 - In analog and digital communications,
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
Vs, and the
noise level, also in microvolts, is Vn, then the
signal-to-noise ratio, S/N, in
decibels is given by the formula
S/N = 20
log10(Vs/Vn)
If Vs = Vn, 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, Vs is greater than Vn, so
S/N is positive. As an
example, suppose that Vs = 10.0 microvolts and
Vn = 1.00 microvolt.
Then
S/N = 20 log10(10.0) = 20.0 dB
which 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 log10(1.30) = 2.28 dB
which is a marginal situation. There might
be some reduction in data
speed under these conditions.
If Vs is less than Vn, 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.
Communications engineers always strive to
maximize the S/N ratio.
Traditionally, this has been done by using the narrowest
possible receiving-system bandwidth consistent
with the data speed
desired. However, there are other methods. In some
cases, spread spectrum
techniques can improve system
performance. The S/N ratio can be increased by providing
the source with a higher
level of signal output power if necessary. In some
high-level systems such as radio
telescopes, internal noise is minimized by lowering the
temperature of the receiving
circuitry to near absolute zero (-273 degrees Celsius or -459
degrees Fahrenheit).
In wireless
systems, it is always important
to optimize the performance of the transmitting and receiving
antennas.
 |
Getting started with signal strength |
| LAST UPDATED: |
03 Aug 2009
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