One of the things that distinguishes analog oscilloscopes from digital instruments is the variable intensity of the displayed trace. Changes in trace brightness and line width result from the fact that light emission from the CRT phosphor coating is inversely proportional to the speed at which the electron beam moves across the phosphor coating. For a sinewave, as shown in Figure 1 for example, the speed of the electron beam across the screen varies over a range of 9.5:1, giving an inversely proportional brightness variation of about 7:1.

Figure 1. Typical analog oscilloscope waveform

Over the years that analog oscilloscopes have been used, operators have learned to interpret and rely upon intensity variations - to the extent that some manufacturers of combined analog and digital oscilloscopes recommend that the user switches to analog mode if there's any doubt about a waveform's true characteristics. With a pure digital instrument, this is, of course, not possible, and some other way has to be found to deal with troublesome waveforms.

Analog problems

These characteristics of analog oscilloscopes - variation of brightness with signal speed and high display resolution - can be perceived as either benefits or failings. For example, in modern electronics, signal edge speeds can be so high that ordinary phosphors generate very little light. The speed variations can also be quite large, so that the rising and falling edges may not be seen at all; alternatively, the slower parts will defocus excessively because of the high overall brightness required to make the edges visible. Some 'exotic' CRTs with photomultiplier faceplates which can display very fast events are available, but they are expensive.

DSO Benefits

There are three DSO benefits which represent the most usual reasons why DSOs are purchased: the ability to capture and display very slow signals; the capture and display of signals which occur only once and which may also be fast; and the fact that the signal is converted to digital form.

Slow signals: Waveform displays are not practicable on an analog oscilloscope at very slow sweep speeds because the combined persistence of the human eye and the CRT phosphor is not long enough. Instead of perceiving a waveform, the user sees only a slowly moving bright dot. The DSO, however, presents a uniform brightness display regardless of how slowly the events take place.

'One off' signals: Users are often faced with the need to measure events such as the effect on a car shock absorber of a bump in the road, or to determine what happens to a product inside a cushioned package when it is dropped. DSOs can capture and display the complex shock waveforms from such events - which by their nature are transient, and may occur only once. Similar challenges are presented by measurements on electrical discharges, high speed pulses in electronic circuitry, or laser systems. Analog oscilloscopes are unsuited to such transient signals. For slow signals, they display the 'moving dot' described above, while for fast signals the amount of light emitted is very small, so that the user has to resort to aids such as viewing hoods, darkened rooms or high speed camera films.

Digital signals: Analog oscilloscopes are basically designed to display waveforms, and some models can perform a limited range of measurements. However, the fact that the waveform is not digitized means that it cannot be stored, transferred to a computer for further analysis, or manipulated to create derived waveforms. There are, of course, still a few analog storage oscilloscopes available, as well as systems combining analog instruments and high-resolution TV cameras, but these are both expensive and unpopular. It's much easier to use a modern DSO which solves all these problems and more.

DSO problems and solutions

Most of the problems that occur with DSOs are caused by the sampling process itself. A common problem, aliasing, has at least two forms: one where the sampling rate is too low compared with the speed of the input signal; and one which is caused by compression of acquired data before it is displayed.

Figure 2. Loss of HF detail due to low sample rate (vector dot-join)

Figure 2 shows how either problem can affect a local area of the waveform. However, it is also important to realize that entire waveforms can be distorted.

A number of different approaches have been developed to counteract the effects of aliasing. 'Glitch detect' is the term usually used to describe the process of running the ADC at its top rate, regardless of timebase speed. This process develops an excess of samples beyond those required for display. Normally, the largest positive (maximum) and negative (minimum) values are selected, and are then used to define a vertical line segment which is displayed instead of a single sample point. There are a number of different types of glitch display which are used.

The maximum and minimum values must be displayed in the same time order in which they occur.

Figure 3. (left) Compressed data without time-ordering. (right) Compressed data with time-ordering

Figure 3 shows the effect of correct and incorrect time-order sorting. It is also possible to decimate the samples derived from the ADC, accepting only one in N, without any attempt made to pick the biggest or smallest value. This approach may, however, create aliasing rather than avoid it. See Figure 4.

Figure 4 (top) Triangular waveform with sample points, showing max-min pairs in that order. Figure 4 (center) Triangular waveform with sample points, showing "1 in N" points (1 in every 4) Figure 4 (bottom) Reconstructed waveform showing loss of peak values

'Display max-min' describes the process of determing what to display from the data that has been stored. In long store instruments, 50,000 data points may have been acquired, even though only 500 points can be displayed. Display max-min is a data-compression technique which chooses maxima and minima from stored data using an algorithm similar to the glitch-detection process, but adapted to prevent display aliasing rather than stored data aliasing.

The TruTrace® Solution

Conventional Digital Storage Oscilloscope waveform with no signal visible within TV frames...

...and now the same signal using TruTrace on a Gould Digital Storage Oscilloscope clearly showing differences in amplitude

In order to address these inherent problems in relation to transient capture and display resolution, a new patented technique called TruTrace has been developed. TruTrace performs an intelligent compression of acquired data which preserves analog-like intensity variations, and hence presents far more information about an acquired signal than is available with a conventional DSO.

In this technique, intensity variation is used to distinguish between those parts of a waveform which change slowly, and those which contain much more rapid activity, just as in analog oscilloscope displays. Each captured data point contributes to the displayed waveform, which is not the case with other commonly used data-compression techniques.

Intensity variation is also used to highlight areas where traces overlap. This is a characteristic of analog oscilloscopes as well. This feature makes it impossible to 'lose' one trace behind another; the display is easier to interpret and the DSO is easier to use.

TruTrace also uses an advanced single-line dot-joining method which allows sharp transients to be represented by the minimum-width line that the display can produce. Earlier systems needed two lines: one to trace from the bottom to the top of the transient, and one to trace from the top back down again. This means that the effective resolution of the TruTrace display is twice that of earlier approaches using the same raster system.

The TruTrace Technique

TruTrace is a data-compression method which operates on each successive single-shot acquisition separately. It applies just as well to a single acquisition as to a series of repeated ones.

The output of TruTrace's data-compression process is a reconstructed waveform in which each dot-joined element comprises a number of line segments of different intensities, rather than a single maximum/minimum line at constant intensity. The overall effect is similar to the variable-intensity of an analog real-time oscilloscope. One of the key benefits of TruTrace is the ability to display waveform details which cannot be seen on other digital storage oscilloscopes. A good example is the detail within video lines which have been compressed by using a slow timebase to view several lines at once. A long store would be able to capture the fine details, but the use of traditional data-compression methods would mean that most of the detail within each line would be lost by the time the trace was displayed. As a result, a typical digital storage oscilloscope will display a group of TV lines as solid bright blocks separated by a small synchronization time.

TruTrace operates on 'frames' of data which are similar to the frames required for the maximum/minimum and '1-in-N' methods of data compression. See Figure 5.

Figure 5. Full waveform memory is divided into data frames prior to processing

The concept of a data frame arises from the conflict between limited display resolution (typically 640 horizontal locations in a VGA system of which only 500 are used) and the need to acquire large amounts of data in order to describe adequately the detailed nature of complex waveforms. The new series of Gould DSOs offers up to 200,000 words of memory per channel, so that there could be 400 times more data captured than could be displayed without some form of compression.

The key to TruTrace is the development of a 'data density histogram'. This consists of 254 bins, 254 being the maximum number of displayable values in the 8-bit acquisition system. Each bin is filled with the number of times that a particular vertical value occurs within the entire dot-joined data frame (assuming that all the data points within the frame have been interconnected in a simple 'join-the-dots' style). See Figure 6.

Figure 6. Each data frame is processed producing a histogram determining the display intensity

This histogram is then drawn along the Y-axis instead of the more usual X-axis, so that the end result represents the relative intensity of all the dot-joined data within the given frame -- were it to be displayed on a high-resolution screen without compression. The more often the waveform segment represented by the data within the given frame crosses through a particular vertical value, the brighter it would appear if displayed on an analog oscilloscope.

From this histogram, it is then possible to develop a series of brightness ranges, each of which comprises a range of contiguous bin values. For example, bin values between 0 and 10 might correspond to brightness level 1, from 11 to 30 to level 2, etc. The vertically drawn histogram then corresponds directly to a single vertical line comprising many segments of different brightness.

To display a complete overview of the entire acquisition memory contents, each data frame is considered in turn, resulting in a display comprising 500 adjacent vertical lines, each with several quantized brightness segments. Every data point within each data frame contributes to the final displayed image. The overall display has the appearance and feel of an analog oscilloscope display.

With TruTrace, seeing really is believing!