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COMMUNICATION SYSTEM EEEB453 Chapter 4(II) NOISE ANALYSIS

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COMMUNICATION SYSTEM EEEB453 Chapter 4(II) NOISE ANALYSIS * Gain and Attenuation Most circuits in are used to manipulate signals to produce a desired relectronic ... – PowerPoint PPT presentation

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Title: COMMUNICATION SYSTEM EEEB453 Chapter 4(II) NOISE ANALYSIS

1
COMMUNICATION SYSTEM EEEB453Chapter 4(II)NOISE
ANALYSIS
2
Gain and Attenuation

Most circuits in are used to manipulate signals
to produce a desired relectronic communication
esult.
All signal processing circuits involve
Gain
Attenuation

3
Gain and Attenuation

Gain means amplification. It is the ratio of a
circuits output to its input.
The number obtained by dividing the output by the
input shows how much larger the output is than
the input.
A positive () dB value indicates that the output
power is greater than the input power which
indicates power gain.

4
Gain and Attenuation

Most amplifiers are also power amplifiers, so the
same procedure can be used to calculate power
gain AP where Pin is the power input and Pout is
the power output.
Power gain (Ap) Pout / Pin
An amplifier is cascaded when two or more stages
are connected together.
The overall gain is the product of the individual
circuit gains.

5
Gain and Attenuation

Example 1
The power output of an amplifier is 6 watts (W).
The power gain is 80. What is the input power?
b) Three cascaded amplifiers have power gains of
5, 2, and 17. The input power is 40 mW. What is
the output power?

6
Gain and Attenuation

Attenuation means a power loss.
Attenuation refers to a loss introduced by a
circuit or component. If the output signal is
lower in amplitude than the input, the circuit
has loss or attenuation.
The letter A is used to represent attenuation
Attenuation A output/input Vout/Vin
A negative (-) dB value indicates that the output
power is less than the input power, which
indicates power loss.
i.e circuits that introduce attenuation have a
gain that is less than 1.
With cascaded circuits, the total attenuation is
the product of the individual attenuations.

7
Gain and Attenuation

Example 2
Calculate the total attenuation
Calculate Vout

8
Decibels

The gain or loss (attenuation) of a circuit is
usually expressed in decibels (dB).
Total gain or attenuation is the algebraic sum of
the individual stage gains in decibels.

AT A1 A2 A3
9
Decibels

Example
Calculate the total gain.

A115dB
A2-20dB
A335dB
AT A1 A2 A3 AT 15 (-20) 35 30dB
10
Signal to Noise Power Ratio (S/N or SNR)

SNR is the ratio of the signal power level to the
noise power level.

SNRdB
SNR
where Vs signal voltage Rin input resistance
Vn noise voltage Rout output resistance

If Rin Rout, then

SNRdB

dBm is an abbreviation for the power ratio in
decibel (dB) of the measured power referenced to
one milliwatt (mW).
It should not be confused with dB, a
dimensionless unit, which is used when measuring
the ratio between two values, such as
signal-to-noise ratio.

Example 3 For an amplifier with an output
signal power of 10W and an output noise power of
0.01W, determine the signal-to-noise power ratio.
Example 4 For an amplifier with an output
signal voltage of 4V, an output noise voltage of
0.005V, and an input and output resistance of
50?, determine the signal-to-noise power ratio.

12
Noise Factor (F) and Noise Figure (NF)

F and NF are figures of merit used to indicate
how much the SNR weaken as a signal passes
through a cct or series of ccts.
Noise Factor, F
Noise Figure, NF parameter to indicate the
quality of a Rx. It is simply the noise factor, F
stated in dB.
NF 10log F dB
NF indicates how much the SNR deteriorates (drop)
as waveform propagates from input to the output
of a cct.
For eg. NF of 6dB means that the SNR at output is
6dB less than the SNR at the input.
If the cct. is noiseless, SNRoutput SNRinput ,
thus F 1 and NF 0dB.

13
Noise Factor (F) and Noise Figure (NF)

An electronic cct amplify signal and noise within
its passband equally well, ideally, the amplifier
is noiseless.
In reality, amplifier in not ideal. It produces
internally generated noise to the waveform, thus
reduce the overall SNR.
For ideal noiseless amplifier,
For non-ideal amplifier that generates an
internal noise, Nd,

The total power gain is simply the product of the
individual gains. (If in dB, sum up the
individual gains)
14

Example 5 For a non-ideal amplifier and the
following parameters,
Input signal power 2x10-10W
Power gain 1,000,000
Input noise power 2x10-18W
Internal noise(Nd) 6x10-12W
determine
Input S/N ratio (dB)
Output S/N ratio (dB)
Noise factor and noise figure

15
Noise Factor (F) and Noise Figure (NF)

When 2 or more amplifiers are cascaded, the total
noise factor is the accumulation of the
individual noise factor.
Friss formula is used to calculate it,
mathematically,
For Friss, NF must be converted to F i.e NFT (dB)
10log FT

16
Example 6 Determine the total noise figure for
a three-cascaded amplifier of 3dB noise figure
and 10dB power gains each.
17
Equivalent Noise Temperature (Te)

Since noise produced from thermal agitation ?
temperature, thermal noise can be expressed in
degree/ watts/ dBm.
From NKTB,then T Kelvin
Te cannot be directly measured. It is used rather
than noise factor in low-noise sophisticated
VHF,UHF, microwave and satellite radio Rx.
Te? F, indicates the reduction in SNR a signal
undergoes as it propagates thro a Rx.
The lower the Te, the better the quality of a Rx.
Mathematically, Te at input Rx
Te T(F-1)Kelvin
Where TO environmental temperature, ref
value of 290K
F noise factor (unitless)
Thus,

Example 7 Determine
Noise figure for an equivalent noise temperature
of 75K (use 290K for the ref temp).
Equivalent noise temperature for a noise figure
of 6dB

19
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