noise and snr calculation

8/10/2019 Noise and SNR calculation

1/43

CHAPTER 6NOISE


2/43

INTRODUCTION

Noise is random energy that interferewith the information signal.

Noise may be defined as any unwantedintroduction of energy tending to

interfere with the proper reception andreproduction of transmitted signal.

In radio receiver, noise may produce hissin the loudspeaker output.

Noise can limit the range of systems. It affects the sensitivity of the receiver.


3/43

NOISE

Electrical noise any undesirable that falls withinthe passband of the signal.

Figure 4 show the effect of noise on electricalnoise.

2 general categories

Correlated noise implies relationship betweenthe signal and the noise, exist only when signalis present.

Uncorrelated noise present at all time,whether there is signal or not.


4/43

Classification ofUncorrelated Noise

NOISE

EXTERNAL INTERNAL

ATMOSPHERIC

NOISE

EXTRATERRESTRIALNOISE

INDUSTRIAL

NOISE

THERMAL

NOISE

SHOT

NOISE

Figure 6.1


5/43

ATMOSPHERIC NOISE Caused by lightning discharges in thunderstorms and other

natural electric disturbances occurring in the atmosphere. Consist of spurious radio signal with components distributed

over a wide range of frequencies. It propagates over the earth in the same way as ordinary

radio waves of the same frequencies. Become less severe at frequencies above 30MHz because: The higher frequencies are limited to line-of-sight

propagation. Nature of the mechanism generating this noise is such

that very little of it is created in the VHF range and

above.


6/43

EXTRATERRESTRIAL NOISE

SOLAR NOISE: Normal condition, there is a constant noise radiation

from the sun, simply because large body at a very highfrequency.

Radiates over a very broad frequency spectrum.

COSMIC NOISE: Stars radiate RF noise in the same manner of sun. The noise received is called thermal noise and

distributed fairly uniformly over the entire sky.


7/43

INDUSTRIAL NOISE

Between 1 to 600 MHz, the intensity noisemade by humans easily outstrips thatcreated by any other source to the

receiver.Sources such as: automobile, aircraft,

electric motors and other heavy machine.

The nature of industrial noise is sovariable that it is difficult to analyze.


8/43

SHOT NOISE

Caused by the random arrival of carriersat the output element of an electronicdevice.

First observed in the anode current of a

vacuum-tube amplifier.The current carriers are not moving in

continuous steady flow.Randomly varying and superimposed onto

any signal present.Sometimes called transistor noise.


9/43


10/43

Thermal noise power is proportional to theproduct of bandwidth and temperature.

Mathematically, noise power is

N=KTB

N = noise power,K=Boltzmanns constant (1.38x10-23J/K)B = bandwidth,T = absolute temperature (Kelvin)(17oCor290K)


11/43

NOISEVOLTAGE

4N

V RkTB

VN/2

VN/2VN R

RI

Noise Source

Figure 4.2 shows the equivalentcircuit for a thermal noise source.

Internal resistanceRIin series

with the rms noise voltage VN

.

For the worst condition, the loadresistanceR = R

I, noise voltage

dropped acrossR =half the noise

source (VR

=VN/2) and

From the final equation The

noise powerPN , developed acrossthe load resistor = KTB

The mathematical expression :

RKTBV

RKTBV

RV

RVKTBN

N

N

NN

4

4

42/

2

22

Figure 6.2 : Noise source

equivalent circuit


12/43

Example 1

Convert the following temperaturesto kelvin:a) 100C

b) 0Cc) -10C

T=aC+273C


13/43

Example 2

Calculate the thermal noise poweravailable from any resistor at roomtemperature (290K) for a bandwidth of1 MHz. Calculate also thecorresponding noise voltage, given that

R = 50.


14/43

Example 3

For an electronic device operating ata temperature of 17oC with abandwidth of 10 kHz, determine

a)Thermal noise power in watts and dBmb)rms noise noise voltage for a 100

internal resistance and 100 load

resistance.


15/43

Example 4

Two resistor of 20k and 50 kareat room temperature (290K). For abandwidth of 100kHz, calculate thethermal noise voltagegenerated by

1.each resistor

2.the two resistor in series

3.the two resistor in parallel


16/43

Correlated Noise

Form of internal noise that is correlatedto the signal and cannot be present in acircuit unless there is a signal.

Produced by nonlinear amplification.All circuits are nonlinear therefore, theyall produce nonlinear distortion.

Nonlinear distortion creates unwanted

frequencies that interfere with the signaland degrade performance.


17/43

Intermodulation DistortionGeneration of unwanted sum and

difference frequencies produced when twoor more signals mix in a nonlinear device.

The sum and difference frequencies are

called cross products.Unwanted cross products can interfere

with the information signal.Cross products are produced when

harmonics as well as fundamentalfrequency mix in a nonlinear device.


18/43

Cont..

Cross products = mf1nf2.

F1 and f2 are fundamental frequency.

F1>f2

M and n are positive integer.


19/43

Correlated Noise-Intermodulation Distortion

f1 f2

V1 V2

f1 f2f1-f2 f1+f2

V1 V2

Vdifference Vsum

Input frequency spectrum Output frequency spectrum

Figure 6.4


20/43

Example 6

For a nonlinear amplifier with 2 inputfrequencies, 3kHz and 8kHz,determine:

a) First 3 harmonics present in theoutput for each input frequency.

b) Cross-product frequencies

produced for values of m and n of 1and 2.


21/43

InterferenceForm of external noise.Means to disturb or detract from.Electrical interference is when

information signals from one sourceproduce frequencies that fall outside theirallocated bandwidth and interfere withinformation signals form another source.

Most interference occur when harmonics

frequencies from one source fall into thepassband of a neighboring channel.


22/43

Review Notes

Gain

Attenuation Both has the ratio output to the input.

in

out

VV

V

input

outputA

Figure 6.5


23/43

Gain

Ratio output to the input.

Output has greater amplitude than theinput

Most amplifiers are power amplifier, thesame procedure can be used to calculatepower gain, Ap.

Ap= P

out/P

in

Figure 6.6


24/43

Attenuation

Refers to loss introducedby a circuit.

Output is less than input.

For cascade circuit, totalattenuation is,

AT=A1x A2x A3..

Voltage divider network

may introduce

attenuation.

in

out

V

VAnAttenuatio

Figure 4.7 Voltage divider

introduces attenuation


25/43

Attenuation can be offset byintroducing gain.

Figure 6.8 Total attenuation in cascaded network

Figure 6.9 Gain offsets theattenuation


26/43

Figure 6.10 Total gain is the product of the individual stage gains and attenuation


27/43

Example 7

What is the gain of an amplifier that

produces an output of 750 mV for 30 Vinput?

Example 8

The power output of an amplifier is 6 W. Thepower gain is 80. What is the input power?

Example 9Three cascade amplifier have power gains of5,2, and 17. The input power is 40 mW. What

is the output power?


28/43

Signal to Noise Ratio (SNR)

Ratio of the signal power level to thenoise power level.

Express in logarithmic function:

n

sP

PSNR

n

s

P

PdBSNR log10)(


29/43

Example 10

1 For an amplifier with an output signalpower of 10W and an output noise powerof 0.01W, determine the SNR.

2 For an amplifier with an output signalvoltage of 4V, an output noise voltage of0.005V and an input and outputresistance of 50, determine the SNR.

( ) d


30/43

Noise Factor (F) and NoiseFigure (NF)

Figures of merit used to indicate howmuch the SNR deteriorates as a signalpasses through a circuit.

Noise factor is simply a ratio of inputSNR to output SNR.

SNRoutput

SNRinputF


31/43

Cont..

NF is noise factor stated in dB.

Used to indicate the quality of a receiver.

SNRoutputSNRinputdBNF log10)(

FdBNF log10)(


32/43

Ideal Noiseless Amplifier

i

i

N

S

inpowerNoise

inpowerSignal

Ideal Noiseless AmplifierAp=power gain

i

i

ip

ip

N

S

NA

SA

inpowerSignal

outpowerSignal

Figure 6.11


33/43

Non ideal amplifier

i

i

N

S

inpowerNoise

inpowerSignal

Nonideal amplifierAp=power gain

Nd=internally generated noise

p

d

i

i

dip

ip

A

N

N

S

NNA

SA

inpowerSignal

outpowerSignal

Figure 6.12


34/43

Example 11For a nonlinear amplifier and the following

parameter, determine:a) Input SNR(dB)b) Output SNR(dB)c) Noise Factor and Noise Figure

Input signal power=2x10-10WInput Noise power=2x10-18W

Power gain=1,000,000Internal noise (Nd)=6x10-12W


35/43

Noise Figure of Cascaded

Amplifier

i

i

N

S

T

i

i

o

o NFN

S

N

S

Ap1NF1

Ap2NF2

Ap3NF3

Input Output

i

i

N

S

Figure 6.13


36/43

Cont..

Total noise factor is the accumulation ofthe individual noise factor.

Friisss formula is used to calculate thetotal noise factor of several cascadedamplifiers.

n

nT

AAA

F

AA

F

A

FFF

2121

3

1

21

111


37/43

Example 12

For 3 cascaded amplifier stages,each with noise figure of 3 dB andpower gain of 10 dB, determine the

total noise figure.

E l


38/43

Equivalent NoiseTemperature (Te)

Hypothetical value that cannot be directlymeasured.

To indicates the reduction in the SNR a signal

undergoes as it propagates through a receiver. The lower Te is the better quality of a receiver.

1 FTTeT

TF e1


39/43

Example 13

Determine:

a) Noise Figure for an equivalent noisetemperature of 75K.

b) Equivalent noise temperature for anoise figure of 6dB.


40/43

Example 14

A voltage divider shown in Figure 6.9hasvalues of R1= 10kand R2= 47k.

1. What is the attenuation?2.What amplifier gain would you need to

offset the loss for an overall gain of 1?


41/43

Example 15

An amplifier has gain of 45,000, which is toomuch for the amplification. With an input

voltage of 20 V, what attenuation factor isneeded to keep the output voltage fromexceeding 100mV?. Let A1= amplifier gain =45,000; A2= attenuation factor; AT= total

gain.


42/43

Example 16A RF sine wave generator whose output impedance is50 is connected to a 50 load using 50 coaxialcable. The generators output amplitude level is setto + 3 dBm. An rms voltmeter is used to measure the

effective voltage, and an oscilloscope is used todisplay the sine wave. Compute the following:

1. The rms voltage measure by the rms voltmeter2. The peak voltage, Vpof the sine wave that should be

displayed on the oscilloscope.3. The peak-to-peak voltage, Vp-pof the sine wave that

should be displayed on the oscilloscope


43/43

Example 17

The input signal to a telecommunicationsreceiver consists of 100W of signal power

and 1W of noise power. The receivercontributes an additional 80W of noise, ND,and has a power gain of 20 dB. Compute theinput SNR, the output SNR and the

receivers noise figure.

noise and snr calculation

Documents