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
cillator frequency can be changed. Silicon
devices once again shine in realizing
fast settling VCOs. Using silicon
bipolar transistors with silicon varactor
diodes, settling times of better than 1
µs (for the frequency to be within 1
MHz) have been demonstrated at Kuband.13
In applications requiring fast
on and off VCO switching, a technique
of quenching the negative resistance
instead of switching the bias on and off
has demonstrated switching times of
better than 1 µs.14
YIG-tuned oscillators (YTO) are
used in test and measurements as
well as in wideband military systems
requiring multi-octave bands of tuning.
YTOs are oscillators of choice
when wideband tuning, high tuning
linearity and good phase noise are simultaneously
required. These oscillators
utilize a yittrium iron garnet
(YIG) (Y3Fe5O11) spherical resonator
placed between two poles of a cylindrically
re-entrant electromagnet.
The resonant frequency of the YIG
resonator in a uniform magnetic field
is a linear function of the magnetic
field strength. YIG resonators offer a
very high Q (> 4000 at 10 GHz),
which linearly increases with frequency.
The practical usable frequency
range of YTO is between 2 and 50
GHz. While the higher frequency is
limited by the magnet saturation and
high power dissipation, the lower limit
is governed by the saturation magnetization
4πMs.
In view of the wideband nature of
these oscillators, active devices in the
negative resistance configuration
have been generally used. Using a
low noise silicon bipolar transistor
and a novel composite feedback architecture
in which double coupling
the YIG sphere as a series feedback
for higher frequencies and as a parallel
feedback for lower frequencies, a
tuning range of 2 to 22 GHz has been
achieved with a phase noise of better
than –130 dBc/Hz at 100 kHz at 10
GHz.15 At higher frequencies, using a
single GaAs FET and a single YIG
sphere frequency range coverage of
20 to 40 GHz was reported with a
phase noise of better than –100
dBc/Hz at 100 kHz at 40 GHz.16
YTOs have been commercially available
up to 50 GHz. In practice it is
becoming more common to achieve
mm-wave frequencies by using
MMIC frequency doublers with low
er frequency YTOs or VCOs. YIGtuned
oscillators are still an attractive
choice when their slower tuning
speed, large size and high power consumption
can be tolerated.
Permanent magnet YTOs are a narrowband
tunable version of the YTO.
These oscillators use a permanent magnet
in place of an electromagnet significantly
reducing the DC power consumption
and size of the oscillator.
Phase noise of –125 dBc/Hz at 100 kHz
has been demonstrated using PMYTOs
in X- and Ku-bands covering 20 percent
bandwidth. These oscillators offer
excellent tuning linearity, good frequency
stability and phase lock/modulation
capability as well