For the design engineer, building communications equipment is a tough challenge. Speeds have increased to the multigigabit range and standards are changing every day. It's no wonder there's a pressing need for improved communications test equipment to validate their designs. Instrument manufacturers know this and are constantly innovating to meet these challenges.
We might consider how these innovations come about. Are research staffs of the big test-equipment companies busy predicting the future, so that they can turn out the perfect product at the right time? Probably not.
To keep pace with new communications developments, test-equipment manufacturers work closely with their customers. Sometimes this means developing test equipment side-by-side with the communications products. Other times it involves listening to customer feedback about the equipment.
We don't intend to cover the entire spectrum of communications test instruments here. Rather, we will cover some trends and examine a few new measurements required by the latest advances in communications technology.
Let's begin with general-purpose digital storage oscilloscopes (DSOs). These instruments, of course, test the complete gamut of circuitry, including communications designs. In order to facilitate communications testing, DSO manufacturers supply software for checking compliance the various communications standards.
One of the trends in this area is to increase the flexibility of the test software. An example is a recent innovation from LeCroy Corp. called a pass/fail mask-creation utility. Known as PolyMask, the utility runs on a PC. Users create pass/fail masks that define signal pass/fail boundaries. These masks are then loaded into a high-performance LeCroy oscilloscope.
With the PolyMask option, users can create complicated test masks, such as those used for testing 100Base-T network signals. Signals can be tested to lie within or outside the mask area.
While high-end general-purpose DSOs have bandwidths in the 1- to 2-GHz range, high-end sampling oscilloscopes have bandwidths in the 30- to 50-GHz range. Those oscilloscopes, referred to as communications analyzers, are targeted at designers who build transmitters for optical networks which operate at rates up to 10 Gbits/s.
As optical transmitter signal speeds increase, it becomes more difficult to distinguish the transmitter signal from the noise. One of the trends in the newest communications analyzers, therefore, is increased signal-acquisition fidelity. The Tektronix CSA8000, for instance, has short-term trigger jitter that's typically less than 1 ps and timebase stability of less than 0.1 ppm.
Communications analyzers, like the general-purpose DSOs, must be able to test to various standards. These instruments usually accept modules, which perform compliance tests against such standards as SONET/SDH, Gigabit Ethernet, and Fibre Channel.
An important feature is a module's ability to test for compliance against several standards. This ability eliminates the process of swapping out modules and recalibrating the test system for each standard tested.
Another trend with these high-end communications analyzers is to integrate all necessary test components into a module rather than connecting to external devices. For example, a sampling module might contain an optical receiver, power meter, and clock-recovery circuit. Abolished, then, is the need for additional cabling and add-on accessories to support these functions.
Besides the Tektronix model mentioned above, Agilent Technologies recently announced its 86100A Infinium Digital Communications Analyzer. Compared to its predecessor, the 86100A has faster throughput and improved accuracy and repeatability.
Let's move away from networks for a moment and turn our attention to wireless communications. A known fact is the global transition from second-generation (2G) wireless phones to third-generation (3G) sets (see "A Bump In The Path To 3G," p. 88). Designers typically use spectrum analyzers to make measurements on their wireless prototypes, both handsets and basestations.
Especially useful in making wireless measurements on CDMA and wideband CDMA signals is the real-time- spectrum analyzer (RTSA). Note the opening theme illustration for an example of measurements taken by this type of instrument. An RTSA differs from a conventional swept-spectrum analyzer. The latter sweeps through a series of frequency increments one at a time. It requires a steady signal, and takes time to accumulate enough information for a display. While time elapses, momentary events may occur unnoticed by the instrument. An RTSA, on the other hand, captures a broad band of frequencies instantaneously and continuously. That's why it can capture wireless signal "bursts" and brief transients.
Right now, Tektronix is the only manufacturer that makes an RTSA. Its top- of-the-line model, the 3086, has a 3-GHz bandwidth and a 30-MHz instantaneous capture capability (Fig. 1). Two new software options equip the 3086 to perform code-domain power and complementary cumulative-distribution-function (CCDF) measurements.
Code-domain power quantifies a basestation's response to instructions from the network. When configured with this option, the 3086 measures code-domain power to the published specifications for W-CDMA experimental version 1.1. This measurement will also be available for analysis of 3GPP signals.
While other instruments can measure code-domain power, Tektronix believes the wide-bandwidth, real-time acquisition and deep-capture memory of the 3086 are perfectly suited for critical 3G measurements. The instrument can record 10 full frames—that's 160 time slots of 100 ms—of information from one trigger event. This gives the designer a wide window of viewable frames. Conventional spectrum analyzers, the company says, require a hit-and-miss, frame-by-frame examination of the data.
"For power control measurements, major mobile equipment manufacturers have been looking for a code-domain measurement tool that captures many W-CDMA frames in one acquisition," says Steve Stanton, product manager for Tektronix. "The 3086 spectrum analyzer with Option 16 is the first solution to really meet that need."
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