2-19Figure 2-11.—Power amplifier transmitter using crossed-field amplifiers.A typical frequency synthesizer consists of a bank of oscillators producing different fixedfrequencies. The outputs of a relatively few fixed oscillators can be mixed in various combinations toproduce a wide range of frequencies. In mti systems the selected oscillator frequencies are mixed with acoherent oscillator frequency to provide a stable reference for the mti circuits. The frequency synthesizeralso produces the local oscillator signals for the receiver system. Because the transmitted pulse changesfrequency on each transmission, the local oscillator signal to the receiver must also change and beincluded in the transmitted frequency. A system of this type is frequency-programmed by select gatesfrom the synchronizer.The detailed operation of frequency synthesizers is beyond the scope of this manual but may befound 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 amodulator pulse (from the first stage modulator) at the same time. The rf pulse is usually slightly widerthan 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 outputof 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 stagemodulator pulse; this also prevents the amplifier from pulsing when no rf is present. Therefore, the secondstage amplifier receives a modulator pulse a short time after the first stage rf arrives at the input. Asshown 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 amplifiersthat amplify the input signals without frequency distortion. Typically, the first stage and the second stageare traveling-wave tubes (twt) and the final stage is a crossed-field amplifier. Recent technological
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business