AM TRANSMITTER PRINCIPLES

In this section we will describe the methods used to apply voice signals (intelligence) to a carrier wave by the process of amplitude modulation (AM).

An AM transmitter can be divided into two major sections according to the frequencies at which they operate, radio-frequency (rf) and audio-frequency (af) units. The rf unit is the section of the transmitter used to generate the rf carrier wave. As illustrated in figure 1-37,

the carrier originates in the master oscillator stage where it is generated as a constant-amplitude, constant-frequency sine wave. The carrier is not of sufficient amplitude and must be amplified in one or more stages before it attains the high power required by the antenna. With the exception of the last stage, the amplifiers between the oscillator and the antenna are called INTERMEDIATE POWER AMPLIFIERS (ipa). The final stage, which connects to the antenna, is called the FINAL POWER AMPLIFIER (fpa).

Figure 1-37. - Block diagram of an AM transmitter.

The second section of the transmitter contains the audio circuitry.

This section of the transmitter takes the small signal from the microphone and increases its amplitude to the amount necessary to fully modulate the carrier. The last audio stage is the MODULATOR. It applies its signal to the carrier in the final power amplifier. In this way, intelligence is included in the radiated rf waveform.

The Modulated Wave

The frequencies present in a signal can be conveniently represented by a graph of the frequency spectrum, shown in figure 1-38. In this graph, each individual frequency is portrayed as a vertical line. The position of the line along the horizontal axis indicates the frequency of the signal. The height of the frequency line is proportional to the amplitude of the signal. The rf spectrum in figure 1-38 shows the frequencies present when heterodyning occurs between frequencies of 5 and 100 kilohertz.

Figure 1-38. - Radio-frequency spectrum.

Radiating energy at audio frequencies (discussed earlier in this chapter) is not practical. The heterodyning principle, however, makes possible the conversion of an af signal (intelligence) to an rf signal (with af intelligence) which can be radiated or transmitted through space.

Look again at figure 1-38. The sum and difference frequencies are located very near the rf signal (100 kilohertz), while the audio signal (5 kilohertz) is spaced a considerable distance away. Because of this frequency separation, the audio frequency can be easily removed by filter circuits, leaving just three radio frequencies of 95, 100, and 105 kilohertz. These three radio frequencies are radiated through space to the receiving station. At the receiver, the process is reversed. The frequency of 95 kilohertz, for example, is heterodyned with the frequency of 100 kilohertz and the sum and difference frequencies are again produced. (A similar process occurs between the frequencies of 100 and 105 kilohertz.) Of the resultant frequencies (95, 100, 105, and 5 kilohertz), all are filtered out except the 5 kilohertz difference frequency. This frequency, which is identical to the original 5 kilohertz audio applied at the transmitter, is retained and amplified. Thus, the 5 kilohertz audio tone appears to have been radiated through space from the transmitter to the receiver.

In the process just described, the 100 kilohertz frequency is referred to as the CARRIER FREQUENCY, and the sum and difference frequencies are referred to as SIDE FREQUENCIES. Since the sum frequency appears above the carrier frequency, it is referred to as the UPPER SIDE FREQUENCY. The difference frequency appears below the carrier and is referred to as the LOWER SIDE FREQUENCY.

When a carrier is modulated by voice or music signals, a large number of sum and difference frequencies are produced. All of the sum frequencies above the carrier are spoken of collectively as the UPPER SIDEBAND. All the difference frequencies below the carrier, also considered as a group, are called the LOWER SIDEBAND.

If the carrier and the modulating signal are constant in amplitude, the sum and difference frequencies will also be constant in amplitude. However, when the carrier and sidebands are combined in a single impedance and viewed simultaneously with an oscilloscope, the resultant waveform appears as shown in figure 1-39. This resultant wave is called the MODULATION ENVELOPE. The modulation envelope has the same frequency as the carrier. However, it rises and falls in amplitude with the continual phase shift between the carrier and sidebands. This causes these signals to first aid and then oppose one another. These cyclic variations in the amplitude of the envelope have the same frequency as the audio-modulating voltage. The audio intelligence is actually contained in the spacing or difference between the carrier and sideband frequencies.

Figure 1-39. - Formation of the modulation envelope.

BANDWIDTH OF AN AM WAVE. - An ideal carrier wave contains a single frequency and occupies very little of the frequency spectrum. When the carrier is amplitude modulated, sideband frequencies are created both above and below the carrier frequency. This causes the signal to use up a greater portion of the frequency spectrum. The amount of space in the frequency spectrum required by the signal is called the BANDWIDTH of the signal.

The bandwidth of a modulated wave is a function of the frequencies contained in the modulating signal. For example, when a 100-kilohertz carrier is modulated by a 5-kilohertz audio tone, sideband frequencies are created at 95 and 105 kilohertz. This signal requires 10 kilohertz of space in the spectrum.

If the same 100-kilohertz carrier is modulated by a 10-kilohertz audio tone, sideband frequencies will appear at 90 and 110 kilohertz and the signal will have a bandwidth of 20 kilohertz. Notice that as the modulating signal becomes higher in frequency, the bandwidth required also becomes greater. As illustrated by the above examples, the bandwidth of an amplitude-modulated wave at any instant is two times the highest modulating frequency applied at that time. Thus, if a 400-kilohertz carrier is modulated with 3, 5, and 8 kilohertz simultaneously, sideband frequencies will appear at 392, 395, 397, 403, 405, and 408 kilohertz. This signal extends from 392 to 408 kilohertz and has a bandwidth of 16 kilohertz, twice the highest modulating frequency of 8 kilohertz.

Musical instruments produce complex sound waves containing a great number of frequencies. The frequencies produced by a piano, for example, range from approximately 27 to 4,200 hertz with harmonic frequencies extending beyond 10 kilohertz. Modulating frequencies of up to 15 kilohertz must be included in the signal to transmit a musical passage with a high degree of fidelity. This requires a bandwidth of at least 30 kilohertz to prevent attenuation of higher-order harmonic frequencies.

If the signal to be transmitted contains voice frequencies only, and fidelity is of minor importance, the bandwidth requirement is much smaller. A baritone voice includes frequencies of approximately 100 to 350 hertz, or 250 hertz. Intelligible voice communications can be carried out as long as the communications system retains audio frequencies up to several thousand hertz. Comparing the conditions for transmitting voice signals with those for transmitting music reveals that much less spectrum space is required for voice communications.

Radio stations in the standard broadcast band are assigned carrier frequencies by the Federal Communications Commission (FCC). When two stations are located near each other, their carriers must be spaced some minimum distance apart in the radio spectrum. Otherwise, the sideband frequencies of one station will interfere with sideband frequencies of the other station. The standard AM broadcast band starts at 535 kilohertz and ends at 1,605 kilohertz. Carrier assignments start at 540 kilohertz and continue in a succession of 10-kilohertz increments until the upper limit of the broadcast band is reached. This adds up to a total of 107 carrier assignments, or CHANNELS, over the entire broadcast band. If stations were assigned to all 107 channels (in a given geographical area), each station would be allotted a channel width of 10 kilohertz. This leaves 5 kilohertz on each side of each carrier for sidebands. Since interference between such closely spaced stations would be nearly impossible to prevent, the FCC avoids assigning adjacent channels to stations in the same area. As a consequence of this policy, one or more vacant channels normally exist between stations in the broadcast band. In the interest of better fidelity, the stations are permitted to use modulating frequencies higher than 5 kilohertz as long as no interference with other stations is produced.

Q.31 What are the two major sections of a typical AM transmitter?
Q.32 When 100 kilohertz and 5 kilohertz are heterodyned, what frequencies are present?
Q.33 What is the upper sideband of an AM transmission?
Q.34 Where is the intelligence in an AM transmission located?
Q.35 What determines the bandwidth of an AM transmission?