LEARNING EVENT 21: MIXER
The mixer circuit heterodynes the 60-MHz angle-modulated subcarrier signal from the modulator and the 7,200
MHz injection signal from the multiplier stages to produce a 7,200-MHz modulated carrier to be used as the
uplink frequency.
LEARNING EVENT 22: TRAVELING-WAVE TUBE
The traveling-wave tube (TWT) is a special type of electron tube which provides amplification for wideband
signals. The operation of the TWT depends upon the technique of velocity-modulating the electron beam inside
the tube structure.
LEARNING EVENT 23: HIGH-POWER AMPLIFIER
1. Another type of velocity-modulated tube, a klystron, is used to provide the final amplification needed to
raise the modulated signal to the desired power level for transmission. This circuit usually operates with very
high voltages and requires a cooling system to reduce the operating temperature.
2. Several fault detection circuits are used to remove the high voltage or the RF drive from the high-power
amplifier when any defect or malfunction occurs.
SECTION V. DIRECT-ANGLE-MODULATED TRANSMITTER
LEARNING EVENT 24: ANGLE MODULATION
All angle-modulated transmitters use either direct or indirect methods for producing the angle modulation. The
modulating signal in the direct method has a direct effect on the frequency of the carrier. In the indirect
method, the modulating signal uses the frequency variations caused by phase modulation. In either case, the
output of the transmitter is an angle-modulated wave, and the receiver cannot distinguish between them. A
simplified directly modulated transmitter is shown in Figure 2-15.
SS0031
2-16
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