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Signal-to-noise ratio (S/N or SNR) definition
Posted by Margaret
Rouse
Contributor(s): John Burke
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).
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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.
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