5-21
peak and average power. The principles of radar are covered in NEETS, Module 18, Radar Principles,
which can be consulted for an explanation of pulsed waves.
Rectangular Pulse
A rectangular wave is used to pulse-modulate the constant frequency rf carrier to produce the pulse
radar output. The rectangular wave is made up of a fundamental frequency and its combined odd and even
harmonics. Figure 5-21 shows the development of a rectangular wave.
Figure 5-21.—Rectangular pulse.
Pulsed Wave Analysis
In amplitude modulation, sidebands are produced above and below the carrier frequency. A pulse is
also produced above and below the carrier frequency, but the pulse is made up of many tones. These tones
produce multiple sidebands that are commonly referred to as SPECTRAL LINES, or RAILS, on the
spectrum analyzer display. Twice as many rails will be in the pulse-modulated output of the radar as there
are harmonics contained in the modulating pulse (upper and lower sidebands), as shown in figure 5-22. In
the figure, the pulse repetition frequency (prf) is equal to the pulse interval of 1/T. The actual spectrum
analyzer display would show the lower lobes (shown below the reference line in the figure) on top
because the spectrum analyzer does not retain any polarity information. Changing the pulse interval, or
pulse width, of the modulation signal will change the amount of rails (prf), or number of lobe minima, as
illustrated in figure 5-23.
