What is Noise ?
Communication is the process of transmitting information from one point to another.
In a communication system, during the transmission of a signal, or while receiving the signal, some unwanted signal enters the communication system via communicating medium, and interferes with the transmitted signal. Such a disturbance is called as Noise.
A noise signal has no pattern and no constant frequency or amplitude. It is quite random and unpredictable.
Measures are usually taken to reduce it, though it can’t be completely eliminated.
Most common examples of noise are −
- Hiss sound in radio receivers
- Buzz sound amidst of telephone conversations
- Flicker in television receivers, etc.
Classification of Noise
The classification of noise is done depending on the type of the source, the effect it shows or the relation it has with the receiver, etc.
There are two main ways in which noise is produced. One is through some external source (known as External Noise) while the other is created by an internal source (known as Internal Noise), within the receiver section.
This noise is produced by the external sources which may occur in the medium or channel of communication.
This noise cannot be completely eliminated. The best way is to avoid the noise from affecting the signal.
Most common examples of external noise are −
- Atmospheric noise (due to irregularities in the atmosphere).
- Extra-terrestrial noise, such as solar noise and cosmic noise.
- Industrial noise.
This noise is produced within the receiver by the receiver components while functioning. The components in the circuits, due to continuous functioning, may produce few types of noise.
Internal noise can be easily evaluated or measured mathematically and can be reduced to a great extent by proper receiver design .
Most common examples of internal noise are −
- Thermal noise or White noise or Johnson noise
- Shot noise (due to the random movement of electrons and holes).
- Transit-time noise (during transition).
- Miscellaneous noise is another type of noise which includes flicker, resistance effect and mixer generated noise, etc.
Signal to Noise Ratio
Signal-to-Noise Ratio (SNR) is the ratio of the signal power to the noise power.
The higher the value of SNR, the greater will be the quality of the received output.
Signal-to-noise ratio at different points can be calculated by using the following formula :
Figure of Merit
The ratio of output SNR to the input SNR is known as the Figure of merit (F). It is denoted by F and it describes the performance of a device.
Since for a receiver , the channel is the input, Figure of merit of a receiver is given as :