5-8
Figure 5-6C.—Examples of ideal two-tone test waveforms.
When the trapezoidal method is used, two opposed triangles appear on the oscilloscope, as shown in
figure 5-6, view C. When equally balanced modulators are used, the triangles are mirror images. Elliptical
or straight-line patterns appear when the phase-distortion check is used.
It is also possible to make a rough operating adjustment by varying the audio drive from the
microphone so that on peak swings a definite value of final plate current is not exceeded. This check
depends upon the initial accuracy of calibration and response characteristics of the ammeter in the final
stage, as well as other factors.
FREQUENCY MODULATION
In frequency modulation, the carrier amplitude remains constant, and the output frequency of the
transmitter is varied about the carrier (or mean) frequency at a rate corresponding to the audio
frequencies. The extent to which the frequency changes in one direction from the unmodulated (carrier)
frequency is called the FREQUENCY DEVIATION.
Deviation in frequency is usually expressed in kilohertz. It is equal to the difference between the
carrier frequency and either the highest or lowest frequency reached by the carrier in its excursions with
modulation. There is no modulation percentage in the usual sense. With suitable circuit design, the
frequency deviation may be made as large as desired without encountering any adverse effects that are
equivalent to the overmodulation in amplitude-modulation transmissions. However, the maximum
permissible frequency deviation is determined by the width of the band assigned for station operation.
In frequency modulation, the equivalent of 100% modulation occurs when the frequency deviation is
equal to a predetermined maximum value. There are several methods of measuring the modulation in
frequency-modulated transmissions.
The frequency-deviation measurement of a frequency-modulated signal is normally performed with
either a spectrum analyzer or with a modulation analyzer. The modulation analyzer method is more
commonly used because of its accuracy. Typical accuracies for a modulation analyzer are within
1%.
Figure 5-7 shows a typical modulation analyzer.
