Radio Frequency Integrated Circuit Design - Webs

8
Voltage-Controlled Oscillators
8.1 Introduction
An oscillator is a circuit that generates a periodic waveform whether it be
sinusoidal, square, triangular as shown in Figure 8.1, or, more likely, some
distorted combination of all three. Oscillators are used in a number of applications
in which a reference tone is required. For instance, they can be used as
the clock for digital circuits or as the source of the LO signal in transmitters.
In receivers, oscillator waveforms are used as the reference frequency to mix
down the received RF to an IF or to baseband. In most RF applications,
sinusoidal references with a high degree of spectral purity (low phase noise) are
required. Thus, this chapter will focus on LC-based oscillators, as they are the
most prominent form of oscillator used in RF applications.
In this chapter, we will first look at some general oscillator properties and
then examine the resonator as a fundamental building block of the oscillator.
Different types of oscillators will then be examined, but most emphasis will be
on the Colpitts oscillator and the negative transconductance oscillator. Both
single-ended and double-ended designs will be considered. This chapter will
also include discussions of the theoretical calculations of the amplitude of
oscillation and the phase noise. Finally, there will be a section on automatic
amplitude control circuitry for oscillators.
8.2 Specification of Oscillator Properties
Perhaps the most important characteristic of an oscillator is its phase noise. In
other words, we desire accurate periodicity with all signal power concentrated
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