BASIC TRANSMITTERS

in the audio frequency spectrum, 10-20,000 Hz. In na- val communications the terms voice communications and audio communications are sometimes used inter- changeably. The audio signal is impressed upon the rf carrier because it is impractical to transmit frequen- cies in the audio range due to their excessive wave- length. Three characteristics of the carrier wave may be varied, or modulated, at an external signal rate: ampli- tude, frequency, and phase. The following paragraphs discuss each type of modulation. Amplitude Modulation (AM) Amplitude modulations the process of combining audio frequency and radio frequency signals so that the amplitude of the radio frequency waves varies at an audio frequency rate. Frequency Modulation (FM) Frequency modulation is a process in which the frequency of the carrier wave is made to vary. An FM signal should remain constant in amplitude and change Frequency-Shift Keying (FSK) Frequency-shift keying is considered a form of FM. It is a digital mode of transmission commonly used in radioteletype applications. In FSK the carrier is present all the time. In a keyed condition, the carrier frequency changes by a predetermined amount called the mark frequency. The unkeyed state is called a space. Phase-Shift Keying (PSK) Phase-shift keying is similar to FSK except that the phase, not the frequency, is shifted. The primary ad- vantage of PSK is that it can be accomplished in an am- plifier stage. Pulse Modulation Pulse modulation is accomplished by varying the characteristics of a series of pulses. This can be done by varying the amplitude, duration, frequency, or posi- tion of the pulses. It can also be done through coding. Pulse modulation is especially suited for use with com- munications systems incorporating time-division mu- tiplexing. BASIC TRANSMITTERS Remember, transmitters are generally divided ac- cording to their type of modulation. In the discussion below, we describe very briefly how each type oper- ates to help you differentiate between them. CW Transmitter A basic CW transmitter is shown in figure 1-3. CW is one of the oldest and least complicated forms of communications. Two advantages of CW are a narrow bandwidth, which requires less power out, and clarity, even under high noise conditions. The major disadvan- only in frequency. Figure 1-3.—Continuous-wave transmitter. 1-5