2-19
Figure 2-11.—Power amplifier transmitter using crossed-field amplifiers.
A typical frequency synthesizer consists of a bank of oscillators producing different fixed
frequencies. The outputs of a relatively few fixed oscillators can be mixed in various combinations to
produce a wide range of frequencies. In mti systems the selected oscillator frequencies are mixed with a
coherent oscillator frequency to provide a stable reference for the mti circuits. The frequency synthesizer
also produces the local oscillator signals for the receiver system. Because the transmitted pulse changes
frequency on each transmission, the local oscillator signal to the receiver must also change and be
included in the transmitted frequency. A system of this type is frequency-programmed by select gates
from the synchronizer.
The detailed operation of frequency synthesizers is beyond the scope of this manual but may be
found in the technical manuals for most frequency scan radar systems.
The first rf amplifier receives the pulses of the selected frequency from the synthesizer and a
modulator pulse (from the first stage modulator) at the same time. The rf pulse is usually slightly wider
than the modulator pulse which prevents the amplifier tube from pulsing when no rf energy is present.
Most pulsed rf amplifiers will oscillate at an undesired frequency if pulsed without an rf input. The output
of the first rf amplifier is an amplified rf pulse that is the same width as the first stage modulator pulse.
The second stage modulator is designed to produce a pulse slightly narrower than the first stage
modulator pulse; this also prevents the amplifier from pulsing when no rf is present. Therefore, the second
stage amplifier receives a modulator pulse a short time after the first stage rf arrives at the input. As
shown in figure 2-11, the same procedure is repeated in the third and final stage.
The amplifiers in this type of power-amplifier transmitter must be broad-band microwave amplifiers
that amplify the input signals without frequency distortion. Typically, the first stage and the second stage
are traveling-wave tubes (twt) and the final stage is a crossed-field amplifier. Recent technological
