LEARNING EVENT 17: BASEBAND CIRCUITS
1. The input amplifier is a linear amplifier used to prepare the baseband signal for modulation. The baseband
circuits must alter the signals so they have the correct frequency and amplitude relationship. The amplitude and
frequency of the baseband signal must be accurately controlled to properly modulate the subcarrier frequency.
The output of the amplifier is coupled to the baseband-shaping circuits.
2. Preemphasis, deemphasis, and limiter circuits are used as baseband-shaping circuits. The limiter circuits
remove the portions of the baseband signal that exceed the predetermined limits. The preemphasis and
deemphasis circuits operate on the high frequencies in such a manner as to improve the overall S/N of the
baseband signal.
3. The baseband amplifier increases the power level of the baseband signal and isolates the shaping circuit
from the modulation amplifier.
LEARNING EVENT 18: MODULATOR
The prepared baseband signal and the unmodulated 60-MHz subcarrier signal are combined in the modulation
amplifier to form the modulated-subcarrier signal. The indirect modulation process in this amplifier involves
amplitude, phase, and frequency modulation techniques.
LEARNING EVENT 19: TRANSMITTER INJECTION STAGES
1. The transmitter injection voltages are usually generated by a separate frequency-generating subsystem. This
subsystem usually contains a highly stable frequency standard, amplifiers, and frequency synthesizers. In
addition to the injection output used in the transmitter, the subsystem also provides synchronizing voltages for
other subsystems within the communications system.
2. The transmitter injection voltage can also be developed by a single highly stable oscillator. For example,
the stage illustrated in Figure 2-14 could be a single crystal oscillator stage operating at a frequency of 60 MHz.
LEARNING EVENT 20: FREQUENCY MULTIPLIERS
The frequency of the transmitter injection voltage is multiplied by a series of multiplying stages to a value of
approximately 7,200 MHz-the frequency required for transmission. The individual circuits used to perform this
multiplication generally do not multiply the frequency by more than 6. To perform the multiplication of 120, as
shown in Figure 2-14, three stages of multiplication can be used-x6, x4, and x5.
2-15
SS0031
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