b. Radiotelephone Transmitter.
Radiotelephone and CW transmitters are
similar in many respects. They differ mainly in the method of adding information
to the carrier.
The CW transmitter adds information to the carrier with code by
means of a key, while a radiotelephone transmitter modulates the carrier with sound
energy by means of a modulator.
(1) Figure 2-8 illustrates a simple radiotelephone transmitter.
A
modulator is added to the improved CW transmitter, and a microphone
replaces the key. The functions of the power supply, oscillator, power
amplifier, and antenna are the same in both transmitters.
The
modulator uses the voice energy (audio signal) from the microphone to
vary the power to the power amplifier at an audio rate.
Figure 2-8.
A simple radiotelephone transmitter.
(2) The addition of a buffer amplifier and a speech amplifier (fig. 2-9)
improves the performance of the transmitter.
The speech amplifier
strengthens the voice signal to drive the modulator.
The buffer
amplifier isolates the oscillator from the final RF amplifier, thus
stabilizing the oscillator frequency.
c. Amplitude Modulation.
Amplitude modulation is one method of adding
information to the RF carrier.
This is done by varying the amplitude of the
carrier with the audio signal from the microphone.
Figure 2-9 shows the stages
requited and the waveforms produced by the process.
(1) The carrier is generated by the oscillator and amplified by the buffer
amplifier.
(2) The microphone transforms voice (audio) signals into voltages.
The
audio signal voltage is amplified by the speech amplifiers
309 L2
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