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
Where:
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
Summary
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.
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