Electrical Indicating and Test Instruments 1 Introduction 161

Personal Computer-Based Oscilloscope

A PC-based oscilloscope consists of a hardware unit that connects to a standard PC via either a USB or a parallel port. The hardware unit provides signal scaling, analogue-to- digital conversion, and buffer memory functions found in a conventional oscilloscope. More expensive PC-based oscilloscopes also provide some high-speed digital signal processing functions within the hardware unit. The host PC itself provides the control interface and display facilities.

The primary advantage of a PC-based oscilloscope over other types is one of cost; the cost saving is achieved because use of the PC obviates the need for a display unit and front control panel found in other forms of oscilloscopes. The larger size of a PC display compared with a conventional oscilloscope often makes the output display easier to read. A further advantage is one of portability, as a laptop plus add-on hardware unit is usually smaller and lighter than a conventional oscilloscope. PC-based oscilloscopes also facilitate the transfer of output data into standard PC software such as spreadsheets and word processors.
Although PC-based oscilloscopes have a number of advantages over converntional oscilloscopes, they also have disadvantages. First, electromagnetic noise originating in PC circuits requires the hardware unit to be well shielded in order to avoid corruption of the measured signal.
Second, signal sampling rates can be limited by the mode of connection of the hardware unit into the PC.

Summary

This chapter looked at the various ways of measuring electrical signals that form the output of most types of measuring instruments. We noted that these signals were usually in the form of varying voltages, although a few instruments have an output where either the phase or the frequency of an electrical signal changes. We observed that varying voltages could be measured either by electrical meters or by one of several forms of oscilloscopes. We also learned that the latter are also able to interpret frequency and phase changes in signals.
Our discussion started with electrical meters, which we found now mainly existed in digital form, but we noted that analogue forms also exist, which are mainly used as meters in control panels. We looked first of all at the various forms of digital meters and followed this with a presentation on the types of analogue meters still in use.
Our discussion on oscilloscopes also revealed that both analogue and digital forms exist, but we observed that analogue instruments are now predominantly limited to less expensive versions used in education markets. However, because the students at which this book is aimed are quite likely to meet analogue oscilloscopes for practical work during their course, we started off by looking at the features of such instruments. We then went on to look at the four alternative forms of digital oscilloscope that form the basis for almost all oscilloscopes used professionally. We learned that the basic form is known as a digital storage oscilloscope and that even this is superior in most respects to an analogue oscilloscope. Where better performance is needed, particularly if the observed signal has fast transients, we saw that a new type known as a digital phosphor oscilloscope is used. A third kind, known as a digital sampling oscilloscope, is designed especially for measuring very high-frequency signals. However, we noted that this could also measure voltage signals that were up to 1 volt peak- to-peak in magnitude. Finally, we looked at the merits of PC-based oscilloscopes. In addition to offering oscilloscope facilities at a lower cost than other forms of oscilloscopes, we learned that these had several other advantages but also some disadvantages.