COMMUNICATION SYSTEMS Barriers to Human Communication”X Language”X Distance Electronic Communications The transmission, reception, and processing of information using electronic circuits. History mid-19 th century! V James Clark Maxwell studied electromagnetic wave and predicted that it can be propagated through free space. 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 k Hz, 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 k Hz (22 k Hz for youngsters, 12 k Hz 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 Range Designations 30! V 300 Hz ELF (Extremely Low Frequencies) 0. 3! V 3 k Hz VF (Voice Frequencies) 3! V 30 k Hz VLF (Very Low Frequencies) 30! V 300 k Hz LF (Low Frequencies) 0. 3! V 3 MHz MF (Medium Frequencies) 3! V 30 MHz HF (High Frequencies) 30! V 300 MHz VHF (Very High Frequencies) 0.
3! V 3 GHz UHF (Ultra High Frequencies) 3! V 30 GHz SHF (Super High Frequencies) 30! V 300 GHz EHF (Extremely High Frequencies) 0. 3! V 3 THz Infrared 3! V 30 THz Infrared 30! V 300 THz Infrared 0. 3! V 3 PHz Visible Light 3! V 30 PHz Ultraviolet 30! V 300 PHz X-rays 0. 3! V 3 EHz Gamma rays 3! V 30 EHz Cosmic rays”‘ FCC stands for Federal Communications Commission Wavelength, f”U The length that one cycle of an electromagnetic wave occupies in space, also, the distance between similar points in a repetitive wave.
where k = 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”X noise”X bandwidth Bandwidth, BW unit: 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 measure of how much source information can be carried through the system in a given period of time. Hartley! |s Law Information Capacity f~N Bandwidth “e time Signal Analysis Factors Affecting the Signal”X Distortion! V signal alteration due to imperfect response of the system to the desired signal.” X Interference! V contamination of extraneous signal usually man-made to a form similar to the desired signal.” X Noise! V random, undesirable high frequency spikes.” X Attenuation! V decrease in signal level. Types of Signals”X Baseband Signal”X Modulated Signal Types of Intelligence Transmitted”X Analog “X Digital Advantages of Digital Over Analog”X easier to multiplex”X improved noise immunity”X better performance”X easier to interface Disadvantages of Digital Over Analog”X need for large (wide) bandwidth”X need for synchronization”X need for additional equipment”X restriction to wired topology Kinds of Signal Representations”X Time-Domain! V amplitude vs. time”X Frequency-Domain! V amplitude vs. frequency Classifications of Signals: According to Period”X Periodic! V signals that keep on repeating at a regular interval.” X Non-periodic! V signals that keep on changing continuously and has no definite period. According to Form of Electrical Signal”X Sinusoidal! V has the form”X Non-sinusoidal! V sum of series of sinusoidal According to Symmetry”X Even Symmetry! V a periodic waveform that is symmetrical about the vertical (amplitude) axis “X Odd Symmetry! V a periodic waveform that is symmetrical about a line midway between the vertical and the negative horizontal axes.” X Half-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) “X one-way-only, receive-only, transmit-only Half-duplex (FDX) “X two-way-alternate, either-way, over-and-out system Full-duplex (FDX) “X two-way simultaneous, both-way lines Full / full Duplex (F/FDX) “X transmit and receive simultaneously but not necessarily between the same two locations. Echo plex”X the 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 returning. 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 base band frequency onto a carrier at any modulation level, to preserve the information signal as much as possible FCC Emission Designations First Symbol Second Symbol Third Symbol – Amplitude Modulation, Double Sideband Full Carrier – Independent Sideband – Vestigial Sideband – Frequency Modulation – Phase Modulation – Single Sideband Full Carrier – Single Sideband Suppressed Carrier – Pulse Amplitude Modulation – Pulse Width (Duration) Modulation – Pulse Position Modulation – Unmodulated Carrier – Unmodulated Pulses R – Single Sideband Reduced Carrier 0 – 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 digital A – Telegraphy (manual) B – Telegraphy (automatic) C – FacsimileD – Telemetry (Data) E – Telephony (Sound Broadcasting) F – Television (video signal) N – No Information – 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 transmittedChannelIt is the medium by which the electronic signal is sent from one point to another. Unguided Media”X or wireless communication, transport electromagnetic waves without using a physical conductor. Kinds of Wave Propagation Ground Wave Sky Wave Space Wave Guided Media”X those that provide a conduit from one device to another. Copper (Twisted-pair Cable, Coaxial Cable, Parallel Line) Fiber Optic CableWaveguideReceiversReceivers 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 signalNoiseAny 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”X hiss / static “X snow / confetti “X bit errors”X signal 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. Inter modulation 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 earth! |s atmosphere. Extraterrestrial Noise! V consists of electrical signals that originate from outside Earth! |s atmosphere.
Solar Noise! V directly from the sun! |s 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. where PN = noise power (W) T = Temperature of the conductor (K) B = bandwidth of the noise spectrum (Hz) k = Boltzmann! |s Constant = Noise Voltage where VN = rms noise voltage T = Temperature of the conductor (K) B = bandwidth of the noise spectrum (Hz) k = Boltzmann! |s Constant = R = equivalent resistance generating the noise for combinations of resistances Series Parallel Power Spectrum Density or Noise Density”X average noise power per Hertz of bandwidth”X a figure that determines the amount of noise contained in a specified bandwidth. Signal-to-Noise Ratio”X a relative measure of the desired signal power to the noise power. In decibel form: where PS = signal power PN = noise power VS = signal voltage VN = noise voltage Noise Factor where Si = 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”X the significant effect of reactive circuits on noise is their limitation on frequency response.” X the equivalent bandwidth to be used in noise calculations with reactive circuits is where B 3 dB = half power bandwidth Equivalent Noise Temperature where Teq = equivalent noise temperature To = reference absolute temperature = 290 K F = noise factor Noise Due to Amplifiers in CascadeFriiss! | Formula Over-all noise factor of n stages Over-all noise temperature of n stages Over-all noise resistance where A = voltage gain (ratio) G = power gain (ratio) Shot Noise”X a form of internal noise, which is due to the random variations in current flow in active devices such as tubes, transistors and diodes.
Where iN = 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-k Hz bandwidth? 2. An amplifier operating over the frequency range of 455 k Hz to 460 k Hz has a 200 kfC input resistance. What is the rms noise voltage at the input to the amplifier if the ambient temperature is 17 cXC? 3. Two resistors, 5 kfC and 20 kfC are at 27 cXC.
Calculate the thermal noise power and voltage for a 10 k Hz bandwidth. a) for each resistor, b) for their series combination, and c) for their parallel combination. 4. Three matched amplifier’s are available to amplify a low level signal, they have the following characteristics. Amplifier Power Gain Noise Factor 6 dB 1.
5 B 12 dB 2 C 20 dB 4 The amplifier’s are to be connected in cascade. Calculate the lowest overall noise factor obtainable noting the order in which the amplifier’s must be connected.