ANALYSIS OF AN AM WAVE. - A significant amount of information concerning the basic principles of amplitude modulation can be obtained from a study of the properties of the modulation envelope.

A carrier wave which has been modulated by voice or music signals is accompanied by two sidebands; each sideband contains individual frequencies that vary continuously. Since a wave of this nature is nearly impossible to analyze, you can assume in the following sections that the modulating signal, unless otherwise qualified, is a single-frequency, constant-amplitude sine wave.

PERCENT OF MODULATION IN AN AM WAVE. - The degree of modulation is defined in terms of the maximum permissible amount of modulation. Thus, a fully modulated wave is said to be 100-PERCENT MODULATED. The modulation envelope in figure 1-40, view (A), shows the conditions for 100-percent sine-wave modulation. For this degree of modulation, the peak audio voltage must be equal to the dc supply voltage to the final power amplifier. Under these conditions, the rf output voltage will reach 0 on the negative peak of the modulating signal; on the positive peak, it will rise to twice the amplitude of the unmodulated carrier.

Figure 1-40A. - Conditions for 100-percent modulation.

When analyzed, the modulation envelope consists of the unmodulated rf carrier voltage plus the combined voltage of the two sidebands. The combined sideband voltages are approximately equal to the rf carrier voltage since each sideband frequency contains one-half the carrier voltage, as shown in view (B). This condition is known as 100-percent modulation and the maximum modulated rf voltage is twice the carrier voltage. The audio-modulating voltage can be increased beyond the amount required to produce 100-percent modulation. When this happens, the negative peak of the modulating signal becomes larger in amplitude than the dc plate-supply voltage to the final power amplifier. This causes the final plate voltage to be negative for a short period of time near the negative peak of the modulating signal. For the duration of the negative plate voltage, no rf energy is developed across the plate tank circuit and the rf output voltage remains at 0, as shown in figure 1-41, view (A).

Figure 1-40B. - Conditions for 100-percent modulation.

Figure 1-41A. - Overmodulation conditions.

Look carefully at the modulation envelope in view (A). It shows that the negative peak of the modulating signal has effectively been limited. If the signal were demodulated (detected in the receiver), it would have an appearance somewhat similar to a square wave. This condition, known as OVERMODULATION, causes the signal to sound severely distorted (although this will depend on the degree of overmodulation).

Overmodulation will generate unwanted (SPURIOUS) sideband frequencies. This effect can easily be detected by tuning a receiver near, but somewhat outside the desired frequency. You would likely be able to tune to one or more of these undesired sideband frequencies, but the reception would be severely distorted, possibly unintelligible. (Without overmodulation, no such unwanted sideband frequencies would exist and you would be able to tune only to the desired frequency.) These unwanted frequencies will appear for a considerable range both above and below the desired channel. This effect is sometimes called SPLATTER. These spurious frequencies, shown in view (B), cause interference with other stations operating on adjacent channels. You should clearly understand that overmodulation, and its attendant distortion and interference is to be avoided.

Figure 1-41B. - Overmodulation conditions.

In addition to the above problems, overmodulation also causes abnormally large voltages and currents to exist at various points within the transmitter. Therefore, sufficient overload protection by circuit breakers and fuses should be provided. When this protection is not provided, the excessive voltages can cause arcing between transformer windings and between the plates of capacitors, which will permanently destroy the dielectric material. Excessive currents can also cause overheating of tubes and other components.

Ideally, you will want to operate a transmitter at 100-percent modulation so that you can provide the maximum amount of energy in the sideband. However, because of the large and rapid fluctuations in amplitude that these signals normally contain, this ideal condition is seldom possible. When the modulator is properly adjusted, the loudest parts of the transmission will produce 100-percent modulation. The quieter parts of the signal then produce lesser degrees of modulation.

To measure degrees of modulation less than 100 percent, you can use a MODULATION FACTOR (M) to indicate the relative magnitudes of the rf carrier and the audio-modulating signal. Numerically, the modulation factor is:

To illustrate this use of the equation, assume that a carrier wave with a peak amplitude of 400 volts is modulated by a 3-kilohertz sine wave with a peak amplitude of 200 volts. The modulation factor is figured as follows:

If the modulation factor were multiplied by 100, the resultant quantity would be the PERCENT OF MODULATION (%M):

By using the correct equation, you can determine the percent of modulation from the modulation envelope pattern.

This method is useful when the percent of modulation is to be determined using the pattern on the screen of an oscilloscope. For example, assume that your oscilloscope is connected to the output of a modulator circuit and produces the screen pattern shown in figure 1-42. According to the setting of the calibration control, each large division on the vertical scale is equal to 200 volts. By using this scale, you can see that the peak carrier amplitude (unmodulated portion) is 400 volts. The peak amplitude of the carrier is designated as e_o in figure 1-42.

Figure 1-42. - Computing percent of modulation from the modulation envelope.