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Agilent
Spectrum Analysis Amplitude
and Frequency Modulation
Application Note 150-1
Table of contents
Chapter 1. Modulation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Chapter 2. Amplitude modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Modulation degree and sideband amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
Zero span and markers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
The fast fourier transform . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
Special forms of amplitude modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Single sideband . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Chapter 3. Angle modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
Bandwidth of FM signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
FM measurements with the spectrum analyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
AM plus FM (incidental FM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
2
Chapter 1. Modulation methods
Modulation is the act of translating some low-fre-
quency or baseband signal (voice, music, and data)
to a higher frequency. Why do we modulate signals?
There are at least two reasons: to allow the simulta-
neous transmission of two or more baseband signals
by translating them to different frequencies, and to
take advantage of the greater efficiency and smaller
size of higher-frequency antennae.
In the modulation process, some characteristic of
a high-frequency sinusoidal carrier is changed in
direct proportion to the instantaneous amplitude
of the baseband signal. The carrier itself can be
described by the equation:
e = A cos (t + )
where:
A = peak amplitude of the carrier,
= angular frequency of the carrier in radians per second,
t = time, and
= initial phase of the carrier at time t = 0.
In the expression above, there are two properties of
the carrier that can be changed, the amplitude (A)
and the angular position (argument of the cosine
function). Thus we have amplitude modulation and
angle modulation. Angle modulation can be further
characterized as either frequency modulation or
phase modulation.
3
Chapter 2. Amplitude modulation Since the modulation is symmetrical,
Emax