Figure 5.
Harmonic generation using a multivibrator.
1-7.
HOW THE HETERODYNE METHOD WORKS
section To do this, the selected
harmonic signal is modulated with a
a. The heterodyning process is a
carrier signal.
The frequency of the
modulation process.
When it is used
carrier must be such a value that the
in a frequency synthesizing circuit,
output of the first mixer is 400 kHz.
it serves as a filtering process as
For
example,
to
make
the
lower
well.
A typical circuit that uses
sideband signal output from the first
heterodyning to synthesize frequencies
mixer 400 kHz when the input 2.54 MHz,
is shown in figure 6.
the carrier oscillator must produce
2.14 MHz, as shown by the following
b. Notice
that
the
circuit
in
calculations.
figure
6
is
divided
into
three
sections.
2,540,000 - 2,140,000 = 400,000
(1) Harmonic generating section.
Then, to insure that the output signal
from the first mixer is actually 400
(2) Filtering section.
kHz
critical
filter
circuits
are
connected to the input and output of
(3) Output section.
the 400 kHz amplifier.
This 400 kHz
signal then modulates the same carrier
c. The harmonic generating section
signal
that
is
modulated
by
the
works similarly to the circuit shown
selected 2.54 MHz harmonic signal.
in figure 5. The filtering section is
This
second
modulation
process
divided into two parts--a variable
produces a sideband signal that is
filter circuit and a heterodyning
exactly equal to the original 2.54 MHz
circuit.
The filter selects one of
harmonic
signal.
The
difference
the harmonics and the heterodyning
between this output signal and the
circuit purifies this signal so that
original harmonic signal is that the
no other harmonic, or any of the har-
output
signal
is
free
from
all
monic by-products, get to the output
harmonics, noise and spurious signals.
327 L1
6
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