SIGNAL TO NOISE RATIO BASIC INFORMATION AND TUTORIALS

Maintaining an adequate ratio of signal power to noise power is essential for any communication system, though the exact definition of “adequate” varies greatly. Obviously there are two basic ways to improve S/N: increase the signal power or reduce the noise power.

Increasing signal power beyond a certain point can cause problems, particularly where portable, battery powered devices are concerned. Reducing noise power requires limiting bandwidth and, if possible, reducing the noise temperature of a system.

The system bandwidth must be large enough to accommodate the signal bandwidth, but should be no larger than that. Some modulation schemes are more efficient than others at transmitting information with a given power and bandwidth.

Noise Figure and Noise Temperature

The noise temperature of a complex system is not necessarily equal to the actual operating temperature, but may be higher or lower. The noise temperature for electronic systems is often found by way of the noise figure, so let us look briefly at that specification.

Noise figure describes the way in which a device adds noise to a signal and thereby degrades the signal-to-noise ratio. It is defined as follows:

NF =(S/N)i / (S/N)o

where

(S/N)i = signal-to-noise ratio at the input

(S/N)o = signal-to-noise ratio at the output

All of the above are expressed as power ratios, not in decibels. When a device has multiple stages, each stage contributes noise, but the first stage is the most important because noise inserted there is amplified by all other stages.

Again, all these are ratios, not in decibels. The noise figure for the system is usually specified in dB in the usual way:

NF(dB) = 10 log NF (1.7)

Converting noise figure to noise temperature is quite easy:

Teq = 290(NF − 1) (1.8)

where

Teq = equivalent noise temperature in kelvins

NF = noise figure as a ratio (not in dB)

The noise temperature due to the equipment must be added to the noise temperature contributed by the antenna and its transmission line to find the\ total system noise temperature. We’ll see how that is done after we have looked at receivers, antennas, and transmission lines separately.