1. Communication System
Ass. Prof. Ibrar Ullah
BSc (Electrical Engineering)
UET Peshawar
MSc (Communication & Electronics Engineering)
UET Peshawar
PhD (In Progress) Electronics Engineering
(Specialization in Wireless Communication)
MAJU Islamabad
E-Mail: ibrar@cecos.edu.pk
Ph: 03339051548 (0830 to 1300 hrs)
1
14. Noise
In any real physical system, when the signal voltage arise at the
demodulator, it will be accompanied by a voltage waveform which varies
with time in an entirely unpredictable manner. This unpredictable voltage
wave form is a random process called noise.
Types of Noise
Most man made electro-magnetic noise occurs at frequencies below
500 MHz. The most significant of these include:
• Hydro lines
• Ignition systems
• Fluorescent lights
• Electric motors
Therefore deep space networks are placed out in the desert, far from
these sources of interference.
14
15. Types of Noise (cont..)
• There are also a wide range of natural noise sources which cannot be so
easily avoided, namely:
• Atmospheric noise - lighting < 20 MHz
• Solar noise - sun - 11 year sunspot cycle
• Cosmic noise - 8 MHz to 1.5 GHz
• Thermal or Johnson noise. Due to free electrons striking
vibrating ions.
• White noise - white noise has a constant spectral density over a
specified range of frequencies. Johnson noise is an example of
white noise.
• Gaussian noise - Gaussian noise is completely random in nature
however, the probability of any particular amplitude value
follows the normal distribution curve. Johnson noise is Gaussian
in nature.
• Shot noise - bipolar transistors (caused by random variations in
the arrival of electrons or holes at the output electrodes of an
amplifying device)
• Transit time noise - occurs when the electron transit time across a
junction is the same period as the signal.
15
• Of these, only Johnson noise can be readily analyzed and compensated
16. Noise power
• The noise power is given by:
Pn = kTB
• Where:
• k = Boltzman's constant (1.38 x 10-23 J/K)
• T = temperature in degrees Kelvin
• B = bandwidth in Hz
• If the two signals are completely random with respect to each
other, such as Johnson noise sources, the total power is the
sum of all of the individual powers:
16
17. Noise power (Cont..)
•
A Johnson noise of power P = kTB, can be thought of as a noise
voltage applied through a resistor, Thevenin equivalent.
An example of such a noise source may be a cable or transmission line.
The amount of noise power transferred from the source to a load, such
as an amplifier input, is a function of the source and load impedances
17
18. Noise power (Cont..)
The rms noise voltage at maximum power transfer is:
Observe what happens if the noise resistance is resolved into two components:
18
19. Noise Figure
• The terms used to quantify noise
:
• Signal to noise ratio: It is either unit-less or specified in
dB. The S/N ratio may be specified anywhere within a system.
Noise Factor (or Noise Ratio):
(unit less)
19
20. Noise Figure (cont..)
• This parameter (i.e. Noise Figure ) is specified in all high
performance amplifiers and is measure of how much noise
the amplifier itself contributes to the total noise. In a
perfect amplifier or system, NF = 0 dB. This discussion
does not take into account any noise reduction techniques
such as filtering or dynamic emphasis.
20
21. Noise Figure (cont..)
• Friiss' Formula & Amplifier Cascades
• It is interesting to examine an amplifier cascade to see how
noise builds up in a large communication system.
Amplifier gain can be defined as:
Therefore the output signal power is:
21
22. Noise Figure (cont..)
and the noise factor (ratio) can be rewritten as:
The output noise power can now be written:
From this we observe that the input noise is increased by the noise ratio and
amplifier gain as it passes through the amplifier. A noiseless amplifier would
have a noise ratio (factor) of 1 or noise figure of 0 dB. In this case, the input
noise would only be amplified by the gain since the amplifier would not
contribute noise.
Friiss' Formula
22
23. Model Paper
Communication system CU-510 5th semester
Deptt. Of Electrical Engineering, CECOS Univ.
Total Time: 3 hrs
Q # 1 Time: 30 minutes
(1):Is frequency modulation a linear modulation?
A) Yes
B) No
(2): Does Dirichlet’s condition require a signal to be absolutely integrable?
A) Yes B) No
(3): suitable measure for this signal is -----------
23
