2-18
To examine the operation of the transmitter, we will trace the signal through the entire circuit. The
local oscillator shown at the left of figure 2-10 is a very stable rf oscillator that produces two cw rf
outputs. As shown, the cw output is sent to the receiver system; the cw output is also one of the two rf
signals fed to the mixer amplifier by way of the two BUFFER AMPLIFIER STAGES. The buffer
amplifiers raise the power level of the signal and also isolate the local oscillator.
The COHERENT OSCILLATOR (COHO) is triggered by the system trigger and produces as its
output an rf pulse. This rf pulse is fed directly to the mixer amplifier.
The mixer-amplifier stage receives three signals: the coherent rf pulse, the local oscillator cw rf
signal, and a dc modulating pulse from the low-voltage modulator. The coherent and local oscillator
signals are mixed to produce sum and difference frequency signals. Either of these may be selected as the
output. The modulator pulse serves the same purpose as in the keyed-oscillator transmitter, because it
determines the pulse width and power level. The mixer stage functions only during the modulator pulse
time. Thus the mixer amplifier produces an output of rf pulses in which the frequency may be either the
sum or difference of the coherent and local oscillator signals.
The mixer-amplifier feeds the pulses of rf energy to an intermediate power amplifier. This amplifier
stage is similar to the buffer-amplifier stage except that it is a pulsed amplifier. That is, the pulsed
amplifier has operating power only during the time the modulator pulse from the low-voltage modulator
is applied to the stage. The amplified output signal is fed to a second intermediate power amplifier that
operates in the same manner as the first.
From the second intermediate power amplifier, the signal is fed to the KLYSTRON POWER
AMPLIFIER. This stage is a multicavity power klystron. The input rf signal is used as the exciter signal
for the first cavity. High-voltage modulating pulses from the high-voltage modulator are also applied to
the klystron power amplifier. These high-voltage modulating pulses are stepped up across a pulse
transformer before being applied to the klystron. All cavities of the klystron are tunable and are tuned for
maximum output at the desired frequency.
Provisions are made in this type of transmitter to adjust the starting time of the modulating pulses
applied to the coherent oscillator, mixer amplifier, intermediate power amplifiers, and klystron power-
amplifier. By this means the various modulator pulses are made to occur at the same time.
This transmitter produces output rf pulses of constant power and minimum frequency modulation
and ensures good performance.
Q22. What is the primary advantage of power-amplifier transmitters over keyed-oscillator
transmitters?
Q23. In the power amplifier shown in figure 2-10, what two signals are mixed to produce the output
signal?
Q24. What type of klystron is used as the final stage of a power-amplifier transmitter?
Figure 2-11 is a block diagram of a power-amplifier transmitter that uses a FREQUENCY
SYNTHESIZER to produce the transmitted frequency rather than the heterodyning mixer. The frequency
synthesizer allows the transmitter to radiate a large number of discrete frequencies over a relatively wide
band. Such a system is commonly used with frequency-scan search radars that must transmit many
different frequencies to achieve elevation coverage and to compensate for the roll and pitch of a ship.
