Measurements technics
RadioRadar 
Measurements technics
Oscilloscope Display Quality Impacts Ability to Uncover Signal Anomalies Agilent 6000 Series Scope versus LeCroy WaveSurfer 400 Series Scope
Application Note 1553
Introduction
The quality of your oscilloscope’s display can make a big difference in your ability to troubleshoot your designs effectively. If your oscilloscope has a low-quality display, you may not be able to see critical signal anomalies. A scope that is capable of showing signal intensity gradations can reveal important waveform details, including signal anomalies, in a wide variety of both analog and digital signal applications.
This application note compares display quality for a variety of analog and digital signals using Agilent’s 6000 Series mixed signal oscilloscope (MSO) and LeCroy’s WaveSurfer 400 Series scope. In some examples, we also show the results displayed on a traditional analog oscilloscope. We also discuss a methodology for quantifying display quality to make it easier to compare scope displays objectively.
The third dimension: intensity modulation
Engineers traditionally think of digital storage oscilloscopes (DSOs) as two-dimensional instruments that graphically display only voltage versus time. But there is actually a third dimension to a scope: the z-axis. This third dimension shows continuous waveform intensity gradation as a function of the frequency of occurrence of signals at particular X-Y locations. In analog oscilloscope technology, intensity modulation is a natural phenomenon of the scope’s vector-type display, which is swept with an electron beam. Due to early limitations of digital display technology, this third dimension, intensity modulation, was missing when digital oscilloscopes began replacing their analog counterparts. Now, it is making a comeback.
Display intensity gradation can be extremely important when you are looking for signal anomalies, especially when you are viewing complex-modulated analog signals such as video, read-write disk head signals, and digitally controlled motor drive signals. Intensity gradation is also helpful in a wide variety of mixed-signal applications found in embedded microprocessor and microcontroller technologies common in the automotive, industrial, and consumer markets. But even when you are viewing purely digital waveforms, intensity gradation can show statistical information about edge jitter, vertical noise, and the relative occurrence of anomalies.
Recently, all major digital oscilloscope vendors have begun to provide z-axis intensity gradation – with varying degrees of success – in order to emulate the display quality of an analog oscilloscope.
Complex-modulated analog signal applications
If you are working with complex-modulated signals, you need a scope with sufficient display quality to let you look at the big picture and then zoom in to see the details.
Composite video signals
Many engineers are familiar with standard NTSC or PAL composite video signals, which are complex-modulated analog signals. Figure 1 shows one frame of composite video photographically captured from an analog oscilloscope's display. Even though the display “flickers” when you view this waveform at 5 ms/div, there is important information embedded within the displayed waveform envelope. An experienced video design engineer can quickly determine the quality of analog signal generation from this display.
Figure 2 shows an older digital oscilloscope display without z-axis intensity modulation. Although this scope has sufficient sample rate and memory depth to capture details of this signal even at 5 ms/div, all captured points are displayed with the same display intensity. Waveform detail within the signal’s envelope is visually lost. Given a choice between analog oscilloscope technology and older digital display technology, it’s no surprise that today’s video labs are filled with analog oscilloscopes!
Figure 1. Full frame of composite video displayed on a 100-MHz analog oscilloscope
Figure 2. Full frame of composite video displayed on an older digital oscilloscope without intensity gradation capability
However, the visual quality of an analog oscilloscope’s display has finally been matched in a digital oscilloscope. Figure 3 shows the real-time capture of a video signal using Agilent’s 6000 Series oscilloscope. This scope uses Agilent’s proprietary MegaZoom III technology that provides up to 256 levels of color intensity gradation for each pixel based on deep memory acquisitions (up to 8 MB) mapped to a high-resolution display (XGA). This digital oscilloscope can display a repetitive analog signal with quality similar to (or perhaps better than) an analog oscilloscope, and it can also capture, display, and store complex single-shot signals with the same visual resolution. This is where conventional analog oscilloscopes fall short of their digital counterparts. Analogscopes can only display repetitive waveforms. Figure 4 shows a zoomed-in/windowed display of a single line of the composite video captured from the same acquisition that is shown in Figure 3. But since analog oscilloscopes can’t digitally store waveforms, we are unable to show a similar zoomed-in single-shot display using an analog oscilloscope.
Figure 3. Full frame of composite video displayed on Agilent’s 60000 Series scope
Figure 4. Zoomed-in display of a single line of video using Agilent’s 6000 Series scope
Figure 5 shows an attempt to repetitively capture this same composite video signal using a LeCroy WaveSurfer 400 Series oscilloscope. This oscilloscope has a special user-selectable display mode called Analog Persistence, which is a technique used by LeCroy in an attempt to emulate the display quality of an analog oscilloscope. With analog persistence, the intensity of digitized points decays over a user-specified persistence time. But with a minimum persistence setting of 0.5 seconds, the scope’s effective waveform update rate is just two waveforms per second. Although the quality of LeCroy’s analog persistence mode far exceeds that of older digital scopes (Figure 2), we think it falls short of the quality of either Agilent’s MegaZoom III technology or of an analog scope’s display quality. We encourage you to judge for yourself. But just like an analog oscilloscope, analog persistence in the LeCroy WaveSurfer 400 Series oscilloscopes is primarily a repetitive mode of operation. And although analog persistence does provide minimal levels of intensity grading for real-time acquisition displays, this display mode does not provide a display of continuous waveforms, but only displays “dots” as shown in Figure 6.
Figure 5. Full frame of composite video using the LeCroy WaveSurfer scope
Figure 6. Zoomed-in display of a single line of video using the LeCroy WaveSurfer scope
Digitally controlled motor drive signal Another example of a complex analog signal is a digitally controlled motor drive signal. A one-time start-up cycle of a motor would be classified as a single-shot phenomenon. Figure 7 shows how Agilent’s 6000 Series mixed signal oscilloscope (MSO) is able to reliably capture one phase of this single-shot start-up motor drive signal. You can also use this scope’s digital/logic channels to synchronize and trigger the waveform capture based on the digital control signals of the motor. This capability can be extremely important when you attempt to synchronize acquisitions on not only power-up sequences, but also on particular motor positioning commands. Although not shown, this oscilloscope could just as easily capture all three phases of the motor drive signals simultaneously using its four channels of analog acquisition.
Also shown in Figure 7 are two zoomed-in images taken from the same single-shot acquisition. With this scope’s MegaZoom III technology, we can see a bright vertical vector (near the center of the display) in the middle image after zooming-in by a factor of 100. Further waveform expansion (20,000:1) on the pulse-width modulated (PWM) burst reveals a glitch, as shown in the image on the right.
Figure 7. Motor drive signal start-up sequence with digital control signal triggering and various levels of zoom to reveal “runt” pulse using Agilent’s 6000 Series MSO
As shown in Figure 8, the LeCroy WaveSurfer oscilloscope is able to capture the start-up cycle of motor drive signal with some levels of intensity grading. However, when zooming in to observe waveform details, this oscilloscope lacks sufficient acquisition and display resolution to clearly show the signal anomaly that the Agilent oscilloscope captured and displayed. When zoomed in by a factor of 20,000 on this single-shot waveform capture, Figure 9 shows significantly less signal detail as compared to the Agilent 6000 Series oscilloscope. With just 2 M points of acquisition memory, the LeCroy scope captures this waveform with one-fourth the sample rate of the Agilent scope, which provides up to 8 M points of acquisition memory.
No attempt was made to capture this signal on an analog scope since our signal was single-shot and a conventional analog oscilloscope is incapable of storing a single-shot waveform. At this time base setting (100 ms/div), you would observe nothing more than a streak of light traveling across the analog scope’s display. And of course zooming in a stored waveform would be impossible with analog oscilloscope technology.
Figure 8. Start-up motor drive signal capture using a LeCroy WaveSurfer scope
Figure 9. Zooming in by a factor of 20,000 on the LeCroy WaveSurfer scope (note: dots have been enhanced for clarity)
