Noise, Power, Voltage Concepts, Computations and Sample Pronlems

NOise calculations and Power
sample problems

1. NOISE
ONLINE LECTURE NOTES

2. TOPIC LEARNING OUTCOMES
Identify the basic groups of noise, analyze and solve problems on noise.
Relate results of noise calculations to the performance of communication systems.

3. DEFINITION OF NOISE
Any unwanted form of energy tending to interfere the proper and easy reception
and reproduction of signals.
It degrades the signal’s fidelity, completeness and clarity.

4. EXTERNAL NOISE
1.ATMOSPHERIC NOISE
Caused by lightning discharges in thunderstorms and other natural electric disturbances
occurring in the atmosphere.
Consists of spurious radio signals with components distributed over a wide range of
frequencies.
Atmospheric noise becomes less severe at frequencies above 30 MHz because:
a) the higher frequencies are limited to the line of sight propagation
b) the nature of the mechanism generating this noise is such that very little of it is created
in the VHF range and above.

5. EXTERNAL NOISE
2. Extraterrestrial Noise/Space Noise
Solar noise –caused by the constant noise radiation from the sun and covers a very broad
frequency spectrum.
Cosmic noise/Galactic noise –caused by distant stars which have very high temperatures.
Also called thermal black –body noise distributed fairly uniformly over the entire sky.
3. Industrial Noise
Caused by sources such as automobile and aircraft ignition, electric motors and switching
gears, leakage from high voltage power lines, heavy electric machines, fluorescent lights, etc.
Often called Radio Frequency Interference (RFI) or impulse noise because it comes primarily
from sudden on/off events.

6. INTERNAL NOISE
Noise created by any of the active or passive devices found in receivers.
1. Thermal Agitation Noise
Noise generated in a resistance or resistive component of any impedance.
Due to the rapid and random motion of the molecules, atoms and electrons of which
any such resistor is made up.
Also called white or Johnson noise.

7. INTERNAL NOISE
2. Shot Noise
caused by random motions in the arrival of electrons or holes at the output electrode of an
amplifying device and thus appears a randomly varying noise current superimposed on the
output.
3. Transit –time Noise
Caused when the time taken by an electron from emitter to collector at a transistor
becomes comparable to the period of the signal being amplified at frequencies in the upper
VHF range and beyond.
A high frequency noise which goes on increasing with frequency at a rate that approaches 6
dB/octave and this random noise then quickly predominates over other forms.

8. INTERNAL NOISE
4. Miscellaneous Noise
Flicker/Modulation Noise –Low frequency noise. It is proportional to emitter current and
junction temperature but inversely proportional to frequency. It may be ignored above 500
Hz.
Resistance Noise –This is due to the base, emitter, and collector internal resistances with
the base resistance making the largest contribution.
Noise Mixers –mixers are noisier than amplifiers using identical devices except at
microwave frequencies. This is caused by the following reasons:
a) conversion transconductance is much lower
b) image frequency rejection is inadequate

18. SAMPLE PROBLEMS
1.Two resistors, 5kΩand 20kΩ, are at 20
0
C. Calculate the thermal noise power and
voltage for a 10kHz BW. (K=1.38 x 10
-23
joules/Kelvin)
a)For each resistorb) for their series combination c) for their parallel
combination

20. 2.Thefirststageofatwo–stageamplifierhasavoltagegainof10,a600Ωinputresistor,a
1600Ωequivalentnoiseresistanceanda27kΩoutputresistor.Forthesecondstage,these
valuesare25,81kΩ,10kΩ,and1MΩrespectively.Calculatetheequivalentinputnoise
resistanceofthistwostageamplifier.

21. 3.Calculatethesignalvoltageandequivalentnoisevoltageappearingattheinput
terminalsofanoisyamplifierwithexternalsourceforaneffectivenoiseBWof10kHzand
atroomtemperature(290
0
K)whereR
NA=400Ω,R
1=600Ω,Rs=50Ω,andEs=1.0µV,R
2=
1KΩ,andA=10.

22. 4.Calculate the signal –to –noise power ratio of problem no. 3.

23. 5.AthreestageamplifiersystemhasaneffectivenoiseBWof314kHz,determinedbyan
LCtunedcircuitatitsinput,andoperatesat22
0
C.Thefirststagehasapowergainof14dB
andanNFof3dB.Thesecondandthirdstagesareidenticalwithpowergainsof20dBand
NF=8dB.Theoutputloadis300Ω.Theinputnoiseisgeneratedbya10kΩresistor.
Calculate:
a)Thenoisestageandpowerattheinputandtheoutputofthissystemassumingideal
noiselessamplifiers.
b)Theoverallnoisefigureforthesystem.
c)Theactualoutputnoisevoltageandpower.