The telecommunications market is demanding and dynamic, with twin forces-rapid expansion and quality expectations-creating an enormous challenge for designers. These engineers work at the cutting edge of network and wireless technology. At the same time, they must produce advanced products at lower costs, and on schedules that are measured in months, and sometimes only weeks. Moreover, the product must work right the first time, with no exceptions, otherwise the opportunity for market domination can be completely lost.
Aware of these relentless demands, telecommunications designers rely heavily on simulation to ferret out the hidden flaws in a device before it goes into production. Simulation does not, however, provide a complete picture of a system's real-world performance. There are just too many variables—both in design and manufacturing—to effectively simulate a product's behavior without the aid of hardware.
The best way to thoroughly examine a design is to couple simulation with rigorous prototype testing. This is accomplished by completely characterizing the device or system on both the physical and protocol layers. The designer also must stress test the product to ensure its operation under such adverse conditions as jitter, bit-pulse impairments, code violations, and noise.
Using an arbitrary waveform generator (AWG) to quickly create an unlimited range of waveform stimuli is one of the best ways to enhance prototype testing of a telecom system. Unlike a bit-error-rate tester (BERT), an AWG can go beyond ideal signals and generate the anomalies, intersymbol effects, jitter, and signal degradation that occur in the real world, helping the telecom designer fully determine the performance of a product under actual operating conditions.
For example, acceptable data communications signal tolerances for network physical-layer testing are well defined by industry standards. However, most current protocol analyzers or BERTs do not test bit pulse extremes as defined by the industry pulse-mask standards. An AWG, on the other hand, simulates serial data signals and tests a device with the pulse extremes to standards. This limit testing is extremely useful in verifying that a device or system truly conforms to a telecom standard.
It is important to note that an AWG does not eliminate the need for a BERT or protocol analyzer. The reason behind this is that a 1-GHz AWG's analog output supports physical-layer testing of communications data rates up to 250 MHz, providing four points per cycle. Instead, an AWG complements existing test tools such as a BERT and digital storage oscilloscope (DSO), helping to extend the range of testing that a designer can perform. This is possible because modern AWGs seamlessly interact with other test and measurement instruments, exchanging pertinent information between them (Fig. 1).
An AWG can be synchronized with a BERT, and used to stimulate a device under test (DUT), with the BERT receiving, analyzing, and displaying the output from the system. A typical prototype test scenario is as follows: First, the designer uses the BERT to perform protocol testing. Then, moving to the physical layer, the designer either selects from the AWG internal library, or downloads simulation vectors directly into the AWG.
Using the isolated bit-pulse impairment features of a modern AWG, the generator then creates those vectors needed for "what if" scenarios and stress testing of the prototype. The AWG creates serial-data signals, for example, to determine if the DUT complies with the specified telecom pulse-mask standard. The designer also quickly tests for different loading and possible worst-case scenarios, such as fading in a wireless design. Testing for peak or region shifting and all types of jitter or jitter modulation can also be performed, including tests for low- and high-frequency jitter, and jitter on an individual or isolated bit.
Streamline Debugging
If there is a performance issue, an AWG significantly streamlines the task of isolating and debugging the problem. In conjunction with a BERT and a DSO, the AWG creates a full debugging environment, eliminating the need for special or custom debug circuitry, or combinations of multiple signal generators. First, the DUT can be stimulated with the BERT's transmitter section. If an error occurs, the advanced logic or mask triggering capabilities of the DSO will trigger and capture the aberrant behavior. In this way, the conditions that caused the error can be recreated by the AWG on a repetitive basis, enabling the designer to quickly zero in on the source of the error. This combination is particularly powerful when the error-generating events occur infrequently.
With all they have to offer, you would think that AWGs would be part of every telecom designer's suite of test tools. But the fact is they are not.
Until just a few years ago, AWGs did not offer the performance required for telecom applications. Fortunately, today's AWGs change all that. The newest AWGs deliver the advanced capabilities required for telecom testing—up to 250-MHz data standards using the analog outputs, and up to 1-GHz parallel data rates when using the digital output. Many new electrical telecom standards now exceed 1 GHz, such as fiber-distributed data interface (FDDI) and Ethernet. Optical standards are going way beyond that, making it is absolutely essential that an AWG be able to keep pace.
Modern AWGs also supply sufficient vertical resolution (some up to 10 bits at 1 GHz) to allow telecom designers to go beyond idealized square waves, and precisely create extreme, real-world conditions such as high-frequency jitter and noise. Using these examples of worst-case telecom signals, the designer can quickly determine the robustness of an application.
Along with enhanced throughput and vertical resolution, leading-edge AWGs support extremely long record lengths. These deep memories let the designer recreate the message stream more completely with its pulse shape and information content. Longer record lengths eliminate having to sacrifice simulation accuracy by providing enough storage capacity for fast, detailed waveforms.
Some of the newest AWGs go beyond simply improving overall performance by providing specialized features tailored for telecom applications. A few even have built-in telecom standards. The AWG 500 Series from Tektronix, for example, supports DS1, DS3, STS1, and many more standards. New, emerging standards also can be added using a floppy disk or the built-in Ethernet port.
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