chapter 1 : introduction to communications system beng 2413 communication principles faculty of...

Chapter 1 : Introduction to Communications System

BENG 2413 Communication Principles Faculty of Electrical Engineering 1

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



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



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



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)



1.2 Communication Transmission/Mode Half-duplex System vs Full-duplex System



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 !



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

(1)

(2)

2

110log10

P

PdB



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



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

(2)

where 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)

in

outdBp

P

PA 10)( log10



1.3.1 dB



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.



1.3.1 dB Ex 3 : Expressing power gain in term of voltage ratio

From

(3)

Substituting (3) into (2),

i.e. (3-1)

Voltage Gain

(3-2)

2in

2out

10V

Vlog10dB

2VP

in

outdBv

V

VA 10)( log20



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

(4)

Ex 4 : Convert a power level of 200 mW to dBm

Ex 5 : Convert a power level of 30 dBm to an absolute power

001.0log10 10

PdBm



1.3.2 dBm



1.3.3 Bel

A Bel is one-tenth of a decibel

(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.

in

out

P

PBel 10log



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 dBmc) the dB gain of each of the 3 stagesd) the overall gain in dB



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.



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.



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.



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,

(6)

where λ= wavelength (m), c = velocity of light (3 x 108 m/s),

f = frequency (Hz) Total electromagnetic wavelength spectrum is shown below.

f

c



1.4 Electromagnetic Frequency Spectrum



1.4 Electromagnetic Frequency Spectrum Ex. 8 : Determine the wavelength in meters for the following frequencies:

1 kHz, 100 kHz and 10 MHz



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.



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)

where I = information capacity (bits per second)

B = bandwidth (Hz)

t = transmission time (seconds)

tBI



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,

(8)

or

(9)

N

SBI 1log 2

N

SBI 1log32.3 10



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.

Chapter 5 : Digital Communication System


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



1.7.1 Uncorrelated noise 2 general categories of uncorrelated noise :

1. External noise – noise that generated outside the device or circuit.

Atmospheric noise- naturally occurring electrical disturbances that originate within earth’s atmosphere such as

lightning.- also known as static electricity.

Extraterrestrial noise- 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).




2. Internal noise – noise that generated within the device or circuit.

Shot 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.

Transit-time noise- 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).




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 :

(6.1)

where N : noise power (watts) B : bandwidth (Hertz) T : absolute temperature (kelvin) .......... T = ºC + 273º

KTBN



1.7.1 Uncorrelated noise Thermal / random noise

- equivalent circuit for a thermal noise source when the internal resistance of the source R1 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 :

(6.2)

thus rms noise voltage can be define as

(6.3)

R

V

R

VKTBN

NN

4

2/ 22

RKTBVN 4



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 :




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)

(6.4)

where

all in rms value.

100% lfundamenta

higher

v

vTHD

244

33

22 .... nhigher vvvvv




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 !



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.



1.8 Noise Parameters1.8.1 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

(6.5)

in logarithmic function

(6.6)

in terms of voltages and resistance

(6.7)

in the case Rin = Rout, (6.7) can be reduced to

(6.8)

n

s

P

P

N

S

n

s

P

PdB

N

Slog10)(

outn

ins

RV

RVdB

N

S

/

/log10)(

2

2

n

s

V

VdB

N

Slog20)(



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

(6.9)

Noise figure is the noise factor stated in dB and is a parameter to indicate the quality of a receiver

(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

out

in

NS

NSF

)/(

)/(

out

in

NS

NSFNF

)/(

)/(log10log10



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.



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.



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

(6.11)N

NT

AAA

F

AA

F

A

FFF

...

1...

11

2121

3

1

21



1.8.2 Noise Factor and Noise Figure Noise Figure in Cascaded Amplifier (continue)

- the Total Noise Figure(6.12)

When using Friss’ formula, the noise figures must be converted to noise factors !!!

TT FNF log10



1.9 Examples Ex 1 : Convert the following temperatures to Kelvin : 100º C, 0º C and -10º C.



1.9 Examples Ex 2 : For and electronic device operating at a temperature of 17º C, with a

bandwidth of 10 kHz, determinea. thermal noise power in watts and dBm.b. rms noise voltage for a 100 Ω load resisstance.



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.



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 signal-to-noise power ratio.



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



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.

chapter 1 : introduction to communications system beng 2413 communication principles faculty of...

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