Radio receiver noise floor

- an overview or tutorial about the basics of the noise floor or a receiver, what it is and how the noise floor affects the performance of a receiver and may affect the design of fixed and mobile radio communications systems.

Noise is a fact of life. Despite the best efforts of any design engineers, there is always some background noise present in any radio receiver used for any radio communications system. The noise emanates from many sources, and although the design of the receiver is optimised to reduce it some will always be present.

Accordingly a concept that is very useful in many elements of signal theory and hence in radio receiver design is that of a noise floor. The noise floor can be defined as the measure of the signal created from the sum of all the noise sources and unwanted signals within a system.

When designing a radio receiver for any radio communications system it is necessary to ensure that the performance of the radio receiver matches the performance required. For some radio communications systems, typically those operating on frequencies below about 30 MHz, the level of noise from the antenna system may be relatively high. In these cases, it is of no use to have an ultra-low noise radio receiver. However in applications such as VHF and UHF fixed or mobile radio communications systems where the levels of received noise are much lower, then a low noise radio receiver is more useful.

Elements of a radio receiver that affect noise floor

In order to reduce the levels of noise and thereby improve the sensitivity of the radio receiver, the main element of the receiver that requires its performance to be optimised is the RF amplifier. The use of a low noise amplifier at the front end of the receiver will ensure that its performance will be maximised. Wither for use at microwaves or lower frequencies, this RF amplifier is the chief element in determining the performance of the whole receiver. The next most important element is the first mixer.

Radio receiver noise floor

While noise can emanate from many sources, when looking purely at the receiver, the noise is dependent upon a number of elements. The first is the minimum equivalent input noise for the receiver. This can be calculated from the following formula:

P     =     k T B

P is the power in watts
K is Boltzmann's constant (1.38 x 10^-23 J/K)
B is the bandwidth in Hertz

Using this formula it is possible to determine that the minimum equivalent input noise for a receiver at room temperature (290K) is -174 dBm / Hz.

It is then possible to calculate the noise floor for the receiver:

Noise floor     =     -174   +   NF +   10 log Bandwidth

Where NF is the noise figure
dBm is the power level expressed in decibels relative to one milliwatt

The concept of noise floor is valuable in many radio communications systems and enables the radio receiver design and performance to be matched to the requirements of the overall system.

By Ian Poole

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