TABLE OF CONTENTS Signal-generator circuits produce RF sine-wave (or other waveform) output for purposes of testing, troubleshooting, and alignment. Although the subject of designing top-notch signal generator circuits can be quite deep, it is possible to reduce the possibilities to a small number for people whose needs are less stringent. This chapter looks at RF signal generators that can be used in a wide variety of applications on the electronic and radio workbenches. But first, take a look at some generic types of oscillator circuits.
The two major categories of oscillator circuits in electronics textbooks are relaxation oscillators and feedback oscillators. The relaxation oscillator uses some sort of voltage-breakdown device, such as a neon glow-lamp or unijunction transistor. The feedback oscillator (Fig. 10-1) uses an amplifier circuit and a feedback network to start and sustain oscillations on a particular frequency. Most oscillators that are useful for sine-wave signal generator circuits are of the feedback oscillator class.
The requirements for sustained oscillator, called Nyquist s criteria, are (1) the loop gain between feedback network losses and amplifier gain must be greater than or equal to one at the frequency of oscillation and (2) the feedback signal must be inphase with the input signal at the frequency of oscillation. The second of these criteria means that the feedback signal must be phase-shifted 360 , of which 180 is usually obtained from the inversion of the amplifier and 180 from the frequency-selective feedback network.
Feedback oscillators...
