is mixed with the transmit frequency from the reflex klystron after the transmit frequency has been coupled
through an isolator and a directional coupler. The output from the harmonic mixer is a 34-MHz IF that is
amplified and coupled to the phase detector.
3-14.
PHASE-LOCK LOOP
a. The inputs to the phase detector consist of a modulated subcarrier and a nominal IF of 34 MHz, which
is the difference between the transmit frequency and the multiplied 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.
b. 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.
c. 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.
d. 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 klystron's modulated
output signal or to compress the signal into a narrower bandwidth.
3-15.
POWER AMPLIFIER
a. The input stages 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.
b. 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.
c. The harmonic filter absorbs the unwanted harmonic energy from the signal to be transmitted.
344 L3
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