TABLE OF CONTENTS Ian Hickman
Signal sources play an important role in electronic test and measurements, but their use is far from limited to that. They form an essential part of many common types of equipment. For example, a stabilized power supply needs an accurate DC voltage source as a reference against which to compare its output voltage. Many pieces of electronic equipment incorporate an audio-frequency signal source as an essential part of their operation, from the mellifluous warble of a modern push-button telephone to the ear-shattering squeal of a domestic smoke detector. And RF sources oscillators form an essential part of every radio transmitter and of virtually every receiver. So let's start with the DC signal source or voltage reference circuit.
The traditional voltage reference was the Weston standard cell, and these are still used in calibration laboratories. However, in most electronic instruments nowadays, from power supplies to digital voltmeters (DVMs), an electronic reference is used instead.
A Zener diode exhibits a voltage drop, when conducting in the reverse direction, which is to a first approximation independent of the current flowing through it, i.e., it has a low slope resistance. Thus if a Zener diode is supplied with current via a resistor from say the raw supply of a power supply (Figure 24.1(A), the voltage variations across the Zener both AC due to supply frequency ripple and DC due to fluctuations of the mains voltage will be substantially less than on the raw supply, provided that the value of the resistor is...
