Those 6- and 8-GHz digital storage oscilloscopes (DSOs) that were sufficient a year or two ago are now becoming overrun. Silicon and motherboard designers' scopes seemingly have lost the ability to handle the lightning-fast speeds of the latest and next-generation computer buses.
For instance, new high-end PCs feature a Serial ATA (SATA) bus for drives and a PCI Express motherboard. As a result, they promise faster-than-ever speeds for chip-to-chip, graphics, and other I/O functions.
In addition, cell phones, cordless phones, and other consumer and military devices operate in the extreme UHF to microwave frequencies, requiring DSOs with wide bandwidth and high sampling speeds. Designers are looking toward bandwidths of 10 GHz and beyond in their scopes to keep up with the devices they're creating (Fig. 1).
Not all design engineers need a light-speed DSO with a $50,000 to $130,000 price tag, though. Motor drives, audio devices, industrial I/O boards, light dimmers, and other relatively "low-tech" devices probably require a DSO of less than 100-MHz bandwidth. But engineers still dream about practical features that make their jobs easier, even with the less glitzy sampling speeds and bandwidth features. Scope manufacturers are fulfilling these dreams in their latest products.
DSOs offer several features not available in analog scopes. These features are sometimes no more expensive than the analog scope you may have used eons ago. In the dark ages, engineers used an awkward Polaroid camera to freeze a waveform on the screen—somewhat tricky if they were waiting for that all-elusive, fleeting transient—and the triggering wasn't set just right. Along came analog storage technologies, which made it possible to trap a transient for an extended time on the screen, but not forever.
Today, the DSO detects and displays the transient and writes it to memory, hard drive, or network file location. DSOs serve up a variety of options for triggering. Some provide the means of analyzing data internally on Microsoft Windows-based platforms or externally on the software of your choice (Fig. 2). The possibilities are almost limitless.
TRIGGERING NEEDS IMPROVEMENT Many designers believe triggering is a pretty important feature in a DSO, but the feature needs further development. Stan Katz, senior embedded designer at Control Technology Corp., lists flexible triggering and easy setup of triggering options among the functions and features he would value in a new DSO. He also cites the ability to "trigger on multiple events on different inputs, e.g., trigger on axis-2 positive edge or on axis-1 negative edge, whichever happens first."
Senior staff engineer Kevin Crawley of Keithley Instruments would like to see improved cross-channel triggering and keying on analog levels, as well as a trigger that would automatically save to a file each time an event happens. Thus, data could be gathered during overnight testing.
Some scopes already have these triggering and data-saving capabilities, remarks Boyd Shaw, a product manager with Yokogawa Corp. of America's Test and Measurement Division. In his company's scopes, an "OR" trigger works across several channels. It would, say, trigger any time the signal on channel 1 rises above 2 V, channel 2 falls below 7.8 V, or channel 4 rises above 6.2 V.
The user sets rising/falling edge conditions and trigger-level conditions independently for each channel of interest. The scope then continuously monitors each input signal and compares it to the triggering conditions set. Afterward, if and when the trigger condition is met on any channel, the instrument triggers (i.e., saves that segment of data).
Second, an "Action on Trigger" performs one or more activities every time a trigger occurs (Fig. 3). Actions might include saving the data to a floppy drive or printing a hard copy of the screen image. If the instrument is attached to a network via Ethernet, it could save data directly to a network drive or send an e-mail indicating that a trigger occurred. The ability to automatically save data each time a trigger occurs is ideal when running tests over extended periods.
According to Jerry Murphy, Agilent's manager of mixed-signal oscilloscopes, customers often design equipment with a combination of digital and analog signals. But using a logic analyzer to monitor signals in these devices is like using an elephant gun to kill a fly, he adds.
Giorgio Decker of Elkron was one of these customers. "We had a big problem in our design regarding transmission synchronization on many devices," he says. "Usually, we analyzed these signals with a logic analyzer. But in debugging serial synchronous interfaces (data in, data out, and clock), it was really not easy since the problem was random and there was no way to trigger on it. We had to look at eight different channels in which the anomaly could appear in any one of them without a timing relationship."
Borrowing another engineer's 54621D mixed-signal scope solved the problem (Fig. 4). Unlike a logic analyzer, the scope was easy to set up. The scope also made it possible to see the waveform related to eight transmission lines simultaneously in a continuous mode with a slow time base (a transmission every second). It also stored the image when the problem happened. The scope's MegaZoom feature even let Decker see a single bit in the entire pattern, allowing a complete analysis of the problem.
The ability to trigger on patterns is an optional part of Tektronix's new Pinpoint triggering system, says Colin Shepard, vice president, oscilloscope products. The name Pinpoint summarizes the precision with which the new trigger system can isolate individual events or combinations.
Customer demand brought about this system, and it's been applied throughout Tek's high-performance scope family. Pinpoint triggering on the TDS7704B provides a jitter spec of less than 1.2 ps rms and less than 170-ps glitch capture for isolating events. The system offers full capabilities and flexibility on both A- and B-triggers.
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