Page 214 - Complete Wireless Design

Source: Complete Wireless Design

Chapter
4

Oscillator Design

Oscillator design is perhaps one of the least understood practices of wireless
engineering in general, and is definitely considered to be the most complex.
Indeed, until Randall W. Rhea released his groundbreaking book Oscillator
Design and Computer Simulation in 1990, oscillator design was strictly a hit-
or-miss affair for many engineers. As anyone in RF is well aware, it is quite
easy to design an oscillator—just design a poor amplifier and turn on the
power, and it will probably begin to oscillate. But the problem is to design an
oscillator that will oscillate at the desired frequency and amplitude, that will
start reliably and not wander, that will not be plagued with spurious
responses and harmonics, that will not be excessively affected by normal
changes in temperature, and that will be consistent in operation when built
over a long production run.
This chapter will concentrate on the design, simulation, and verification of
voltage-controlled oscillators (VCOs), LC oscillators, and crystal oscillators
over a wide range of frequencies. But first, a memory refresher on basic oscil-
lator theory.

Sine-wave oscillators. When a pulse is applied to a tank circuit, it will ring
at the tank’s resonant frequency, creating a decaying sinusoidal wave (Fig.
4.1). But if amplification from an active device, such as a transistor, is used
to amplify and sustain this output, then an oscillator can be formed. The nat-
ural resonant frequency of the tank circuit is established by the tank’s L and
C components, or:

1
f
r
2 LC
Thus, oscillators will use a small part of their output signal from the active
device in order to send a regenerative, or in-phase, feedback into their own

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