2-21
Figure 2-15B.—Varactor symbol and schematic. SIMPLIFIED CIRCUIT.
Notice the simplicity of operation of the circuit in figure 2-16. An af signal that is applied to the
input results in the following actions: (1) On the positive alternation, reverse bias increases and the
dielectric (depletion region) width increases. This decreases capacitance which increases the frequency of
the oscillator. (2) On the negative alternation, the reverse bias decreases, which results in a decrease in
oscillator frequency.
Figure 2-16.—Varactor fm modulator.
Many different fm modulators are available, but they all use the basic principles you have just
studied. The main point to remember is that an oscillator must be used to establish the reference (carrier)
frequency. Secondly, some method is needed to cause the oscillator to change frequency in accordance
with an af signal. Anytime this can be accomplished, we have a frequency modulator.
Q-7.
How does the reactance-tube modulator impress intelligence onto an rf carrier?
Q-8.
What characteristic of a transistor is varied in a semiconductor-reactance modulator?
Q-9.
What circuit section is required in the output of a multivibrator modulator to eliminate
unwanted output frequencies?
Q-10.
What characteristic of a varactor is used in an fm modulator?
