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4.1.2 Line Configuration
Signal-to-Noise Ratio, often written S/N or SNR, is a measure of signal strength
relative to background noise. The ratio is usually measured in decibels (dB).
If the incoming signal strength in microvolts is V
s
, and the noise level, also in
microvolts, is V
n
, then the signal-to-noise ratio, S/N, in decibels is given by the
formula
S/N = 20 log
10
(V
s
/V
n
)
If V
s
= V
n
, then S/N = 0. In this situation, the signal borders on unreadable, because
the noise level severely competes with it. In digital communications, this will
probably cause a reduction in data speed because of frequent errors that require the
source (transmitting) computer or terminal to resend some packets of data.
Ideally, V
s
is greater than V
n
, so S/N is positive. As an example, suppose that V
s
=
10.0 microvolts and V
n
= 1.00 microvolt. Then
S/N = 20 log
10
(10.0) = 20.0 dB
This results in the signal being clearly readable. If the signal is much weaker but still
above the noise -- say 1.30 microvolts -- then
S/N = 20 log
10
(1.30) = 2.28 dB
This is a marginal situation. There might be some reduction in data speed under these
conditions.
If V
s
is less than V
n
, then S/N is negative. In this type of situation, reliable
communication is generally not possible unless steps are taken to increase the signal
level and/or decrease the noise level at the destination (receiving) computer or
terminal.
To select the direction and target SNRM of line, click on the Line Confing link in the
left navigation bar, show as below.
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