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
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 negaCOVER
FEATURE
▲ Fig. 3 A 36 GHz InGaP/GaAs HBT
MMIC VCO (2.1 × 1.3 mm sq.).21
(courtesy of FBH, Berlin, Germany)
Z_resonator
SERIES FEEDBACK
PARALLEL FEEDBACK
Z_load
Z_match
Z_resonator
Z_load
Z_match
▲ Fig. 4 Generalized oscillator
configurations using three-terminal devices.
tive 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 different
types of oscillators
Oscillators are also regarded asDC-to-RF converters. A typical oscillatorconsists of an active device and apassive frequency-determining resonantelement. The active device canbe a two-terminal device like a Gunnor IMPATT diode or more commonlya three-terminal device including ajunction bipolar transistor, metalsemiconductor FET or more recentdevices using newer semiconductormaterials. In order to generate a highfrequency signal an active device withsufficient gain to compensate forfeedback loop losses is necessary. Oscillationconditions need to be satisfiedfor the circuit containing the activedevice and passive element. Twodifferent topologies are used for thispurpose, as shown in Figure 4 intheir generalized form. A parallelfeedback oscillator is the one inwhich the frequency-determining elementis used as a feedback elementbetween the input and output in orderto generate necessary instabilityand a negative resistance oscillator isthe one in which reflection gain at agiven terminal is used to satisfy theoscillation condition when connectedto a frequency-determining elementwith the proper phase conditionWhile both techniques are commonlyused for microwave signal generation,the parallel feedback approachis more suitable for narrowband, lowernoise tunable oscillators and the negaCOVERFEATURE▲ Fig. 3 A 36 GHz InGaP/GaAs HBTMMIC VCO (2.1 × 1.3 mm sq.).21(courtesy of FBH, Berlin, Germany)Z_resonatorSERIES FEEDBACKPARALLEL FEEDBACKZ_loadZ_matchZ_resonatorZ_loadZ_match▲ Fig. 4 Generalized oscillatorconfigurations using three-terminal devices.tive resistance configuration is used forwideband tunable oscillators.Microwave oscillators can be dividedinto many different types basedon frequency bandwidth, type of resonatorused or type of active deviceused. Resonators largely determinefrequency tuning range, stability andnoise performance of the oscillator,and are commonly used to define differenttypes of oscillators
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