device is a crystal.
However, as
corrective
action,
which
continues
stable as a crystal may be, there are
until the phase difference is zero.
still
some
variations.
These
In other words, this constant phase
variations
are
multiplied
in
the
error checking holds the oscillator
harmonic frequencies.
Therefore, the
output frequency constant.
higher the harmonic frequency selected
as output frequency, the greater the
c. The temperature of the component
variation in that frequency.
For
parts of an oscillator circuit has a
example, let's assume that a specific
great
effect
on
the
oscillator's
circuit produces multiple signals by
output frequency.
Controlling the
harmonic generation. Assume also that
temperature of these component parts
a
100
kHz
crystal
controlled
helps to control the accuracy and
oscillator (often referred to as the
stability
of
the
output
signal.
master oscillator) supplies the signal
However, you need to control the
from
which
the
harmonic
signals
temperature of only a few key items.
originate.
Now, if this oscillator
These items are the crystal and the
frequency output varies as little as
amplifying
device
or
0.1 percent, the oscillator output
electron tube).
You control the
frequency is between 99.9 and 100.1
temperature by placing them in an
kHz. That's a 200 Hz variation, which
inclosure called an oven.
This oven
is well within certain acceptable
can be a simple metal box, or it can
standards. However the 100th harmonic
be an elaborate insulated inclosure
of the original 100 kHz signal (10
with
temperature
sensitive
control
MHz) varies between 9.99 and 10.01
circuits.
MHz. That's a 20 kHz variation, which
1-10. FREQUENCY
ADD,
SUBTRACT,
is far beyond an acceptable tolerance
MULTIPLY AND DIVIDE CIRCUITS
for many applications. Therefore, the
master oscillator that supplies the
signal input to the harmonic generator
Most
frequency
synthesizing
circuit must be stabilized.
This
circuits
are
made
up
of
several
oscillator,
referred
to
as
a
different
types
of
circuits
that
stabilized master oscillator (SMO), is
either add, subtract, multiply, or
controlled not only by a crystal, but
divide the frequency of the signal
also by special frequency controlling
applied to the input.
Therefore,
circuits.
you'll see many block diagrams of
synthesizing
circuits
that
are
1-9.
METHODS
OF
STABILIZING
THE
designated with a mathematical symbol
MASTER OSCILLATOR
such as +, -, x, or . Make sure that
you know what type circuit each symbol
a. There are several different ways
designates
by
studying
the
to stabilize an oscillator, but the
explanations given below.
two most common methods are:
a. Add Circuit.
A modulator and
(1) Phase error detecting (phase
filter
circuit
combination
that
lock).
selects the upper sideband for the
output. For example, if a 1 kHz and a
(2) Temperature control.
3 kHz signal is applied to the input
of a modulator and a 4 kHz appears at
b. The phase error, or phase lock
the output, then this is an add
method, shown by the block diagrams in
circuit.
figures 7 and 8 compares the phase of
the output signal with one or more
b. Subtract Circuit.
A modulator
reference
signals.
The
phase
and filter circuit combination that
difference,
if
any,
between
the
selects the lower sideband as the
reference signal and output signal
output.
For example, if the input
produces a dc error voltage.
This
signals are 1 kz and 3 kHz, and the
error voltage is then applied to a
output signal is 2 kHz, it's a
reactance circuit that changes the
subtract circuit.
reactance of the oscillator circuit.
The error voltage determines if the
c. Multiply
Circuit.
Harmonic
reactance of the circuit is increased
generating and filter circuit that
or
decreased.
This
increase
or
selects one of the harmonic signals.
decrease determines whether the output
For example, a circuit that selects a
frequency is increased or decreased.
100
kHz
signal
from
a
10
kHz
In
any
case,
the
change
is
a
oscillator is a multiply by 10 circuit
327 L1
8
Previous Page
