6.
Radio frequency (RF) signal generators.
a. A typical radio frequency signal generator contains, in addition to the
necessary power supply, three main sections; an oscillator circuit, a modulator,
and an output control circuit.
The internal modulator modulates the radio
frequency signal of the oscillator. In addition, most RF generators are provided
with connections through which an external source of modulation of any desired
waveform may be applied to the generated signal.
Metal shielding surrounds the
unit to prevent the entrance of signals from the oscillator into the circuit under
test by means other than through the output circuit of the generator.
b. A block diagram of a representative RF signal generator is shown in Figure
3-3.
The function of the oscillator stage is to produce a signal which can be
of oscillator circuit used depends on the range of the frequencies for which the
generator is designed. In low frequency signal generators, the resonating circuit
consists of a group of coils combined with a variable capacitor. One of the coils
has a selector switch attached to the capacitor to provide an LC circuit that has
the correct range of resonant frequencies.
Figure 3-3.
Block diagram of RF signal generator
c. The function of the modulating circuit is the production of audio (or
video) voltage which can be superimposed on the RF signal produced by the
oscillator.
The modulating signal may be provided by an audio oscillator within
the generator, or it may be derived from an external source.
In some signal
generators, either of these methods of modulation may be used.
In addition, a
means of disabling the modulator section is used whereby the pure unmodulated
signal from the oscillator can be used when it is desired.
d. The type of modulation used depends on the application of the particular
signal generator. The modulating voltage may be either a sine wave, a square wave,
or pulse of varying duration.
In some specialized generators, provision is made
for pulse modulation in which the RF signal can be pulsed over a wide range of
repetition rates and at various pulse widths.
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