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# Noise

Submitted By qiela
Words 817
Pages 4
Telecommunication Engineering Principles

Introduction

Why need to understand noise?
 Limit

the performance of a device or system

Limit the sensitivity of measurements, and the transmission rate to lower the bit error rate

 Efficient

product development requires

quantification of noise from components
 calculation of noise effects on system performance

One of the main impact on system cost
Incompetent noise specifications can irritate customers

Relationship of Noise and Decibels

What is decibels and noise?
 Decibel

(dB)

Uses ratios and logarithms
Compress wide span of magnitude into smaller and easier-tomanipulate range of numbers
Uses to measure one signal against another signal of defined value Simplifies the measurement of relative signal gain and loss
Allows total gain through multiple stage to be calculated by simple addition

Relationship of Noise and Decibels
 Noise

Interfered or unwanted signal
 Measures in decibels in relation to the desired signal
 Produces its own noise in any circuit
 The overall system performance is calculated by adding the amount of noise from the circuit itself with the noise of the received signal

Decibels for Power and Voltage

Power and dB
 To

compare two power signal/level dB  10 log10

 Uses

P
Po

two power value instead of single absolute value such as watt
 Use base 10 logarithm, to compress wide range of signal values into a much smaller range

Decibels for Power and Voltage

Voltage and dB
 To

compare two voltage signal dB  20 log10

 Uses

V
Vo

the factor of 20 as the multiplier

Caution:
When using the dB scale with voltage and power, the voltage or current must be measured at the same point in a circuit

Decibels Calculation

Example:
What is the dB value for a signal at 10W compared to one at 0.5W?

10W dB  10 log10
0.5W
 13.01dB

Decibels Calculation

dB scale can be NEGATIVE value, when a signal is loss
Example:
A signal enters the circuit with a value of 5V and is attenuated to 0.1V. The input and output resistances are the same. What is the dB ratio that shows the gain in magnitude? Answer:
-33.98dB

Decibels Calculation

dB scale can also be ZERO, when the two signals have equal power or voltage values
P

 0dB
 10 log
Po
 dB  
V
20 log
 0dB

Vo

for power for voltage

Another case is when the power or voltage is doubled
P 

dB  10 log 2    10 log 2

P0 

 3.010

Decibels Calculation

3dB corresponds to a power factor of approximately to 2
a

power factor of one-half and another one-half is from second signal

Voltage ratio of 2 is very close to 6 dB
 V  dB  20 log 2    20 log 2
 V 
0 

 6.021

Decibels Calculation

Most of the case, dB is used by the communication system, but at some point, the exact value of power or voltage must be calculated
For power,

P  P0 10

dB value
10

P0 
10

For voltage,
V  V0 10

dB value
20

P dB value
10

V

V0 
10

dB value
20

Decibels Calculation

Examples (for power):
 What is the original power, when a signal has been amplified by 13dB at 1.2W?
1.2
P0  1.3  0.06W
10

If a 0.2W signal comes into a circuit with a gain of
22dB, what is the output power?
P  0.2 10 2.2  31.7W

Decibels Calculation

dB scale is called as dBm, when the common reference is
1mW for low power circuit
Means 0dB point reference is 1mW
Example:
 A signal of 200 mW expressed in dBm is
200mW
dB  10 log
 23dBm
1mW
 One ten-thousandth of a watt would be
0.0001W
dB  10 log
 10dBm
0.001W

Decibels Calculation

For high power circuit, 1W is commonly used as the reference value, dBW
Example:
 If an audio amplifier have an output of 7dBW, what is the input power?

P  1100.7  5.01W

Note that
0dBm  30dBW
30dBm  0dBW

Decibels Calculation

Similarly, the voltage ratio dB scale has also some standard value
The most common is 1V, denoted by dBV
Example:
 A signal of 8.2 V corresponds to
8.2
20  log
 18.3dBV
1

System Level Decibels Analysis

Most of practical communication systems consist of many stages
When a signal passes from one stages to another, it undergoes some changes in signal level and characteristics Example:

System Level Decibels Analysis
The output power at each stage are:
1. Pre-amplifier
P  3 10 20 /10  300 W

2. Main amplifier
a) If gain setting is 10dB

P  3 1030 /10  3000W
b) If gain setting is 20dB
P  3 10 40 /10  30000W

System Level Decibels Analysis
3. Antenna (based on the main amplifier)
(a) For +45dB
P  3 10 45 /10  94868W  94.8mW

(b) For +55dB
P  3 1055 /10  948683W  949mW

Thank You For Your
Attention

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