Comparing analog and digital oscilloscopes for troubleshooting - HP 54600 Series oscilloscopes - Technical

[NOTE: SOME FORMULA'S HAVE BEEN OMITTED]

The analog oscilloscope has remained the troubleshooter's instrument of choice even though the digital oscilloscope has replaced it for laboratory analysis. However, the analog oscilloscope has limitations, especially in digital troubleshooting.

Taking these two characteristics of the analog CRT into account, a map of useful operating area can be developed. Mg. 1 shows a plot of sweep speed as a function of input trigger repetition rate. Let's examine this map to see how the limits were established. Holding the input signal repetition rate constant at a point where a bright crisp display will be obtained and decreasing the sweep speed, we will reach a point where the display starts to flicker. Contining to decrease the sweep speed will cause the flicker to increase to a point where the display is no longer useful. It will eventually decay to a moving bright dot. The point at which the flicker becomes so bad as to cause the display to be unusable is the left-hand limit of the operating area. Holding the sweep speed constant at a point where the light output and flicker problems are minimized and varying the signal repetition rate, we see that as the input signal repetition rate is decreased the display becomes dimmer. Once a point is reached where the display in no longer viewable in room light, the lower limit of the operating area is reached. This limit increases as the sweep speed is increased.

Another limitation of analog oscilloscopes is their inability to look ahead of the trigger point. Laboratory-quality analog oscilloscopes have delay lines in their vertical systems to ensure that the trigger point is displayed, but this does not solve the problem when the trigger event is at the end of the event to be displayed. This is a very common case in digital system analysis. Another limitation of analog based oscilloscopes is that their measurements are based on analog ramps with accuracy specified as a percent of full scale. Often greater accuracy is required to determine the cause of problems in digital systems.

Digital Oscilloscopes

The digital oscilloscope has addressed these problems. Digital oscilloscopes make their measurements with digital precision, and they have the ability to view events that occur before the trigger. In most cases they have the ability to place the trigger event at the end of the display window, giving the user a full screen of negative-time or pretrigger information.

With their digital storage displays, digital oscilloscopes have solved the flicker and low light output problems. The present generation of digital oscilloscopes offers a bright display but it is rather static. The displays of these oscilloscopes are more like a slide projector show than a video show. That is, they are very high-fidelity, but changes in the waveform are difficult to observe. Higher sample rates offer a partial solution to this problem, but the limiting factor is the processing time required to create the image. The present state of the art in digital oscilloscopes results in an image throughput rate of less than 100 waveforms per second. For this reason these instruments have been widely accepted in research and development labs, but they have not replaced the analog oscilloscope in troubleshooting and adjustment applications.

Effect on HP 54600 Design

The market research gave us a clear picture of the needs of oscilloscope users who were continuing to use analog oscilloscopes even when the inherent limitations of these oscilloscopes became problems. These users placed such high value on the interactive display and direct access controls of analog oscilloscopes that they were willing to live with their other limitations. The design goals of the HP 54600 project were to produce a digital oscilloscope that would have the look and feel of analog with the power of digital and at a price that was comparable to high-quality analog oscilloscopes. The HP 54600 Series oscilloscopes are designed for users who know that they need the advantages of a digital oscilloscope but are unwilling to sacrifice the real-time performance of analog oscilloscopes.

This design goal resulted in a three-processor architecture that relieves the CPU of the acquisition and display functions and places those tasks under the control of two custom integrated circuits. This new architecture provides a display throughput of more than one million points per second, producing a digital oscilloscope with display and control responsiveness equal to that of analog oscilloscopes.

Fig. 2 is a plot of sweep speed as a function of trigger repetition rate for the HP 54600. The digital display system never flickers, so the oscilloscope is fully useful at sweep speeds as slow as 5 seconds per division. Since the display is refreshed by a custom processor instead of the sweep ramp, the brightness does not decay as the sweep speed is increased or as delayed sweep is used. The only part of the map that is not covered is the high-speed single shot area. This coverage is limited by the 20-mega-sample-per second analog-to-digital converter system.

The logarithmic vertical axis of Fig. 2 masks the single-shot limitation of the HP 54600 analog-to-digital converter system. Single-shot captures are useful for signals with bandwidths of 2 MHz or less.

HP 54600 displays of slowly changing signals are presented free of flicker. Precision timing of control and handshake signals can be performed at sweep speeds that required a viewing hood or camera with analog oscilloscopes. Figs. 3 and 4 are analog oscilloscope and HP 54600 displays of the same delayed sweep application.

References

1. Principles of Cathode-Ray Tubes, Phosphors, and High-Speed Oscillography, Application Note 115, Publication number 5952-2044, Hewlett-Packard Co., 1970.

2. Eliminating Time-Base Errors from Oscilloscope Measurements, Application Note 262, Publication number 5953-'3812, Hewlett-Packard Co., 1978.

COPYRIGHT 1992 Hewlett Packard Company
COPYRIGHT 2004 Gale Group