FREQUENCY TUNABLE
OSCILLATORS
Wideband tunable oscillators are
necessary components for ECM, ESM
and test instrumentation, as well as
many communication systems. These
are characterized by the tuning bandwidth
and linearity, phase noise, settling
time and post tuning drift. The
needs for the specific characteristics
depend upon the application. Compromises
are called for in view of the
fact that all of these parameters cannot
be achieved using a single technology
or technique. In order to achieve
oscillations over a wideband the active
device needs to possess negative resistance
over the band and the frequency
tuning element needs to tune over the
band. Design techniques are then
used to satisfy oscillation conditions
over the band while optimizing one or
more desired parameters. Phase noise
being a function of the carrier frequency
and tuning bandwidth, care
should be taken while comparing frequency
tunable oscillators.
Varactor-tuned oscillators are voltage-tuned
oscillators utilizing Si or
GaAs varactors, typically a metal ntype
schottky barrier, in association
with the active device to generate signals
over a wideband. With the available
technology a little more than octave
band oscillators have been reported
up to Ku-band using multiple
varactors.10 At lower frequencies it is
more practical to achieve an octave
band. VCOs based on discrete devices
have demonstrated more than octave
bands with excellent phase noise up to
4 GHz. Silicon bipolar devices are the
devices of choice due to their lower 1/f
noise and corner frequency and are
commonly used for VCOs up to
X-band. Using a low noise silicon
bipolar transistor with a 10 GHz ft and
40 GHz fmax process and a silicon hyperabrupt
varactor diode, a narrowband
VCO was reported at 10 GHz
with phase noise of –112 dBc/Hz at
100 kHz offset.12 GaAs FET devices
offer wideband oscillations up to mmwave
frequencies with degraded phase
noise. Another technique to achieve
low noise wideband signal generation
at higher frequencies is the use of
push-push oscillators. VCOs covering
9 to 18 GHz have been demonstrated
using silicon bipolar devices.10 VCOs
exceeding octave bandwidth have
been recently developed using coupled
push-push technology. Phase
noise of –118 dBc/Hz at 100 kHz offset
was reported at 4 GHz in a 3 to 6
GHz VCO.11
Frequency settling time is another
important characteristic required in
certain military systems as well as in
test instruments. This represents the
speed and accuracy with which the os