frequency standard output. The output of the phase detector, therefore, is proportional to the modulated
subcarrier input and the difference in frequency between the outputs of the reflex klystron and the multiplied
frequency standard. The output of the phase detector is applied to the repeller plate of the reflex klystron. If the
output of the phase detector is positive, the output frequency of the reflex klystron will decrease; a negative
output from the phase detector will cause the klystron frequency to increase.
2. When an input from the baseband circuits is applied to the phase detector, the phase detector will produce
an output that causes the reflex klystron to deviate from its center frequency. The output of the klystron is fed
through an isolator and a directional coupler and a sample of this signal is applied to the harmonic mixer. The
changing input to the mixer causes the 34-MHz IF signal to change and this variation is applied to the phase
detector as the reference input.
3. When a small frequency difference exists between the modulated subcarrier input and the reference signal,
the output of the phase detector is a sinusoidal voltage. This sinusoidal voltage will modulate the reflex
klystron, thus producing phase lock automatically. The signal from the klystron now only requires
amplification before transmission.
4. The phase-lock-loop circuit consists of the phase detector, reflex klystron, isolator, directional coupler,
harmonic mixer, and IF amplifier. Its purpose is to reduce the deviation of the klystrons modulated output
signal or to compress the signal into a narrower bandwidth.
LEARNING EVENT 28: POWER AMPLIFIER
1. The input states for the klystron power amplifier are a diode switch and a ferrite attenuator that function as
an isolator. The diode switch normally allows the input drive to be coupled to the klystron power amplifier.
However, an input from the protection circuits will cause the diode switch to remove the input drive from the
klystron power amplifier. The ferrite attenuator is a variable attenuator that is used to vary the power level of
the input drive.
2. The klystron power amplifier is a velocity-modulated amplifier that is used to raise the power level of the
RF input signal to the level required for transmission. Power amplifiers used in transmitters are usually
equipped with a heat exchanger or some type of cooling system to remove the excess heat from the amplifier.
3. The harmonic filter absorbs the unwanted harmonic energy from the signal to be transmitted.
LEARNING EVENT 29: FAULT DETECTION CIRCUITS
1. The fault detection system removes the RF drive from the klystron power amplifier if arcing or if an
excessive voltage standing wave ratio exists in the waveguide system.
SS0031
2-18
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