IVQ in Telecommunication
Systems 2730
Technician Diploma
Systems 2730
Technician Diploma
(2730-011) Fundamentals of electronic communication 02
Noise
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Introduction
Information signals travel over communication channel can be distorted due to different reasons. Mixing unwanted signals with the wanted signals may be one reason for such change,
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| Adding Noise to a signal |
Noise can be described as "any unwanted signal that present in a communication system"
Noise is a mixture of sinusoidal signals with different frequencies and aptitudes.
Information Transmission is affected by noise if any frequency component of the noise signal is fallen in to the pass band (frequency spectrum) of the information signal.
Effect of Noise
The transmitter of an communication system generates an very strong wanted signal with a very low noise signal level, hence the Tx output has a very high SNR
As the signal travels over a communication channel the noise signal strength increases while decreasing the wanted signal strength. Therefore , SNR value gradually decreases along the communication channel.
Whatever the communication system, decreasing SNR ( Increasing noise ) affects the communication in different ways.
example:
01 Distort the wanted signal
02 SNR decreases along the medium
03 The receiver cannot reproduce the original information
04 Limits the distance of communication with a given transmit power
05 Produce bit errors in digital system
06 Sometimes wipe out the wanted signal
07 If high noise power is received communication equipment may be burnt.
Note:- Noise from environment can often be dramatically reduced by twisting, shielding, grounding and minimizing wire length.
Effect of noise on analog signals
Since the wanted signal and noise signals in an analog communication system is in analog form noise on an analog communication system cannot be removed
In addition signal amplifying devices are placed to improve the wanted signal, but also the unwanted noise signal are improved
Therefore the noise accumulate along the transmission medium of an analog system
But effect of noise on digital communication system can be minimized by introducing Pulse Re-generators (also known as Regenerative Repeaters) at regular intervals.
But some noise effect cannot be fully eliminated
example : Lightning
Analog Communication System
Decide weather the received signal is above a particular voltage level or below another voltage level, Accordingly new signal is produced
Example: In data transmission,
Noise reduces the achievable bit rate over a given medium
Noise due to lightning may produce bit errors
If the noise level is high compared to the wanted signal level. The information is completely obliterated
Noise Sources
Categorizing noises
Noise can be divided in different ways
- Natural or man-made noise
- White noise or Impulsive noise
- Internal or External Noise
Natural or man-made noise
Noise may arise from different sources and they can be categorize as either man made or natural
by nature noise can be divided in to
- White noise
- Impulsive noise
White Noise
If noise is spread throughout the whole electromagnetic frequency spectrum and noise aptitudes are equal at all the frequencies such a noise is known as a "White Noise".
Example:
- Thermal noise
- Solar noise
Theoretically white noise has an infinite bandwidth
Flicker noise
Experimentally provided that an unwanted signal exists in
electronic components, even the component is kept at 0 Kelvin temperature ( no
thermal noise) and no external voltage is applied ( no shot noise). This noise
is known as Flicker noise.
The origins of flicker noise are somewhat less understood as
compared to thermal noise and shot noise, but predicts that, may be due to
imperfection in the crystalline structure of all materials
Flicker noise is inversely proportional to the frequency at
the signal being observed. Due to this reason flicker noise is also known
as 1/f noise.
Expressing noise level
Noise level in a communication system can be expressed by
- Signal to Noise Ratio (S/N or SNR)
- Noise factor (F)
- Noise Figure (NF
- Noise temperature (Tn)
SNR is a ratio indicting that the relative strength of a
wanted signal and noise in a communication system.
The ratio between signal power and the noise power
is defines as SNR
Signal to Noise Ratio (S/N or SNR) =
Signal power (Ps) / Noise power
(Pn)
SNR can also be expressed in decibels (dB)
S/N = 10
log10 (Signal power/ Noise
power)
Example 01
If the Input
signal power is 55µW and input noise power is 5.5 nW, calculate SNR at the
input of the communication system as a ratio in dB
= 10 log10
( Ps/Pn)
=10 log(55x10-6/5.5x10-4)
=40dB
Calculate the
maximum noise power allowed at the input of a communication receiver in order
to maintain 40 dB SNR at the input for an input signal power of 20 pW
40 = 10 log10
(Ps/Pn)
4 = log (20 x
10-12 / Pn)
= 20 x 10-16
W
Example 03
Determine the signal
power required at the input of a piece of communication equipment in order to
maintain 35dB SNR. That the input power noise is 0.1nW
SNR in dB using Voltage/Current Values
SNR in dB using Voltage/Current Values
S/N = 20 log10 ( Vs/Vn)
S/N = 20 log10 ( Is/In)
Vs - Signal Voltage
Vn – Noise
Voltage
Is – Signal
Current
In- Noise
Current
Example 01
The internal
noise produced by an amplifier with 20 dB gain is negligible. If the noise
voltage at the input of amplifier is 5mV and output signal voltage is 50 V,
calculate the SNR in dB.
Note:
The received
signal of a communication system has two components, the wanted signal and the
unwanted signal.
To
identify/receive information from the received signal, the wanted signal
strength should be higher than the noise signal strength. Otherwise the
information cannot be identified.
Since SNR
determines the accuracy with which received information can be identified.
Communication equipment is designed to produce the highest possible SNR.
To identify
information a minimum gap should be maintained between the wanted signal power
and the noise signal power.
Therefore a
minimum threshold SNR level is established for different services for effective
communication. Beyond which if the noise increases information would be lost.
Example
Typical
threshold SNR limits for different services
Telephone calls
over PSTN 35dB
Television
system 50dB
Satellite 53dB
Noise factor
(F)
Nose factor is
the ratio of input S/N power ratio to the output S/N power ratio
Noise factor (F) =input signal to noise power ratio / output
signal to noise power ratio
Noise figure (NF)
The noise factor, expressed in decibels is known as the
noise figure
Noise figure (NF) = 10 log10 (noise factor)
Noise figurer (NF) =10 log10 (input signal to
noise power ratio / output signal to noise power ratio)
Example 01 – past paper 2004 June Q5
- An amplifier has an output signal of 1mV and a noise level of 0.514 mV. Determine the signal to noise ratio (SNR)
- An amplifier has an output signal of 1 watt and the input noise is 0.01 watt. When the output signal is 10 watts and the output noise power is 0.3 Watts calculate the amplifier’s noise figure.
Example 02 – past paper 2004 June Q5
An amplifier with a noise ratio of 4 dB has an input SNR of
20dB. Calculate the output signal noise as,
- A ratio
- dB
Note:
Noise figure (NF) and noise factor (F) are measures of
deration of the signal to noise ratio (SNR) caused by components in an
electronic circuit or device.
These figure indicate how much the signal to noise ratio
deteriorates along a communication system
NF = 10 log10
(S/N) = 10 log10 (S/N)in - 10 log10 (S/N) out
Noise Factor = (S/N in dB)
in - (S/N in dB) out
Output noise power of an
amplified noise factor (F) when the gain is G
Operating bandwidth
of the amplifier – B
Temperature –
T0
PN in
= KT0B
PS out
= GPS in
F = (S/N)in
/ (S/N)out
F = PNout/GKT0B
PNout
= FGKT0B
Noise
Temperature (Tn)
Tn is
another method of expressing noise introduced by an electronic component or
source
Given as a
temperature value in Kelvin
more
NOISE
With reference to an electrical system, noise may be defined as any unwanted form of energy which tends to interfere with proper reception and reproduction of wanted signal.
OR
Noise is random, undesirable electrical energy that enters the communications system via the communicating medium and interferes with the transmitted message. However, some noise is also produced in the receiver.
Atmospheric noise accordingly consists of spurious radio signals with components spread over a wide frequency range. These spurious radio waves constituting the noise get propagated over the earth in the same fashion as the desired radio waves of the same frequency. Accordingly at a given receiving point, the receiving antenna picks up not only the signal but also the static from all the thunderstorms, local or remote.
The field strength of atmospheric noise varies approximately inversely with the frequency. Thus large atmospheric noise is generated in low and medium frequency (broadcast) bands while very little noise is generated in the VHF and UHF bands. Further VHF and UHF components of noise are limited to the line-of-sight (less than about 80 Km) propagation. For these two-reasons, the atmospheric noise becomes less severe at Frequencies exceeding about 30 MHz.
Thus noise generated in any resistance due to random motion of electrons i5 called thermal noise or white or Johnson noise.
The analysis of thermal noise is based on the Kinetic theory. It shows that the temperature of particles is a way of expressing its internal kinetic energy. Thus "Temperature" of a body can be said to be equivalent to the statistical rms value of the velocity of motion of the particles in the body. At -273°C (or zero degree Kelvin) the kinetic energy of the particles of a body becomes zero .Thus we can relate the noise power generated by a resistor to be proportional to its absolute temperature. Noise power is also proportional to the bandwidth over which it is measured. From the above discussion we can write down.
Pn ∝ TB
Pn = KTB ------ (1)
Where
Pn = Maximum noise power output of a resistor.
K = Boltzmann’s constant = 1.38 x10-23 joules I Kelvin.
T = Absolute temperature.
B = Bandwidth over which noise is measured.
From equation (1), an equivalent circuit can be drawn as shown in below figure
From equation (2), we see that the square of the rms noise voltage is proportional to the absolute temperature of le resistor, the value of the resistor, and the bandwidth over which it is measured. En is quite independent of the Frequency.
Example
R.F. amplifier is saving an input resistor of 8Kr and works in the frequency range of 12 to 15.5 MHz Calculate the rms noise voltage at the input to this amplifier at an ambient temperature of 17oC?
Solution:
Transit time is (he duration of time that it takes for a current carrier such as a hole or current to move from the input to the output.
The devices themselves are very tiny, so the distances involved are minimal. Yet the time it takes for the current carriers to move even a short distance is finite. At low frequencies this time is negligible. But when the frequency of operation is high and the signal being processed is the magnitude as the transit time, then problem can occur. The transit time shows up as a kind of random noise within the device, and this is directly proportional to the frequency of operation.
A receiver has an input signal power of l.2µW. The noise power is 0.80µW. The signal to noise ratio is
The device under consideration can be the entire receiver or a single amplifier stage. The noise figure F also called the noise factor can be computed with the expression
F = Signal to Noise power Input/Signal to noise power output
You can express the noise figure as a number, more often you will see it expressed in decibels.
OR
Noise is random, undesirable electrical energy that enters the communications system via the communicating medium and interferes with the transmitted message. However, some noise is also produced in the receiver.
Classification of Noise
Noise may be put into following two categories.- External noises, i.e. noise whose sources are external.External noise may be classified into the following three types:
- Atmospheric noises
- Extraterrestrial noises
- Man-made noises or industrial noises.
- Internal noise in communication, i.e. noises which get, generated within the receiver or communication system.Internal noise may be put into the following four categories.
- Thermal noise or white noise or Johnson noise
- Shot noise.
- Transit time noise
- Miscellaneous internal noise.
Explanation of External Noise
Atmospheric Noise
Atmospheric noise or static is caused by lighting discharges in thunderstorms and other natural electrical disturbances occurring in the atmosphere. These electrical impulses are random in nature. Hence the energy is spread over the complete frequency spectrum used for radio communication.Atmospheric noise accordingly consists of spurious radio signals with components spread over a wide frequency range. These spurious radio waves constituting the noise get propagated over the earth in the same fashion as the desired radio waves of the same frequency. Accordingly at a given receiving point, the receiving antenna picks up not only the signal but also the static from all the thunderstorms, local or remote.
The field strength of atmospheric noise varies approximately inversely with the frequency. Thus large atmospheric noise is generated in low and medium frequency (broadcast) bands while very little noise is generated in the VHF and UHF bands. Further VHF and UHF components of noise are limited to the line-of-sight (less than about 80 Km) propagation. For these two-reasons, the atmospheric noise becomes less severe at Frequencies exceeding about 30 MHz.
Extraterrestrial Noise
There are numerous types of extraterrestrial noise or space noises depending on their sources. However, these may be put into following two subgroups.- Solar noise
- Cosmic noise
Solar Noise
This is the electrical noise emanating from the sun. Under quite conditions, there is a steady radiation of noise from the sun. This results because sun is a large body at a very high temperature (exceeding 6000°C on the surface), and radiates electrical energy in the form of noise over a very wide frequency spectrum including the spectrum used for radio communication. The intensity produced by the sun varies with time. In fact, the sun has a repeating 11-Year noise cycle. During the peak of the cycle, the sun produces some amount of noise that causes tremendous radio signal interference, making many frequencies unusable for communications. During other years. the noise is at a minimum level.Cosmic noise
Distant stars are also suns and have high temperatures. These stars, therefore, radiate noise in the same way as our sun. The noise received from these distant stars is thermal noise (or black body noise) and is distributing almost uniformly over the entire sky. We also receive noise from the center of our own galaxy (The Milky Way) from other distant galaxies and from other virtual point sources such as quasars and pulsars.Man-Made Noise (Industrial Noise)
By man-made noise or industrial- noise is meant the electrical noise produced by such sources as automobiles and aircraft ignition, electrical motors and switch gears, leakage from high voltage lines, fluorescent lights, and numerous other heavy electrical machines. Such noises are produced by the arc discharge taking place during operation of these machines. Such man-made noise is most intensive in industrial and densely populated areas. Man-made noise in such areas far exceeds all other sources of noise in the frequency range extending from about 1 MHz to 600 MHzExplanation of Internal Noise in communication
Thermal Noise
Conductors contain a large number of 'free" electrons and "ions" strongly bound by molecular forces. The ions vibrate randomly about their normal (average) positions, however, this vibration being a function of the temperature. Continuous collisions between the electrons and the vibrating ions take place. Thus there is a continuous transfer of energy between the ions and electrons. This is the source of resistance in a conductor. The movement of free electrons constitutes a current which is purely random in nature and over a long time averages zero. There is a random motion of the electrons which give rise to noise voltage called thermal noise.Thus noise generated in any resistance due to random motion of electrons i5 called thermal noise or white or Johnson noise.
The analysis of thermal noise is based on the Kinetic theory. It shows that the temperature of particles is a way of expressing its internal kinetic energy. Thus "Temperature" of a body can be said to be equivalent to the statistical rms value of the velocity of motion of the particles in the body. At -273°C (or zero degree Kelvin) the kinetic energy of the particles of a body becomes zero .Thus we can relate the noise power generated by a resistor to be proportional to its absolute temperature. Noise power is also proportional to the bandwidth over which it is measured. From the above discussion we can write down.
Pn ∝ TB
Pn = KTB ------ (1)
Where
Pn = Maximum noise power output of a resistor.
K = Boltzmann’s constant = 1.38 x10-23 joules I Kelvin.
T = Absolute temperature.
B = Bandwidth over which noise is measured.
From equation (1), an equivalent circuit can be drawn as shown in below figure
Example
R.F. amplifier is saving an input resistor of 8Kr and works in the frequency range of 12 to 15.5 MHz Calculate the rms noise voltage at the input to this amplifier at an ambient temperature of 17oC?
Solution:
Shot Noise
The most common type of noise is referred to as shot noise which is produced by the random arrival of 'electrons or holes at the output element, at the plate in a tube, or at the collector or drain in a transistor. Shot noise is also produced by the random movement of electrons or holes across a PN junction. Even through current flow is established by external bias voltages, there will still be some random movement of electrons or holes due to discontinuities in the device. An example of such a discontinuity is the contact between the copper lead and the semiconductor materials. The interface between the two creates a discontinuity that causes random movement of the current carriers.Transit Time Noise
Another kind of noise that occurs in transistors is called transit time noise.Transit time is (he duration of time that it takes for a current carrier such as a hole or current to move from the input to the output.
The devices themselves are very tiny, so the distances involved are minimal. Yet the time it takes for the current carriers to move even a short distance is finite. At low frequencies this time is negligible. But when the frequency of operation is high and the signal being processed is the magnitude as the transit time, then problem can occur. The transit time shows up as a kind of random noise within the device, and this is directly proportional to the frequency of operation.
MISCELLANEOUS INTERNAL NOISES Flicker Noise
Flicker noise or modulation noise is the one appearing in transistors operating at low audio frequencies. Flicker noise is proportional to the emitter current and junction temperature. However, this noise is inversely proportional to the frequency. Hence it may be neglected at frequencies above about 500 Hz and it, Therefore, possess no serious problem.Transistor Thermal Noise
Within the transistor, thermal noise is caused by the emitter, base and collector internal resistances. Out of these three regions, the base region contributes maximum thermal noise.Partition Noise
Partition noise occurs whenever current has to divide between two or more paths, and results from the random fluctuations in the division. It would be expected, therefore, that a diode would be less noisy than a transistor (all other factors being equal) If the third electrode draws current (i.e.., the base current). It is for this reason that the inputs of microwave receivers are often taken directly to diode mixers.Signal to Noise Ratio.
Noise is usually expressed as a power because the received signal is also expressed in terms of power. By Knowing the signal to noise powers the signal to noise ratio can be computed. Rather than express the signal to noise ratio as simply a number, you will usually see it expressed in terms of decibels.A receiver has an input signal power of l.2µW. The noise power is 0.80µW. The signal to noise ratio is
Signal to Noise Ratio = 10 Log (1.2/0.8)
= 10 log 1.5
= 10 (0.176)
= 1.76 dB
= 10 log 1.5
= 10 (0.176)
= 1.76 dB
Noise Figure
Noise Figure F is designed as the ratio of the signal-to-noise power at the input to the signal to noise power at the output.The device under consideration can be the entire receiver or a single amplifier stage. The noise figure F also called the noise factor can be computed with the expression
F = Signal to Noise power Input/Signal to noise power output
You can express the noise figure as a number, more often you will see it expressed in decibels.
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