OSCILLATOR TOPOLOGIES
AND TYPES
Oscillators are also regarded as
DC-to-RF converters. A typical oscillator
consists of an active device and a
passive frequency-determining resonant
element. The active device can
be a two-terminal device like a Gunn
or IMPATT diode or more commonly
a three-terminal device including a
junction bipolar transistor, metal
semiconductor FET or more recent
devices using newer semiconductor
materials. In order to generate a high
frequency signal an active device with
sufficient gain to compensate for
feedback loop losses is necessary. Oscillation
conditions need to be satisfied
for the circuit containing the active
device and passive element. Two
different topologies are used for this
purpose, as shown in Figure 4 in
their generalized form. A parallel
feedback oscillator is the one in
which the frequency-determining element
is used as a feedback element
between the input and output in order
to generate necessary instability
and a negative resistance oscillator is
the one in which reflection gain at a
given terminal is used to satisfy the
oscillation condition when connected
to a frequency-determining element
with the proper phase condition.2
While both techniques are commonly
used for microwave signal generation,
the parallel feedback approach
is more suitable for narrowband, lower
noise tunable oscillators and the negative resistance configuration is used for
wideband tunable oscillators.
Microwave oscillators can be divided
into many different types based
on frequency bandwidth, type of resonator
used or type of active device
used. Resonators largely determine
frequency tuning range, stability and
noise performance of the oscillator,
and are commonly used to define diferent types of oscillators.