increased noise that occurs at the higher frequencies. The receiver will be equipped with a
deemphasis circuit to restore the signal to its original amplitude. This compensation tends to
equalize the signal-to-noise ratio across the frequency band. A preemphasis network is also
called a high-pass filter circuit.
(3) The baseband amplifier provides the required amplification for the baseband signals prior to
modulation. In some modulators, the amplifier network comprises additional shaping circuits,
such as: clippers, limiters, preemphasis networks, deemphasis networks, and filters. These
additional shaping circuits are used to reduce the overall noise and to improve the baseband
signal.
c. Direct- and indirect-angle-modulation techniques are employed in various types of microwave
communication terminals.
(1) If a direct-angle modulation technique is used, the baseband signals, after preparation, are applied
directly to the subcarrier oscillator circuit to produce the desired type of angle modulation. If low
values of oscillator frequency and frequency deviation exist, it may be necessary to include a
number of multiplier stages to raise them to the desired values.
(2) If indirect-angle modulation techniques are used, the baseband signals and the output of the
subcarrier oscillator are applied to a modulation-amplifier circuit to produce the desired type of
angle modulation.
d. After modulation, the signal generally undergoes additional amplification in an RF amplifier stage.
The signal is then coupled to the exciter-translator unit in the transmitter.
Section II. INDIRECT-ANGLE-MODULATED TRANSMITTER
3-3. INTRODUCTION
Most transmitters amplify and multiply an angle-modulated signal up to the desired frequency and power
level. The block diagram of a simple indirect-angle-modulated transmitter is shown in figure 106
3-4. BASEBAND CIRCUITS
a. The input amplifier is a linear amplifier used to prepare the baseband signal for modulation. The
baseband circuits must alter the signals so that 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.
b. 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 signal-to-noise
ratio of the baseband signal.
344 L3
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