6 - Electrical indicating and test instruments

Abstract

This chapter discusses various types of digital meter and how they are basically modified forms of the digital voltmeter (DVM), irrespective of the quantity that they are designed to measure. Digital meters have been developed to satisfy a need for higher measurement accuracies and a faster speed of response to voltage changes than can be achieved with analog instruments. Analog meters are relatively simple and inexpensive, and are often used instead of digital instruments, especially when cost is of particular concern. Analog meters are electromechanical devices that drive a pointer against a scale. Electronic voltmeters differ from all other forms of analog voltmeters in being active rather than passive instruments. In its basic form, it is an analog instrument and is often called an analog oscilloscope to distinguish it from digital storage oscilloscopes, which have emerged more recently. However, it is not a particularly accurate instrument and is best used where only an approximate measurement is required. The cathode ray oscilloscope is probably the most versatile and useful instrument available for signal measurement. The stream of electrons is focused onto a well-defined spot on a fluorescent screen by an electrostatic focusing system that consists of a series of metal discs and cylinders charged at various potentials. Digital storage oscilloscopes consist of a conventional analog cathode ray oscilloscope with the added facility that the measured analog signal can be converted to digital format and stored in computer memory within the instrument.