Alternating Current and Transformers, you learned that total inductance decreases as additional inductors
are added in parallel. Because this introduced reactance effectively reduces inductance, the frequency of
the oscillator increases to a new fixed value.
Now lets see what happens when a modulating signal is applied. The magnitude of the introduced
reactance is determined by the magnitude of the superimposed current through the tank. The magnitude of
Ip for a given E1 is determined by the transconductance of V1. (Transconductance was covered in NEETS,
Module 6, Introduction to Electronic Emission, Tubes, and Power Supplies.) Therefore, the value of
reactance introduced into the tuned circuit varies directly with the transconductance of the reactance tube.
When a modulating signal is applied to the grid of V1, both E1 and I
change, causing transconductance to
vary with the modulating signal. This causes a variable reactance to be introduced into the tuned circuit.
This variable reactance either adds to or subtracts from the fixed value of reactance that is introduced in
the absence of the modulating signal. This action varies the reactance across the oscillator which, in turn,
varies the instantaneous frequency of the oscillator. These variations in the oscillator frequency are
proportional to the instantaneous amplitude of the modulating voltage. Reactance-tube modulators are
usually operated at low power levels. The required output power is developed in power amplifier stages
that follow the modulators.
The output of a reactance-tube modulated oscillator also contains some unwanted amplitude
modulation. This unwanted modulation is caused by stray capacitance and the resistive component of the
RC phase splitter. The resistance is much less significant than the desired XC, but the resistance does
allow some plate current to flow which is not of the proper phase relationship for good tube operation.
The small amplitude modulation that this produces is easily removed by passing the oscillator output
through a limiter-amplifier circuit.
Semiconductor Reactance Modulator.The SEMICONDUCTOR-REACTANCE MODULATOR
is used to frequency modulate low-power semiconductor transmitters. Figure 2-12 shows a typical
frequency-modulated oscillator stage operated as a reactance modulator. Q1, along with its associated
circuitry, is the oscillator. Q2 is the modulator and is connected to the circuit so that its collector-to-
emitter capacitance (CCE) is in parallel with a portion of the rf oscillator coil, L1. As the modulator
operates, the output capacitance of Q2 is varied. Thus, the frequency of the oscillator is shifted in
accordance with the modulation the same as if C1 were varied.