Biomedical Engineering Reference
In-Depth Information
t
t
r
F
(a)
90% A
τ
A
50% A
10% A
T
(b)
20 dB/decade
40 dB/decade
F
F
F
1
2
Figure 4.1
A pulse train with the characteristics shown in (
a
) produces a spectrum with an enve-
lope that can be approximated by the nomogram of (
b
).
For nonperiodic trains, the nomogram must be modi
ed to account for the broadband
nature of the source. To do so, a nomogram of the spectral density envelope of the signal
can be de
fi
fi
ned for a unity bandwidth of 1 MHz by
6
20 log(
A
τ
)
f
f
1
dB
H
V
z
dB
H
A
z
(
V
or
I
)
or
M
f
1
20 log(
A
)
4
20 log[
f
(MHz)]
f
f
2
A
s
)
τ
f
2
20 log
14
40 log[
f
(MHz)]
f
(
Depending on its internal impedance, a circuit carrying such a pulse train will create in its
vicinity a
fi
field that is principally electric or magnetic. At a greater distance from the
source, the
field becomes electromagnetic, regardless of the source impedance. If there is
a coupling mechanism, which can be either conduction or radiation, some or all of the
frequency components in the digital pulse train's spectrum will be absorbed by some
“victim” receiver circuit.
fi
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