Electrical Indicating and Test Instruments 1 Introduction 161

Display position control
This allows the position at which a signal is displayed on the screen to be controlled in
two ways. The horizontal position is adjusted by a horizontal position knob on the oscilloscope front panel, and similarly a vertical position knob controls the vertical position. These controls adjust the position of the display by biasing the measured signal with d.c. voltage levels.

Digital Storage Oscilloscopes

Digital storage oscilloscopes are the most basic form of digital oscilloscopes but even these usually have the ability to perform extensive waveform processing and provide permanent storage of measured signals. When first created, a digital storage oscilloscope consisted of a conventional analogue cathode ray oscilloscope with the added facility that the measured analogue signal could be converted to digital format and stored in computer memory within the instrument. These stored data could then be reconverted to analogue form at the frequency necessary to refresh the analogue display on the screen, producing a nonfading display of the signal on the screen.

While examples of such early digital oscilloscopes might still be found in some workplaces, modern digital storage oscilloscopes no longer use cathode ray tubes and are entirely digital in construction and operation. The front panel of any digital oscilloscope has a similar basic layout to that shown for an analogue oscilloscope in Figure 7.11, except that the controls for “focusing” and “intensity” are not needed in a digital instrument. The block diagram in Figure 7.12 shows typical components used in the digital storage oscilloscope. A typical commercial instrument was also shown earlier in Figure 5.1. The first component (as in an analogue oscilloscope) is an amplifier/attenuator unit that allows adjustment of the magnitude of the input voltage signal to an appropriate level. This is followed by an analogue-to-digital converter that samples the input signal at discrete points in time. The sampled signal values are stored in the acquisition memory component before passing into a microprocessor. This carries out signal processing functions, manages the front panel control settings, and prepares the output display. Following this, the output signal is stored in a display memory module before being output to the display itself. This consists of either a monochrome or a multicolor liquid crystal display (see Chapter 8). The signal displayed is actually a sequence of individual dots rather than a continuous line as displayed by an analogue oscilloscope. However, as the density of dots increases, the display becomes closer and closer to a continuous line. The density of the



Figure 7.12. Components of a digital storage oscilloscope.

dots is entirely dependent on the sampling rate at which the analogue signal is digitized and the rate at which the memory contents are read to reconstruct the original signal. As the speed of sampling and signal processing is a function of instrument cost, more expensive instruments give better performance in terms of dot density and the accuracy with which the analogue signal is recorded and represented. Nevertheless, the cost of computing power is now sufficiently low to mean that all but the least expensive instruments now have a display that looks very much like a continuous trace.

In addition to their ability to display the magnitude of voltage signals and other parameters, such as signal phase and frequency, most digital oscilloscopes can also carry out analysis of the measured waveform and compute signal parameters such as maximum and minimum signal levels, peak-peak values, mean values, r.m.s. values, rise time, and fall time. These additional functions are controlled by extra knobs and push buttons on the front panel. They are also ideally suited to capturing transient signals when set to single-sweep mode. This avoids the problem of the very careful synchronization that is necessary to capture such signals on an analogue oscilloscope. In addition, digital oscilloscopes often have facilities to output analogue signals to devices such as chart recorders and output digital signals in a form compatible with standard interfaces such as IEEE488 and RS232.
The principal limitation of a digital storage oscilloscope is that the only signal information captured is the status of the signal at each sampling instant. Thereafter, no new signal information is captured during the time that the previous sample is being processed. This means that any signal changes occurring between sampling instants, such as fast transients, are not detected. This problem is overcome in the digital phosphor oscilloscope.