Exactly as we have learned to expect in the PC arena, this year’s digital storage oscilloscopes (DSOs) offer higher performance and more features–without a significant increase in price–than provided by the prior generation. And just as is the case with the ubiquitous PCs, presentation clarity and data-handling capability continue to improve.
Except for some PC card-based versions, single-channel DSOs no longer exist, dual-channel instruments have become predominant and many users now opt for four-, eight- and even higher channel DSOs. But as more channels vie for screen space, there can be confusion and misinterpretation. To preserve presentation clarity, many scopes now offer multi-color displays so a different color can be assigned to each trace.
Since all information captured by the DSO is converted into digital form, data is easily stored or transmitted to a computer for analysis. To facilitate such transfers, many oscilloscopes are now outfitted with external ports or accept transportable storage media.
When viewing only two traces, it may be nice to have a color display but, with few exceptions, it is certainly not essential. However, when four or more traces must be observed, color differentiation is decidedly helpful.
“Color displays greatly enhance the usability of a DSO,” commented Kim Hartman at Tektronix. “Overlaid waveforms become more readily distinguishable and measurement results are color-matched with their corresponding waveform. This is especially useful if several waveforms are being evaluated concurrently or if new information is being compared with previously captured data.”
When traces are crossing, it may be particularly difficult to distinguish between the progression of several intertwined waveforms unless each is in a different color. The alternative, commonly applied when using monochrome DSOs, separates the traces by partitioning the screen and placing each trace in a separate window. For four traces, each attenuated signal would take up 25% of the screen.
“The cost of the split-screen technique is the loss of signal-interaction information and display dynamics,” said Pete Cipriani, Senior Product Marketing Specialist at Gould Instrument Systems. “The user also has smaller traces to observe and is using less resolution of the A/D converter. In contrast, a color display uses the full screen for each signal.”
Judicious use of color may also help in other ways. For instance, displays can be personalized by assigning color to group related signals, types of measurements or events, Mr. Cipriani added.
But not everyone is endorsing color displays. “For 95% of the scope users, color is something they like but has no real value to them,” remarked Mike Lauterbach, Director-Product Management at LeCroy. “Most engineers ponder the color-no color issue, but the real trade-off is color or high resolution. Given the choice between a 7″ color display with 640 x 480 pixel resolution or a 9″ 810 x 696 amber display, most customers choose the high-resolution amber display.”
DSO-PC Data Interchange
In many of today’s automated test applications, DSOs are controlled from and report measurement results to a PC or a local area network. In this situation, a wired interconnection is essential. But when the PC is used primarily in a post-processing or test-report preparation role, a wireless data exchange via a removable storage media is more appropriate.
To accommodate the wired data exchange, IEEE 488 or RS-232 ports are used most often. Using the RS-232 interconnection is tempting since it is economical and every PC is already outfitted with this port. However, being a serial interface, data transfer via RS-232 is very slow. It is most appropriate for sending limited control commands, transmitting short files or transferring data to a plotter.
The IEEE 488 interface offers faster data transfer because it is a parallel bus. While many DSOs provide an IEEE 488 port, either as a standard or an optional feature, PCs must be outfitted with a plug-in board to achieve the PC bus-to-IEEE 488 bus data conversion. Although this entails additional cost, it is often a worthwhile expenditure.
“IEEE 488 provides many benefits, especially whenever multiple-instrument test systems are being configured,” said Wim Nederhoff, Product Manager at Fluke. “The IEEE 488 bus allows one PC to interface with many instruments at fast data transfer rates. However, in very large systems where many instruments share the same bus, each instrument has to wait for permission to send data to the controller, reducing the effective data transfer rate.”
Many of today’s DSOs are used in data acquisition applications, some collecting data on four, eight or as many as 32 channels. Much of this collected data is post-analyzed on a PC. The transfer of voluminous multichannel or long data-stream information is best accomplished by a portable storage media, such as a floppy disk, PCMCIA memory card or PCMCIA hard disk.
A portable storage media may also hold sample known-good waveforms for field-service troubleshooting. To determine which type of storage media would be most appropriate for your application, data length and transfer rate are the two main factors to be considered.
Table 1 shows data capacity, transfer rate and typical waveform transfer speeds for three popular storage media. To determine waveform file sizes, it is assumed that a channel trace uses 1 byte per sample plus approximately 360 bytes for a waveform descriptor; a processed trace uses two bytes per sample.
Since DSO data is not innately available in a DOS-compatible format, a conversion program is usually required. This may be generated by writing a C or BASIC program that uses oscilloscope commands. Alternatively, an inexpensive software package available from the oscilloscope supplier may be used, such as the Windows-based HP BenchLink Scope software.
More users now opt for deep memory because it makes it easier to find elusive fault-producing aberrations and because memory costs less. But more DSOs now contain features that simplify catching offending signal anomalies, including intermittents, through intelligent triggering capabilities. An example is LeCroy’s Exclusion Trigger, which automatically differentiates between abnormal and normal signals.
Buffer- or segmented- memories are also being used to compress long records or provide a match between data acquisition speed and data transfer-rate capabilities. Marianne McTigue, HP 54500 Serial Product Marketing Engineer at Hewlett-Packard, described how a feature such as this can help:
“The HP 54520 and HP 54540 Series scopes have a segmentable memory feature that allows you to capture and store sequential single short waveform data into an internal buffer at a rate faster than the data can be transferred over the GPIB interface. For instance, a series of laser pulses could be acquired without capturing the dead time between the pulses.”
Many DSOs are now configurable or application-oriented or aim to replace an entire data acquisition and analysis system. A case in point is the Gould DataSYS™ 765 Direct Recording Oscilloscope. It has an internal hard drive, thermal plotter and software for real-time and post-storage analysis.
Similarly, the Nicolet 2580 is a single box solution. “It includes a built-in PC with an active matrix color thin-film transistor (TFT) display, TeamPro software for control, analysis and documentation; and space for up to 24 input channels,” explained Mike Hoyer, Applications Engineer at Nicolet.
Another all-inclusive system configurable with up to 32 channels is the Hi-Technique HT600. It also features a TFT active matrix color touch screen and an internal PC with three unused ISA slots.
But common to almost all new offerings is the fact that users get more for their money. While individual prices may not necessarily be lower, several DSOs provide twice the bandwidth than the models they supersede and others offer more features. A listing of current scopes is provided in the accompanying chart.
Copyright 1996 Nelson Publishing Inc.