for microwave systems
Updated July 4,
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As many amplifier designers will
tell you, designing an oscillator is easy, there are infinite combinations
of components that will oscillate when you don't want them to. But
designing a useful oscillator with s stable and controllable output
is another story indeed.
The conditions for oscillation
are simple, the loop gain of a network has to be unity or more,
and the total phase shift through the feedback loop must be an integer
of 360 degrees. This type of feedback is known as positive, or regenerative
feedback. In practice the feedback network usually has a resonance,
which determines the output frequency.
Oscillators are a fundamental
building block of almost every electronic system, analog or digital.
Oscillators provide the signal source for all microwave systems,
including both transmitters and receivers.
Here are some oscillator definitions:
Local oscillator (LO)
The signal source in used in a receiver to downconvert the received
signal to an intermediate frequency. One of these days we will start
a chapter on receivers!
local oscillator (STALO)
controlled oscillator (VCO)
An oscillator that provides a signal whose frequency is controllable
using an analog voltage signal.
Crystal oscillator (XO)
This topic already has it's own
Dielectric resonator oscillator
Generally a piece of test equipment that is able to sweep a CW signal
across a specific band, either in a a frequency modulation mode.
A good sweep oscillator uses a phase-locked loop to ensure that
the signal frequency is accurate and stable. Such a device is often
called a "synthesizer".
Here are some specifications
you need to consider when procuring different types of oscillators:
Expressed in parts per million, this tells you the worst-case error
you can expect when you command your oscillator to a specific frequency.
The 10 cent crystal in a cheap digital watch is accurate to 23 ppm
if it loses one minute per month.
This is a measure of the frequency drift over time. As increasing
number of users crowd the electromagnetic spectrum, the need for
frequency stability becomes greater every year.
Short term stability
Since many oscillators are so temperature dependent for accuracy
often ovens are used to stay at a fixed temperature, some equipment
that uses oscillators has a lengthy warm-up time. For example, it
may take overnight for a network analyzer to stabilize after you
plug it in, and this is the fault of the sweep oscillator.
Especially with an oven-stabilized oscillator, the dissipation
when you first throw the switch can be a lot higher than the
A harmonic frequency is any multiple of the intended frequency.
A ten GHz oscillator may have 20 GHz, 30 GHz, 40 GHz... output signals
at low power levels; these are the second, third and fourth harmonic.
Subharmonic frequencies are signals that occur at 1/2, 1/3,
1/4... the intended signal.
Spurious signals are seemingly not related to the fundamental
output frequency, and can be either below or above it.