TABLE OF CONTENTS The frequency of a superheterodyne receiver is determined by its LOs. Even the DDFS is controlled by the oscillator in its frequency standard. Our discussion of the oscillator is drawn generally from [7.11]. There are also many texts on oscillators (References [7.29] to [7.33]), and extensive references for further reading are available in them, as well as from [7.11] and [7.34].
An electronic oscillator converts dc power to a periodic output signal (i. e., ac power). If the output waveform is approximately sinusoidal, the oscillator is referred to as a sinusoidal or harmonic oscillator. There are other oscillator types, often referred to as relaxation oscillators, whose outputs deviate substantially from sinusoidal. Sinusoidal oscillators are used for most radio applications because of their spectral purity and good noise sideband performance. Oscillator circuits are inherently nonlinear; however, linear analysis techniques are useful for the analysis and design of sinusoidal oscillators. Figure 7.95 is a generic block diagram of an oscillator. It is a feedback loop, comprising a frequency-dependent amplifier with forward loop gain G( j2 ?f) and a frequency-dependent feedback network H(2 ?f) . The output voltage is given by
| (7.54) |  |
To sustain oscillation, the output V O must be nonzero even when the input signal V 1 is zero. Because G( j2 ?f) is finite in practical circuits, the denominator may be zero at some frequency f 0, leading to...
