We can express the amount of noise present in a signal in many different ways. We need not usually the absolute power of the noise, but the power of the noise relative to the power of the signal itself is important. Keeping this reason in view, one of the most commonly used ways of expressing the amount of noise is the signal-to- noise power ratio. A ratio of two powers is most conveniently stated on the decibel scale. As the power is directly proportional to voltage squared, the signal-to- noise ratio is defined as:
In equation shown above, vs is the rms signal voltage and vn is the rms noise voltage. The equation has the standard form that is used for expressing a ratio of powers in decibels.
The SNR is largely used to determine the degree to which a circuit, for example an amplifier, may introduce additional noise into an already noisy signal. If SNRIN and SNROUT are the SNRs of the signal at the input and output, the noise value of the amplifier is shown by:
NF= SNRIN/SNROUT
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