TABLE OF CONTENTS Charles Kingsford-Smith
Agilent Technologies
Lake Stevens, Washington
This chapter covers the more commonly used types of signal source instruments introduced in Chap. 7. It surveys operating principles of these types and provides some guidance in understanding the specifications provided by manufacturers of these instruments.
As used here, the term sine-wave oscillators refers to oscillator circuits which naturally produce sinusoidal waveforms. These circuits consist of an ac amplifier with a positive feedback path from output to input. In the feedback path is a filter network: Its input-output (transfer) gain is very low except close to the desired frequency of oscillation. Hence the ability of the circuit to operate as an oscillator (that is, to operate regeneratively by producing its own amplifier input) is restricted to a narrow band around this frequency.
This important class of instrument began in the late 1920s when the need was recognized for producing radio receiver test signals with accurate frequencies (1 percent) and amplitudes (1 or 2 dB) over wide ranges. The basic block diagram of these instruments remained nearly unchanged until about 1970, when frequency synthesizer circuits began to usurp free-running oscillators in the waveform generator section of the instruments.
Figure 16.1 shows a simplified form of a typical rf sine-wave oscillator used in a signal generator. An amplifier producing an output current proportional to its input voltage (transconductance amplifier) drives a tunable filter circuit consisting of a parallel LC resonant circuit with a...
