal generator, creating complex radar patterns that simulate the radar signature of an aircraft. Tight channel synchronization and low phase noise are keys to the success of this routine. The sequencers permit mixing triggers, waveform loops, and breakpoints without losing synchronization. All this expands the ability to develop complex waveforms.
Simulating Tones From a Satellite
An Arb can be used with a microwave signal generator to simulate the tones from a communications satellite for testing receiving stations. “It drives the upconverters to generate the proper number of tones at the proper frequencies. The channel of interest at each point in the test sequence can be left blank for injection of a live signal. The choice of Arbs depends on the number of tones and their spacing. This same technique also is used for testing other types of telecommunications systems that send multiple streams on the same link.
MEMS Driver System
In a special application, an assembly of Arbs was configured to drive a Microelectromechanical structure (MEMS). The MEMS Driver System is provided as a standard product at Pragmatic Instruments. It consists of two standard Arbs, a driver amplifier, and special driver software to control the little electrostatically actuated microengines. The MEMS timing is critical, and Arbs fit the application perfectly.
Disk Drive Simulation
“The read/write communications on a disk drive can be simulated with an Arb for design verification,” according to Satoshi Takahashi, product line manager at SonyTek. “Synchronized analog and digital signals can be generated for mixed-signal component testing, and panel LCD and plasma HDTV drive signals can be output.”
Useful Features
Arb users appreciate software that can generate and edit waveforms. Also, the ability to capture signals in a DSO and import them into memory is very useful. Being able to phase-lock several channels also comes in handy in many applications, as does noise-generation capability.
Another useful feature is waveform sequencing to create a long and complex pattern. This is especially important in communications applications. Users also like the opportunity to choose waveforms from a standard library.
The ability to import waveforms from communications simulation software is very useful, according to Mike Lauterbach, director of product management at LeCroy. “The arb is able to generate outputs with single-point resolution. The operator can control edge timing to 100-ps resolution.”
Especially in video tests, the user needs a fast interface between the PC and the Arb. A single television frame in 16-bit binary format occupies 1.34 MB.
“A functional test requires a number of these frames in a sequence and failure to maintain the output rates makes testing slow and frustrating,” noted Lokesh Duraiappah, measurements product manager at National Instruments. “The NI 5431 Arb has 16 MB of on-board memory. Its linking and looping capability enables you to create a complex sequence with several Insertion Test Lines (ITL) in each video frame for verifying equipment accuracy.”
The ability to form a master/slave relationship with precise timing opens new applications for the Arb. Andrew Dawson, product manager of board-level products and advanced measurement systems at Gage Applied, said that up to eight CompuGen 1100s can be used together. Each channel receives a clock and trigger from the master but is programmed with its distinct pattern length and conditioning.
One customer needed a signal for high-frequency audio transformer test. Since the common-mode content had to be eliminated, the company used two Arbs in a master/slave configuration and loaded identical patterns of opposite polarities. In another application, in-phase and quadrature (I and Q) signals were captured, filtered, and loaded into a master/slave set of Arbs. Using this combination, the customer could experiment with different filter transfer functions in developing a communications system.
Trends
Increasingly, the user whose test article is a considerable distance from the Arb is concerned about noise pickup over the analog transmission line. The Gage Applied solution is the CompuGen 3250, a derivation of the 1100 instrument. These units output bit-parallel 32-bit samples at rates up to 50 MHz, and the user converts the noise-free data to analog values at the receiving end.
A driver in the development of new Arbs is the explosion in telecommunications, with the need to test new components, subsystems, and complete systems. Digital video is coming onto the scene as well with its new set of test requirements.
“Soon the high-end Arbs will have sample rates of up to 4.0 GS/s with bandwidths greater than 800 MHz,” predicted Joseph Martorano, product marketing manager at Tektronix. “Vertical resolution will be 14 or possibly even 16 bits. More attention will be given to mixed-signal simulation capabilities. Also, even though high signal output levels have been traded for sampling rates in the past, we can expect new PIN drivers to give designers both voltage and speed.”
Lest we forget, the low-speed flexible Arb still attracts a great deal of attention. Tom Sarfi, technical support manager at VXI Technology, noted that the company’s VM3616A module has 16 channels and runs at up to 100 kS/s. “Half of our customers use it as a high-density digital-to-analog converter (DAC) and the other half have the waveform creation capability to generate heart patterns, simulate automobile ignition control signals, gate higher-frequency Arbs, and do other necessary but not exotic tasks.”
Arb Comparisons
Characteristics of several Arbs currently on the market, showing the versatility of these handy instruments, are listed in the chart that accompanies this article. (See the August 2001 issue of Evaluation Engineering.)
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August 2001