The MULTIVIBRATOR MODULATOR uses an astable multivibrator with a modulating voltage
inserted in series with the base return of the multivibrator transistors.
The VARACTOR FM MODULATOR uses a VARACTOR. This is a specially designed diode that
has a certain amount of capacitance between the junction that can be controlled by reverse biasing.
In PHASE MODULATION the carriers phase is caused to shift at the rate of the modulating audio.
The amount of phase shift is controlled by the amplitude of the modulating wave.