Tektronix 1L30, Instruction Manual

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Operating Instructions— Type H 3 0
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(A ) Narrow band FM. Note the resemblance to AM.
CARRIER 100 KHz/CM
DISPERSION 2 GHz
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(C) Carrier almost at a null.
(D )* Carrier at null.
Fig. 2-15. Frequency modulated display. At first carrier null, index o f modulation is 2.4; so ratio of deviation to rate is 2.4. Rate is 0.8
cm X 100 kHz/cm = 80 kHz. Deviation 2.4 X 80 kHz = 192 kHz.
Pulse Modulated Signal Spectra
W hen a C W signal is pulse modulated, the carrier is
periodically turned on and off. The on period is determined
by the modulating pulse width; the off time is related to
the pulse repetition time or frequency. The carrier is usually
modulated by a rectangular-shaped pulse.
A symmetrical square wave is composed of its fundamental
frequency plus the odd harmonics. If the relative amplitudes
and phase of the harmonics are changed, a number of wave
shapes are produced; rectangular, trapezoidal, sawtooth,
etc. The spectrum of the square wave or any pulse shape,
therefore, is displayed according to its frequency compo­
nents and their amplitudes on a spectrum analyzer. Com­
mon pulse forms and their spectra are described in Ref­
erence Data for Radio Engineers, 4th edition. Chapter 35.
ITT 1956.
Fig. 2-16A illustrates a theoretical voltage spectrum of
a rectangulary-pulse, pulse-modulated oscillator The main
lobe and the side lobes are shown as groups of spectral lines
extending above and below the baseline. The number of
these side lobes for a truly rectangular pulse approaches
infinity. Any two adjacent side lobes are separated on the
frequency scale by a distance equal to the inverse of the
modulating pulse width.
Fourier theory shows that adjacent lobes are 180° out
of phase; however, since the spectrum analyzer is insensitive
to phase, only the absolute value of the spectrum is dis­
played and appears as illustrated in Fig. 2-16B.
Fig. 2-17 illustrates the relative effects the pulse width
and pulse repetition frequency have on a pulsed RF spec­
trum.
Since the spacing between the spectral lines of the pulsed
RF spectrum is a function of the PRF (pulse repetition fre­
quency), the spectrum analyzer resolution bandwidth should
be less than the PRF to respond to one frequency compo­
nent. In most instances this is impractical. The spectrum
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