
40-1
40.1 Introduction
A signal is usually dened by a time-varying function carrying some sort of information. Such a func-
tion most oen represents a time-changing electric or magnetic eld, whose propagation can be in free
sp
ace
or in di
electric
ma
terials
co
nstrained
by co
nductors
(w
aveguides,
co
axial
ca
bles,
et
c
.). A sig
nal
is sa
id
to be pe
riodic
if it re
peats
it
self
ex
actly
a
er
a gi
ven
ti
me
T ca
lled
th
e
pe
riod
. e in
verse
of th
e
pe
riod
T, m
easured
i
n
s
econds,
i
s
t
he
f
requency
f m
easured
i
n
h
ertz
(
Hz)
.
A pe
riodic
sig
nal
ca
n
al
ways
be re
presented
in te
rms
of a su
m
of se
veral
(p
ossibly
in
nite)
sin
usoi-
dal
sig
nals,
wi
th
su
itable
am
plitude