Chapter 1 - BENG 2413
Download
Report
Transcript Chapter 1 - BENG 2413
Chapter 1 : Introduction to Electronic Communications
System
Main purpose of an electronic communications system is to transfer
information from one place to another.
Electronic communications can be viewed as the transmission, reception
and processing of information between two or more locations using
electronic circuit/device.
In this chapter, we will cover
Communication models
Communication transmission modes
Power measurement in electronics communication
Electromagnetic frequency spectrum
Communication bandwidth
Information capacity
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
1
1.1 Basic Communication Model
Basic communication models shows the communication flows between 2
points.
Source – sender of the information
Sink – receiver that receive the information
Channel – transmission path/medium of the information between the source and sink
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
2
1.1 Basic Communication Model
Communication system model
Transmission channel – physical link between the communicating parties
Modulator – transform the source signal so that it is physically suitable for the
transmission channel
Transmitter – introduce the modulated signal into the channel (also act as amplifier)
Receiver – Detect the signal on the channel and amplify it (due to the attenuation)
Demodulator – Get the source signal (original) from the received signal and pass it
to the recipient
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
3
1.2 Communication / Transmission Mode
Communication system can be designed for transmitting information in one
or both direction. Generally, the mode of communication can be divided
into 3 types :
Simplex System : the system capable of sending information in one direction
only where only the sender can send the information and only the recipient can
receive the information. (e.g. TV & radio broadcasting)
Half-duplex System : the system capable to carry information in both direction,
but only one direction is allowed at a time. The sender transmits to the intended
receiver, and then reverse their roles. (e.g. walkie-talkie, 2-way intercom)
Full-duplex System : Information can be carried in both direction at the same
time. The 2 directions of information travel are independent of each other. (e.g.
ordinary/mobile phone systems, computer systems)
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
4
1.2 Communication Transmission/Mode
Half-duplex System vs Full-duplex System
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
5
1.3 Power Measurement (dB, dBm & Bel)
Magnitudes of communication signals span a very wide range causing a
drawbacks as follow :
Extremely large scale (graph/drawing)
Hard calculation (too big vs too small numbers)
Prone to errors (e.g. 0.0001 vs 0.00001)
Hard to compare the signals
As a solution, logarithmic scale is used !
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
6
1.3.1 Decibel (dB)
Used to measure the ratio between 2 values – value to be measured relative
to a reference value
In the electronic communication field, decibel is normally used to define
the power ratios between 2 signals
To express relative gain and lose of the electronic device/circuit
Describing relationship between signal and noise
In the common usage, it also used to express the ratios of voltage and
current
If 2 powers are expressed in the same units (e.g. watt, miliwatt), their
ratio is a dimensionless quantity that can be expressed in decibel form as
follow
P1
dB 10 log 10
P2
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
(1)
7
1.3.1 Decibel (dB)
Where
P1 : power level 1 (watts)
P2 : power level 2 (watts)
the dB value is for the power of P1 with respect to the reference power P2
the dB value shows the difference in dB between power P1 and P2
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
8
1.3.1 Decibel (dB)
In the case to measure the power gain or loss of any electronic circuit or
device, equation (1) can be written as follow
Pout
Ap ( dB ) 10 log 10
Pin
where
(2)
Ap(dB) : power gain (unit in dB) of Pout with respect to Pin
Pout : output power level (watts)
Pin
: input power level (watts)
Pout/Pin : absolute power gain (unitless)
Positive (+) dB value indicates the output power is greater than the input
power, which indicates power gain or amplification
Negative (-) dB value indicates the output power is less that the input power
which indicates power loss or attenuation
If Pout = Pin, the absolute power gain is 1, which means dB power gain is 0
(referred as unity power gain)
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
9
1.3.1 dB
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
10
1.3.1 dB
Ex 1 : Convert the absolute power ratio of 200 to a power gain in dB.
Ex 2 : Convert a power gain Ap = 30 dB to an absolute power ratio.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
11
1.3.1 dB
Ex 3 : Expressing power gain in term of voltage ratio
From
PV 2
(3)
Substituting (3) into (2),
i.e.
Voltage Gain
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Vout 2
dB 10 log 10
2
Vin
Vout
Av ( dB) 20 log 10
Vin
Chapter 1 : Introduction to Communications System
(3-1)
(3-2)
12
1.3.2 dBm
A dBm is a unit of measurement used to indicate the ratio of power level
with respect to a fixed reference level. With dBm, the reference level is 1
mW (miliwatts).
dBm unit can be expressed as follow
P
dBm 10 log 10
0.001
Ex 4 : Convert a power level of 200 mW to dBm
Ex 5 : Convert a power level of 30 dBm to an absolute power
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
(4)
13
1.3.2 dBm
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
14
1.3.3 Bel
A Bel is one-tenth of a decibel
Pout
Bel log 10
Pin
(5)
The Decibel unit was originated from the Bel unit, in honor of Alexander
Graham Bell.
Bel unit compressed absolute ratios of 0.00000001 to 100000000 to a
ridiculously low range of only 16 Bel (-8 Bel to + 8 Bel).
Difficult to relate Bel unit to true magnitudes of large ratios and impossible
to express small differences with any accuracy.
To overcome this, Bel was simply multiplied by 10, creating a decibel.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
15
1.3.4 Power levels, Gains and Losses
When power levels are given in watts and power gains are given as
absolute values, the output power is determined by multiplying the input
power with the power gains.
Ex 6 : Given a 3 stages system comprised of two amplifiers and filter. The
input power Pin = 0.1 mW. The absolute power gains are AP1 = 100, AP2 =
40 and AP3 = 0.25. Determine
a) the input power in dBm
b) output power (Pout) in watts and dBm
c) the dB gain of each of the 3 stages
d) the overall gain in dB
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
16
1.3.4 Power levels, Gains and Losses
Ex 7 : For a 3-stages system with an input power Pin = -20 dBm and the
power gains of the 3-stages as AP1 = 13 dB, AP2 = 16 dB and AP3 = -6 dB,
determine the output power (Pout) in dBm and watts.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
17
1.4 Electromagnetic Frequency Spectrum
Communicating the information between two or more location is done by
converting the original information into electromagnetic energy and then
transmitting it to the receiver where it is converted back to its original for
The electromagnetic energy is distributed throughout infinite range of
frequencies
The total electromagnetic frequency spectrum with the approximate
locations of various services is shown below.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
18
1.4 Electromagnetic Frequency Spectrum
The spectrum is divided into bands, with each band having a different
name and boundary.
The radio frequency band (30Hz ~300GHz) is divided into narrower band
as follow.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
19
1.4 Electromagnetic Frequency Spectrum
Wavelength : is the length that one cycle of electromagnetic wave occupies
in space. It is inversely proportional to the frequency of the wave and
directly proportional to the velocity of propagation.
Wavelength can be defined as follow,
c
(6)
f
where λ= wavelength (m), c = velocity of light (3 x 108 m/s),
f = frequency (Hz)
Total electromagnetic wavelength spectrum is shown below.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
20
1.4 Electromagnetic Frequency Spectrum
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
21
1.4 Electromagnetic Frequency Spectrum
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
22
1.4 Electromagnetic Frequency Spectrum
Ex. 8 : Determine the wavelength in meters for the following frequencies:
1 kHz, 100 kHz and 10 MHz
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
23
1.5 Bandwidth
Bandwidth of an information signal is the difference between the highest
and the lowest frequency contained in that signal.
Bandwidth of a communication channel is a difference between the highest
and the lowest frequency that the channel will allow to pass through it.
Bandwidth of a communication channel must be equal or greater than the
bandwidth of the information.
Ex : voice signals contain frequencies between 300 Hz ~ 3000 Hz. For that
a voice signal communication channel must have a bandwidth of 2700 Hz
or greater.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
24
1.6 Information Capacity
Information capacity is a measure of how much information can be
propagated through a communication system.
It can be expressed in the function of bandwidth and transmission time.
It represents the number of independent symbols that can be carried
through a system in a given unit of time
Based on Hartley’s Law,
(7)
IB t
where I = information capacity (bits per second)
B = bandwidth (Hz)
t = transmission time (seconds)
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
25
1.6 Information Capacity
In 1948, Claude E. Shannon published what is called as Shannon limit for
information capacity defined as follow
Based on this law, the information capacity of any communication channel
is related to its bandwidth and the signal-to-noise ratio.
The higher the signal-to-noise ratio, the better the performance and the
higher the information capacity is.
Mathematically, it is defined as,
or
S
I B log 21
N
S
I 3.32 B log 101
N
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
(8)
(9)
26
1.6 Information Capacity
where I = information capacity (bits per second)
B = bandwidth (Hz)
S/N = signal to noise power ratio (unitless)
Ex 9 : For a standard telephone circuit with a signal-to-noise ratio of 1000
(30 dB) and a bandwidth of 2.7 kHz, determine the Shannon limit for
information capacity.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 1 : Introduction to Communications System
27
1.7 Noise – Representation, types & source
Definition – any undesirable electrical energy that falls within the passband of
the signal.
Effect of noise on the electrical signal :
2 general categories of noise :
Correlated noise – noise that exists only when a signal is present.
Uncorrelated noise – noise that presents all the time whether there is a signal or not
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
28
1.7.1 Uncorrelated noise
2 general categories of uncorrelated noise :
1. External noise – noise that generated outside the device or circuit.
Atmospheric noise
-
-
Extraterrestrial noise
-
-
naturally occurring electrical disturbances that originate within earth’s atmosphere such as
lightning.
also known as static electricity.
consists of electrical signal that originate from outside earth’s atmosphere and therefore also
known as deep-space noise.
2 categories of extraterrestrial noise.
i – solar noise – noise that generated directly from the sun’s heat.
ii – cosmic noise / black-body noise – noise that is distributed throughout the galaxies.
Man-made noise
- noise that is produced by mankind.
- source : spark-producing mechanism (commutators in electrical motors, automobile ignition
systems, ac power generating/switching equipment, fluorescent lights).
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
29
1.7.1 Uncorrelated noise
2 general categories of uncorrelated noise :
2. Internal noise – noise that generated within the device or circuit.
Shot noise
-
-
Transit-time noise
-
-
caused by the random arrival of carriers (holes and electrons) at the output element of an
electronic device.
shot noise is randomly varying and is superimposed onto any signal present.
irregular, random variation due to any modification to a stream of carriers as they pass from
the input to the output of a device.
this noise become noticeable when the time delay takes for a carrier to propagate through a
device is excessive.
thermal / random noise
- noise that is produced by mankind.
- source : spark-producing mechanism (commutators in electrical motors, automobile ignition
systems, ac power generating/switching equipment, fluorescent lights).
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
30
1.7.1 Uncorrelated noise
2 general categories of uncorrelated noise :
2. Internal noise – noise that generated within the device or circuit.
Thermal / random noise
-
-
-
associated with the rapid and random movement of electrons within a conductor due to
thermal agitation.
also known as Brownian noise, Johnson noise and white noise.
uniformly distributed across the entire electromagnetic spectrum.
a form of additive noise, meaning that it cannot be eliminated, and it increase in intensity
with the number of devices and with circuit length.
the most significant of all noise sources
thermal noise power can be defined as follow :
N KTB
(6.1)
where N : noise power (watts)
B : bandwidth (Hertz)
T : absolute temperature (kelvin) .......... T = ºC + 273º
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
31
1.7.1 Uncorrelated noise
Thermal / random noise
-
equivalent circuit for a thermal noise source when the internal resistance of the source R 1 is
in series with the rms noise voltage VN
-
for a worst case and maximum transfer of noise power, the load resistance R is made equal
to the internal resistance. Thus the noise power developed across the load resistor :
2
VN / 2
N KTB
R
VN 2
4R
(6.2)
thus rms noise voltage can be define as
VN 4RKTB
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
(6.3)
32
1.7.2 Correlated noise
a form of internal noise that is correlated to the signal and cannot be present in a
circuit unless there is a signal.
produced by a nonlinear amplification resulting in nonlinear distortion.
there are 2 types of nonlinear distortion that create unwanted frequencies that
interfere with the signal and degrade the performance :
1. Harmonic distortion
occurs when unwanted harmonics of a signal are produced through nonlinear
amplification.
harmonics are integer multiples of the original signal. The original signal is the
first harmonic (fundamental harmonic), a frequency two times the fundamental
frequency is the second harmonic, three times is the third harmonic and so on.
Distortion measurements :
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
33
1.7.2 Correlated noise
1. Harmonic distortion
distortion measurements :
- Nth harmonic distortion = ratio of the rms amplitude of Nth harmonic to the rms amplitude
of the fundamental.
- Total Harmonic Distortion (THD)
%THD
vhigher
vfundamental
100
(6.4)
where
vhigher v2 2 v33 v4 4 .... vn 2
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
all in rms value.
34
1.7.2 Correlated noise
2. Intermodulation distortion
intermodulation distortion is the generation of unwanted sum and difference
frequencies produced when two or more signals mix in a nonlinear device (cross
products).
unwanted !
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
35
1.7.3 Other type of noise
1. Impulse noise
characterized by high amplitude peaks of short duration (sudden burst of irregularly
shaped pulses) in the total noise spectrum.
common source of impulse noise : transient produced from electromechanical
switches (relays and solenoids), electric motors, appliances, electric lights, power
lines, poor-quality solder joints and lightning.
2. Interference
electrical interference occurs when information signals from one source produces
frequencies that fall outside their allocated bandwidth and interfere with information
signal from another source.
most occurs in the radio frequency spectrum.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
36
1.8 Noise Parameters
1.8.1 Signal-to-noise Power Ratio
signal-to-noise power ratio (S/N) is the ratio of the signal power level to the
noise power level and can be expressed as
S
Ps
N
Pn
in logarithmic function
S
Ps
(dB) 10 log
N
Pn
(6.5)
(6.6)
in terms of voltages and resistance
Vs 2 / Rin
S
(dB) 10 log 2
N
Vn / Rout
(6.7)
in the case Rin = Rout, (6.7) can be reduced to
BENG 2413 Communication Principles
Faculty of Electrical Engineering
S
Vs
(dB) 20 log
N
Vn
Chapter 5 : Digital Communication System
(6.8)
37
1.8.2 Noise Factor and Noise Figure
Noise factor is the ratio of input signal-to-noise ratio to output signal-to-noise
ratio
( S / N )in
F
(6.9)
( S / N )out
Noise figure is the noise factor stated in dB and is a parameter to indicate the
quality of a receiver
( S / N )in
NF 10 log F 10 log
( S / N )out
(6.10)
Noise Figure in Ideal and Non-ideal Amplifiers
- an electronic circuit amplifies signal and noise within its passband equally well
- in the case of ideal/noiseless amplifier, the input signal and the noise are
amplified equally.
- meaning that, signal-to-noise ratio at input = signal-to-noise ratio at output
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
38
1.8.2 Noise Factor and Noise Figure
Noise Figure in Ideal and Non-ideal Amplifiers (continue)
- in reality, amplifiers are not ideal, adds internally generated noise to the
waveform, reducing the overall signal-to-noise ratio.
- in figure (a), the input and output S/N ratios are equal.
- in figure (b), the circuits add internally generated noise Nd to the waveform,
causing the output signal-to-noise ratio to be less than the input signal-to-noise
ratio.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
39
1.8.2 Noise Factor and Noise Figure
Noise Figure in Ideal and Non-ideal Amplifiers (continue)
- in figure (b), the circuits add internally generated noise Nd to the waveform,
causing the output signal-to-noise ratio to be less than the input signal-to-noise
ratio.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
40
1.8.2 Noise Factor and Noise Figure
Noise Figure in Cascaded Amplifier
- when two or more amplifiers are cascaded as shown in the following figure,
the total noise factor is the accumulation of the individual noise factors.
- Friss’ formula is used to calculate the total noise factor of several cascade
amplifiers
F 2 1 F 3 1
FN 1
(6.11)
FT F 1
...
A1
BENG 2413 Communication Principles
Faculty of Electrical Engineering
A1 A2
A1 A2... AN
Chapter 5 : Digital Communication System
41
1.8.2 Noise Factor and Noise Figure
Noise Figure in Cascaded Amplifier (continue)
- the Total Noise Figure
NFT 10 log FT
(6.12)
When using Friss’ formula, the noise figures must
be converted to noise factors !!!
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
42
1.9 Examples
Ex 1 : Convert the following temperatures to Kelvin : 100º C, 0º C and -10º C.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
43
1.9 Examples
Ex 2 : For and electronic device operating at a temperature of 17º C, with a
bandwidth of 10 kHz, determine
a. thermal noise power in watts and dBm.
b. rms noise voltage for a 100 Ω load resisstance.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
44
1.9 Examples
Ex 3 : For an amplifier with an output signal power of 10 W and output noise
power of 0.01 W, determine the signal-to-noise power ratio.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
45
1.9 Examples
Ex 4 : For an amplifier with an output signal voltage of 4V, an output noise
voltage 0.005 V and an input and output resistance of 50 , determine the signalto-noise power ratio.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
46
1.9 Examples
Ex 5 : For a non-ideal amplifier with a following parameters, determine
a. input S/N ratio (dB)
b. output S/N ratio (dB)
c. noise factor and noise figure
Input signal power = 2 x 10-10 W
Input noise power = 2 x 10-18 W
Power gain = 1000000
Internal noise Nd = 6 x 10-12 W
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
47
1.9 Examples
Ex 6 : For 3 cascaded amplifier stages, each with a noise figures of 3 dB and
power gain of 10dB, determine the total noise factor and noise figure.
BENG 2413 Communication Principles
Faculty of Electrical Engineering
Chapter 5 : Digital Communication System
48