Multivibrator Modulator.Another type of frequency modulator is the astable multivibrator
illustrated in figure 2-13. Inserting the modulating af voltage in series with the base-return of the
multivibrator transistors causes the gate length, and thus the fundamental frequency of the multivibrator,
to vary. The amount of variation will be in accordance with the amplitude of the modulating voltage. One
requirement of this method is that the fundamental frequency of the multivibrator be high in relation to
the highest modulating frequencies. A factor of at least 100 provides the best results.
Figure 2-13.Astable multivibrator and filter circuit for generating an fm carrier.
Recall that a multivibrator output consists of the fundamental frequency and all of its harmonics.
Unwanted even harmonics are eliminated by using a SYMMETRICAL MULTIVIBRATOR circuit, as
shown in figure 2-13. The desired fundamental frequency, or desired odd harmonics, can be amplified
after all other odd harmonics are eliminated in the LCR filter section of figure 2-13. A single frequency-
modulated carrier is then made available for further amplification and transmission.
Proper design of the multivibrator will cause the frequency deviation of the carrier to faithfully
follow (referred to as a "linear" function) the modulating voltage. This is true up to frequency deviations
which are considerable fractions of the fundamental frequency of the multivibrator. The principal design
consideration is that the RC coupling from one multivibrator transistor base to the collector of the other
has a time constant which is greater than the actual gate length by a factor of 10 or more. Under these
conditions, a rise in base voltage in each transistor is essentially linear from cutoff to the bias at which the
transistor is switched on. Since this rise in base voltage is a linear function of time, the gate length will
change as an inverse function of the modulating voltage. This action will cause the frequency to change as
a linear function of the modulating voltage.
The multivibrator frequency modulator has the advantage over the reactance-type modulator of a
greater linear frequency deviation from a given carrier frequency. However, multivibrators are limited to
frequencies below about 1 megahertz. Both systems are subject to drift of the carrier frequency and must,
therefore, be stabilized. Stabilization may be accomplished by modulating at a relatively low frequency
and translating by heterodyne action to the desired output frequency, as shown in figure 2-14. A
1-megahertz signal is heterodyned with 49 megahertz from the crystal-controlled oscillator to provide a
stable 50-megahertz output from the mixer. If a suitably stable heterodyning oscillator is used, the
frequency stability can be greatly improved. For instance, at the frequencies shown in figure 2-14, the