Page 216 - Complete Wireless Design

Oscillator Design

Oscillator Design 215

Figure 4.2 An amplifier with degenerative feedback
cannot oscillate.

Figure 4.3 An amplifier with regenerative feedback can
oscillate.

tions in , and to stabilize the active device over wide temperature variations.
All of these variables must be set by a proper bias network.
A vital parameter of any oscillator circuit will be its Q. A high-Q feedback
oscillator [such as the crystal or surface acoustic wave (SAW) types] will have
a more frequency-stable output than an LC oscillator. This is because vari-
ances in the transistor’s reactances caused by changing V , temperature, as
CC
well as lot variations from transistor to transistor, will cause far less frequen-
cy shifts than a low-Q (LC) oscillator.
The proper choice of each of the components for an oscillator is very impor-
tant, since even the passive components can have a significant impact on
oscillator operation. Unless frequency compensation is desired, the oscilla-
tor’s feedback network capacitors should be NPO for minimum frequency
drift under normal temperature variations. The proper choice of the active
device is also critical. Transistors with a very high f —compared to the oscil-
t
lation frequency—work much better in oscillator circuits than those with
marginal f specifications. This is due to the transistor’s ability, at a high f ,
t t
not only to maintain its 180 degree phase shift at higher frequencies (an
amplifier’s phase shift begins to drop from 180 degrees as frequency increas-
es), but also to have a higher feedback gain.
Most oscillators should be extensively decoupled from any noise and inter-
mittent voltage variations of the power supply, and injection of the oscillator’s
own RF into the mains should be avoided, in all modern wireless applications.

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