J
Fig. 2-13. Spectrum showing series of AM signals.
2 -14
' ■)
4
4
F
- ( -------------
FREQUENCY
Fig. 2-14. Formation of a spectrum. F is the fundamental or carrier
frequency. Ft and F2 are the modulating frequencies.
The amplitude modulated signal spectrum therefore fur
nishes the following information:
1) Fundamental or car
rier frequency, 2) modulation frequency or frequencies, 3)
modulation -percentage, .4) sideband energy distribution
and 5) modulation bandwidth. Other characteristics which
may be evaluated are: Degree of incidental FM (evidenced
by signal jitter), nonlinear modulation and over-modulation.
These, characteristics are illustrated in more detail with other
types of spectrum display patterns.
Frequency Modulated Signal Spectra
W hen a C W signal (Fc) is frequency modulated at a rate
(Fm), it will theoretically produce an infinite number of side
band frequencies. These frequencies are equal to the carrier
frequency plus or minus the modulating frequencies (Fc ±
nFm where n = 1, 2,
3,
. . . etc.). Figure 2-15 illustrates vari
ous degrees of frequency modulation
Frequency modulated signal bandwidth is usually deter
mined by the width of the sidebands containing sufficient
energy to dominate the display. A very approximate cal
culation of the signal bandwidth equals 2 (4FC + Fm) where
AFC is the frequency deviation of the carrier and Fm is the
frequency of the modulating signal.
Frequency deviation
of the carrier is primarily dependent on the modulating
signal amplitude.
This ratio of frequency deviation to modulating frequency
is known as modulation index.
Bessel function and fre
quency spectra for different modulation indices may be
found in the 4th edition of Reference Data for Radio Engi
neers, Chapter 19.
To resolve adjacent sideband components in a frequency
modulated display, the spectrum analyzer resolution band
width should be less than the lowest modulating frequency
in the spectrum, which is the same as the requirements to
resolve an amplitude modulated spectrum.
©
Summary of Contents for 1L30
Page 48: ...si L i y n Maintenance Type 1L30 Fig 4 15 Honeycomb assembly drcui a n d component layout 4 n ...
Page 59: ...Fig 6 1 A Test equipment required for calibration ...
Page 60: ... Calibration Type 1130 ...
Page 120: ...T Y P E I L 3 0 S P E C T R U M A N A L Y Z E R ib i IF SYSTEM BLOCK DIAGRAM 9 1 9 2 ...
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