• It is defined as the ration of signal power to the noise power at the same point.

  ∴ $\frac{S}{N} = \frac{P_s} { P_n}$

Where
$P_s$ = Signal power

$P_n$ = Noise power at the same point.

• The signal to noise ratio is normally expressed in dB and the typical values of S/N ratio range from about 10 dB to 90 dB.

• Higher the values of S/N ratio better the system performance in presence of noise.

• The power can be expressed in terms of signal and noise voltages as follows:

• The signal to noise ration in dB is given by,

Importance of SNR in radio receiver:

• The noise performance and the signal to noise ratio is a key parameter for any radio receiver.

• The signal to noise ratio or SNR as it is often termed is a measure of the sensitivity performance of a receiver.

• There are a number of ways in which the noise performance and the sensitivity of a radio receiver can be measured. The most obvious method is to compare the signal and noise levels for a known signal level, i.e. the signal to noise (S/N) ratio or SNR.

• Obviously the greater the difference between the signal and the unwanted noise, i.e. the greater the S/N ratio or SNR, the better the radio receiver sensitivity performance.