Communication Systems

COMMUNICATION SYSTEMS
Barriers to Human Communication
,XLanguage
,XDistance
Electronic Communications
The transmission, reception, and processing of information using electronic circuits.


History
mid-19th century V James Clark Maxwell studied electromagnetic wave and predicted that it can be propagated through free space.

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1837 V Samuel Morse invented the telegraph.

1876 V Alexander Graham Bell and Thomas A. Watson transmitted human conversation over a functional telephone system.

1888 V Heinrich Hertz radiated electromagnetic energy from a machine he called oscillator.

1894 V Guglielmo Marconi was the first to accomplish wireless transmission.

1908 V Lee DeForest invented the triode vacuum tube.

1933 V Major Howard Armstrong invented frequency modulation.

1948 V William Shockley, Walter Brattain and John Bardeen invented the transistor.


Electromagnetic Spectrum
Electromagnetic Spectrum
The entire range of frequency.


Frequency
The number of times an alternating current goes through its complete cycle per second is known as its frequency. The international unit of measurement of frequency is hertz, abbreviated Hz. The English unit is cycles per second, abbreviated cps (1 Hz = 1 cps). It is rarely used.


To simplify terminology, 1000 Hz is called kilohertz, abbreviated kHz, and 1000000 Hz is called a megahertz, abbreviated MHz.


The vibration rate of sound waves in air may also use the term frequency. When middle C is played on a musical instrument, for example, an air disturbance with a frequency of 262 Hz is set up. The lowest tone that can be heard by human beings is about 15 Hz. The highest audible, audio, or sonic tones are usually 12 to 22 kHz (22 kHz for youngsters, 12 kHz for seniors). A microphone is a device or transducer that can change sound waves in air to an equivalent-frequency ac in wires.


Frequencies that produce sound waves audible to humans are said to be audio frequencies (AF). Frequencies that can be fed to antennas and will radiate electromagnetic and electrostatic waves in space are considered to be radio frequencies (RF).
FCC Band Designations
Frequency RangeDesignations
30 V 300 HzELF (Extremely Low Frequencies)
0.3 V 3 kHzVF (Voice Frequencies)
3 V 30 kHzVLF (Very Low Frequencies)
30 V 300 kHzLF (Low Frequencies)
0.3 V 3 MHzMF (Medium Frequencies)
3 V 30 MHzHF (High Frequencies)
30 V 300 MHzVHF (Very High Frequencies)
0.3 V 3 GHzUHF (Ultra High Frequencies)
3 V 30 GHzSHF (Super High Frequencies)
30 V 300 GHzEHF (Extremely High Frequencies)
0.3 V 3 THzInfrared
3 V 30 THzInfrared
30 V 300 THzInfrared
0.3 V 3 PHzVisible Light
3 V 30 PHzUltraviolet
30 V 300 PHzX-rays
0.3 V 3 EHzGamma rays
3 V 30 EHzCosmic rays
,’ FCC stands for Federal Communications Commission
Wavelength, U
The length that one cycle of an electromagnetic wave occupies in space, also, the distance between similar points in a repetitive wave.


wherek= velocity factor (equal to 1 in free space)
c= velocity of light in free space
= 299793000 m/s ,l 3 ,e 108 m/s
f= frequency
@ high frequencies, wavelength is too short and is usually expressed as Angstrom.


Bandwidth and Information Capacity
Limitations of Communication Systems
,Xnoise
,Xbandwidth
Bandwidth, BWunit:Hz (Hertz)
A portion of electromagnetic spectrum occupied by a system.

Minimum range of frequencies required propagating the source information through the system.

must be sufficiently large (wide) to pass all significant information frequencies.

difference between the upper and lower limit frequencies.


Information Capacity
A measure of how much source information can be carried through the system in a given period of time.


Hartleys Law
Information Capacity N Bandwidth ,e time
Signal Analysis
Factors Affecting the Signal
,XDistortion V signal alteration due to imperfect response of the system to the desired signal.

,XInterference V contamination of extraneous signal usually man-made to a form similar to the desired signal.

,XNoise V random, undesirable high frequency spikes.

,XAttenuation V decrease in signal level.


Types of Signals
,XBaseband Signal
,XModulated Signal
Types of Intelligence Transmitted
,XAnalog
,XDigital
Advantages of Digital Over Analog
,Xeasier to multiplex
,Ximproved noise immunity
,Xbetter performance
,Xeasier to interface
Disadvantages of Digital Over Analog
,Xneed for large (wide) bandwidth
,Xneed for synchronization
,Xneed for additional equipment
,Xrestriction to wired topology
Kinds of Signal Representations
,XTime-Domain V amplitude vs. time
,XFrequency-Domain V amplitude vs. frequency
Classifications of Signals:
According to Period
,XPeriodic V signals that keep on repeating at a regular interval.

,XNon-periodic V signals that keep on changing continuously and has no definite period.


According to Form of Electrical Signal
,XSinusoidal V has the form
,XNon-sinusoidal V sum of series of sinusoidal
According to Symmetry
,XEven Symmetry V a periodic waveform that is symmetrical about the vertical (amplitude) axis
,XOdd Symmetry V a periodic waveform that is symmetrical about a line midway between the vertical and the negative horizontal axes.


,XHalf-wave Symmetry V a periodic waveform is such that for the first half cycle (t=0 to t=T/2) repeats itself except with the opposite sign for the second half cycle (t=T/2 to t=T).


Communication System
The totality of the mechanism that provides transfer of information from one point to another. Includes the components, equipment that is being utilized to execute the communication process.


Basic Block Diagram of a Communication System
Transmission Modes
Simplex (SX)
,Xone-way-only, receive-only, transmit-only
Half-duplex (HDX)
,Xtwo-way-alternate, either-way, over-and-out system
Full-duplex (FDX)
,Xtwo-way simultaneous, both-way lines
Full/full Duplex (F/FDX)
,Xtransmit and receive simultaneously but not necessarily between the same two locations.


Echoplex
,Xthe transmitted information will be sent back in a different form.


Transmitters
A transmitter is a collection of electronic components and circuits designed to convert the information into a form suitable for transmission.


Basic Block Diagram of a Transmitter
Components of a Transmitter
Modulator V accomplishes modulation
Oscillator V produces high frequency carrier
Antenna V radiates the signal to the medium
Transmitter Requirements
Frequency Accuracy and Stability
The accuracy and stability of the transmitter frequency are essentially fixed by the carrier oscillator. The exact requirements vary with the use to which the transmitter is put and are set by government regulatory bodies. Depending on the application, frequency accuracy and stability are specified in Hz or as a percentage of the operating frequency.
Frequency Agility
Frequency agility refers to the ability to change operating frequency rapidly without extensive retuning.


Spectral Purity
All transmitters produce spurious signals, they emit signals at frequencies other than those of the carrier and the sidebands required for the modulation.


Power Output
There are a number of ways to measure power, depending on the modulation scheme.


Efficiency
Transmitter efficiency is important for two reasons. One is energy conservation and heat dissipation.


Modulation Fidelity
A transmitter should be capable of modulating baseband frequency onto a carrier at any modulation level, to preserve the information signal as much as possible
FCC Emission Designations
First SymbolSecond SymbolThird Symbol
A – Amplitude Modulation, Double Sideband Full Carrier
B – Independent Sideband
C – Vestigial Sideband
F – Frequency Modulation
G – Phase Modulation
H – Single Sideband Full Carrier
J – Single Sideband Suppressed Carrier
K – Pulse Amplitude Modulation
L – Pulse Width (Duration) Modulation
M – Pulse Position Modulation
N – Unmodulated Carrier
P – Unmodulated Pulses
R – Single Sideband Reduced Carrier0 – Absence of Any Modulation
1 – Digitally Keyed Carrier
2 – Digitally Keyed Tone
3 – Analog (voice, music)
7 – Multiple Digital Channel
8 – Multiple Analog Channel
9 – Channels with analog and digitalA – Telegraphy (manual)
B – Telegraphy (automatic)
C – Facsimile
D – Telemetry (Data)
E – Telephony (Sound
Broadcasting)
F – Television (video signal)
N – No Information
W – Combination of above
First Symbol (letter) V type of modulation of the main carrier
Second Symbol (number) V nature of modulation
Third Symbol (letter) V type of information being transmitted
Channel
It is the medium by which the electronic signal is sent from one point to another.


Unguided Media
,Xor wireless communication, transport electromagnetic waves without using a physical conductor.


Kinds of Wave Propagation
Ground Wave
Sky Wave
Space Wave
Guided Media
,Xthose that provide a conduit from one device to another.


Copper (Twisted-pair Cable, Coaxial Cable, Parallel Line)
Fiber Optic Cable
Waveguide
Receivers
Receivers are collection of electronic components and circuits that accepts the transmitted message back into a form understandable by human.


Basic Block Diagram of a Receiver
Components of a Receiver
Antenna V picks up the signal from free space
Oscillator V produces high frequency carrier
Demodulator V extracts the information from the modulated signal
Noise
Any unwanted form of energy tending to interfere with the proper and easy reception and reproduction of wanted signals.

Any undesired voltage or current that ultimately ends up appearing in the receiver output.

Results of Noise
,Xhiss/static
,Xsnow/confetti
,Xbit errors
,Xsignal loss
Kinds of Noise
Correlated Noise V mutually related to the signal and cannot be present in a circuit unless there is an input signal and is produced by nonlinear amplification. No signal, no noise!
Harmonic Distortion V unwanted harmonics of a signal are produced.

Intermodulation Distortion V the generation of unwanted sum and difference frequencies (cross products) when two or more signals are amplified in a nonlinear device.

Uncorrelated Noise V present regardless of whether there is a signal present or not.


Kind of Uncorrelated Noise
External Noise V generated outside the device or circuit.

Atmospheric Noise V naturally occurring electrical disturbances that originate within the earths atmosphere.

Extraterrestrial Noise V consists of electrical signals that originate from outside Earths atmosphere.

Solar Noise V directly from the suns heat.

Cosmic Noise V from the stars.

Man-made Noise V produced by manufactured equipment, such as automotive ignition systems, electric motors and generators.

Internal Noise V generated within a device or circuit.
Shot Noise V caused by the random arrival of carriers (holes and electrons) at the output element of an electronic device.

Transit-Time Noise V shows up as a kind of random noise within the device and is directly proportional to the frequency of operation.

Thermal Noise V associated with the rapid and random movement of electrons within a conductor due to thermal agitation.


Noise Computations
Noise Power
the average noise power is proportional to the absolute temperature of the conductor and to the bandwidth or spectrum of the thermal noise.


wherePN= noise power (W)
T= Temperature of the conductor (K)
B= bandwidth of the noise spectrum (Hz)
k= Boltzmanns Constant =
Noise Voltage
whereVN= rms noise voltage
T= Temperature of the conductor (K)
B= bandwidth of the noise spectrum (Hz)
k= Boltzmanns Constant =
R= equivalent resistance generating the noise
for combinations of resistances
Series
Parallel
Power Spectrum Density or Noise Density
,Xaverage noise power per Hertz of bandwidth
,Xa figure that determines the amount of noise contained in a specified bandwidth.


Signal-to-Noise Ratio
,Xa relative measure of the desired signal power to the noise power.


In decibel form:
wherePS= signal power
PN= noise power
VS= signal voltage
VN= noise voltage
Noise Factor
whereSi= input signal power
Ni= input noise power
So= output signal power
No= output noise power
Noise Figure
for ideal noiseless network
for a network that contributes noise
Reactance Noise Effects
,Xthe significant effect of reactive circuits on noise is their limitation on frequency response.

,Xthe equivalent bandwidth to be used in noise calculations with reactive circuits is
whereB3dB= half power bandwidth
Equivalent Noise Temperature
whereTeq= equivalent noise temperature
To= reference absolute temperature = 290 K
F= noise factor
Noise Due to Amplifiers in Cascade
Friiss Formula
Over-all noise factor of n stages
Over-all noise temperature of n stages
Over-all noise resistance
whereA= voltage gain (ratio)
G= power gain (ratio)
Shot Noise
,Xa form of internal noise, which is due to the random variations in current flow in active devices such as tubes, transistors and diodes.


WhereiN= rms noise current
q= charge of an electron =
B= bandwidth over which the noise is observed
I= dc bias current in the device
Problems:
1.What is the shot noise current for a diode with a forward bias of 1.15 mA over a 50-kHz bandwidth?
2.An amplifier operating over the frequency range of 455 kHz to 460 kHz has a 200 kC input resistance. What is the rms noise voltage at the input to the amplifier if the ambient temperature is 17XC?
3.Two resistors, 5 kC and 20 kC are at 27XC. Calculate the thermal noise power and voltage for a 10 kHz bandwidth. a) for each resistor, b) for their series combination, and c) for their parallel combination.

4.Three matched amplifiers are available to amplify a low level signal, they have the following characteristics.

AmplifierPower GainNoise Factor
A6 dB1.5
B12 dB2
C20 dB4
The amplifiers are to be connected in cascade. Calculate the lowest overall noise factor obtainable noting the order in which the amplifiers must be connected.