# COMMUNICATIONS EE 733.

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COMMUNICATIONS EE 733

MAIN TOPICS Angle Modulation FM Equipment Radio-Wave Propagation
Optical Communications Antennas H. Chan, Mohawk College

Angle Modulation Angle modulation includes both frequency and phase modulation. FM is used for: radio broadcasting, sound signal in TV, two-way fixed and mobile radio systems, cellular telephone systems, and satellite communications. PM is used extensively in data communications and for indirect FM. H. Chan, Mohawk College

Comparison of FM or PM with AM
Advantages over AM: better SNR, and more resistant to noise efficient - class C amplifier can be used, and less power is required to angle modulate capture effect reduces mutual interference Disadvantages: much wider bandwidth is required slightly more complex circuitry is needed H. Chan, Mohawk College

Frequency Shift Keying (FSK)
Carrier Modulating signal FSK signal H. Chan, Mohawk College

FSK (cont’d) The frequency of the FSK signal changes abruptly from one that is higher than that of the carrier to one that is lower. Note that the amplitude of the FSK signal remains constant. FSK can be used for transmission of digital data (1’s and 0’s) with slow speed modems. H. Chan, Mohawk College

Frequency Modulation Carrier Modulating Signal FM signal
H. Chan, Mohawk College

Frequency Modulation (cont’d)
Note the continuous change in frequency of the FM wave when the modulating signal is a sine wave. In particular, the frequency of the FM wave is maximum when the modulating signal is at its positive peak and is minimum when the modulating signal is at its negative peak. H. Chan, Mohawk College

Frequency Deviation The amount by which the frequency of the FM signal varies with respect to its resting value (fc) is known as frequency deviation: Df = kf em, where kf is a system constant, and em is the instantaneous value of the modulating signal amplitude. Thus the frequency of the FM signal is: fs (t) = fc + Df = fc + kf em(t) H. Chan, Mohawk College

Maximum or Peak Frequency Deviation
If the modulating signal is a sine wave, i.e., em(t) = Emsin wmt, then fs = fc + kfEmsin wmt. The peak or maximum frequency deviation: d = kf Em The modulation index of an FM signal is: mf = d / fm Note that mf can be greater than 1. H. Chan, Mohawk College

Relationship between FM and PM
For PM, phase deviation, Df = kpem, and the peak phase deviation, fmax = mp = mf. Since frequency (in rad/s) is given by: the above equations suggest that FM can be obtained by first integrating the modulating signal, then applying it to a phase modulator. H. Chan, Mohawk College

Equation for FM Signal If ec = Ec sin wct, and em = Em sin wmt, then the equation for the FM signal is: es = Ec sin (wct + mf sin wmt) This signal can be expressed as a series of sinusoids: es = Ec{Jo(mf) sin wct - J1(mf)[sin (wc - wm)t - sin (wc + wm)t] + J2(mf)[sin (wc - 2wm)t + sin (wc + 2wm)t] - J3(mf)[sin (wc - 3wm)t + sin (wc + 3wm)t] + … . H. Chan, Mohawk College

Bessel Functions The J’s in the equation are known as Bessel functions of the first kind: mf Jo J1 J2 J3 J4 J5 J 0 1 H. Chan, Mohawk College

Notes on Bessel Functions
Theoretically, there is an infinite number of side frequencies for any mf other than 0. However, only significant amplitudes, i.e. those |0.01| are included in the table. Bessel-zero or carrier-null points occur when mf = 2.4, 5.5, 8.65, etc. These points are useful for determining the deviation and the value of kf of an FM modulator system. H. Chan, Mohawk College

Graph of Bessel Functions
H. Chan, Mohawk College

FM Side-Bands Each (J) value in the table gives rise to a pair of side-frequencies. The higher the value of mf, the more pairs of significant side- frequencies will be generated. H. Chan, Mohawk College

Power and Bandwidth of FM Signal
Regardless of mf , the total power of an FM signal remains constant because its amplitude is constant. The required BW of an FM signal is: BW = 2 x n x fm ,where n is the number of pairs of side-frequencies. If mf > 6, a good estimate of the BW is given by Carson’s rule: BW = 2(d + fm (max) ) H. Chan, Mohawk College

Narrowband & Wideband FM
FM systems with a bandwidth < 15 kHz, are considered to be NBFM. A more restricted definition is that their mf < These systems are used for voice communication. Other FM systems, such as FM broadcasting and satellite TV, with wider BW and/or higher mf are called WBFM. H. Chan, Mohawk College

Pre-emphasis Most common analog signals have high frequency components that are relatively low in amplitude than low frequency ones. Ambient electrical noise is uniformly distributed. Therefore, the SNR for high frequency components is lower. To correct the problem, em is pre-emphasized before frequency modulating ec. H. Chan, Mohawk College

Pre-emphasis circuit In FM broadcasting, the high frequency components are boosted by passing the modulating signal through a HPF with a 75 ms time constant before modulation. t = R1C = 75 ms. H. Chan, Mohawk College

De-emphasis Circuit At the FM receiver, the signal after demodulation must be de-emphasized by a filter with similar characteristics as the pre-emphasis filter to restore the relative amplitudes of the modulating signal. H. Chan, Mohawk College

To maintain compatibility with monaural system, FM stereo uses a form of FDM or frequency-division multiplexing to combine the left and right channel information: 19 kHz Pilot Carrier SCA (optional) L+R (mono) L-R L+R kHz .05 15 23 38 53 60 67 74 H. Chan, Mohawk College

To enable the L and R channels to be reproduced at the receiver, the L-R and L+R signals are required. These are sent as a DSBSC AM signal with a suppressed subcarrier at 38 kHz. The purpose of the 19 kHz pilot is for proper detection of the DSBSC AM signal. The optional Subsidiary Carrier Authorization (SCA) signal is normally used for services such as background music for stores and offices. H. Chan, Mohawk College

Block Diagram of FM Transmitter
Modulator Frequency Multiplier(s) Antenna Buffer Power Amp Driver Pre-emphasis Audio H. Chan, Mohawk College

Direct-FM Modulator A simple method of generating FM is to use a reactance modulator where a varactor is put in the frequency determining circuit. H. Chan, Mohawk College

Crosby AFC System An LC oscillator operated as a VCO with automatic frequency control is known as the Crosby system. H. Chan, Mohawk College

Phase-Locked Loop FM Generators
The PLL system is more stable than the Crosby system and can produce wide-band FM without using frequency multipliers. H. Chan, Mohawk College

Indirect-FM Modulators
Recall earlier that FM and PM were shown to be closely related. In fact, FM can be produced using a phase modulator if the modulating signal is passed through a suitable LPF (i.e. an integrator) before it reaches the modulator. One reason for using indirect FM is that it’s easier to change the phase than the frequency of a crystal oscillator. However, the phase shift achievable is small, and frequency multipliers will be needed. H. Chan, Mohawk College

Example of Indirect FM Generator
Armstrong Modulator H. Chan, Mohawk College

H. Chan, Mohawk College

FM Receivers FM receivers, like AM receivers, utilize the superheterodyne principle, but they operate at much higher frequencies ( MHz). A limiter is often used to ensure the received signal is constant in amplitude before it enters the discriminator or detector. The limiter operates like a class C amplifier when the input exceeds a threshold point. In modern receivers, the limiting function is built into the FM IF integrated circuit. H. Chan, Mohawk College

FM Demodulators The FM demodulators must convert frequency variations of the input signal into amplitude variations at the output. The Foster-Seeley discriminator and its variant, the ratio detector are commonly found in older receivers. They are based on the principle of slope detection using resonant circuits. H. Chan, Mohawk College

S-curve Characteristics of FM Detectors
vo Em d fi fIF d H. Chan, Mohawk College

PLL FM Detector PLL and quadrature detectors are commonly found in modern FM receivers. Phase Detector Demodulated output FM IF Signal f LPF VCO H. Chan, Mohawk College

Quadrature Detector Both the quadrature and the PLL detector are conveniently found as IC packages. H. Chan, Mohawk College

Radio waves, infrared, visible light, ultraviolet, X rays, and gamma rays are all different forms of electromagnetic radiation. The waves propagate as transverse electromagnetic waves (TEM) - i.e. the electric field, the magnetic field, and the direction of travel of the waves are all mutually perpendicular. H. Chan, Mohawk College

Transverse Electromagnetic Waves
z Direction of Propagation y Magnetic Field Electric Field x H. Chan, Mohawk College

Speed & Wavelength of em Waves
The speed of propagation () and the wavelength (l) of an electromagnetic wave are given, respectively, by: where c = 3x108 m/s, r = medium’s relative permittivity or dielectric constant, and f = frequency of wave in Hz. H. Chan, Mohawk College

Characteristic Impedance
The characteristic impedance of a medium is the ratio of the electric field intensity and the magnetic field intensity, i.e., Z = E/H. For free space, Zo = 377 W. For other media: where m = medium’s permeability, in H/m and  = medium’s permittivity in F/m H. Chan, Mohawk College

Power Density The power density for em wave in free space is:
PD = E2/Zo or H2Zo or EH in W/m2 For an isotropic radiator, i.e. an antenna that radiates equally well in all directions and perfectly efficient, the power density is: where Pt = total power in W, and r = distance from antenna in m H. Chan, Mohawk College

Electric Field Strength and EIRP
The strength of a signal is more often given by its electric field intensity in V/m: Since a transmitting antenna focuses energy in a specific way, it has “gain” over an isotropic radiator in a particular direction. One can speak of the effective isotropic radiated power, EIRP = PTGT where PT = total transmitter power, and GT = gain of transmitter antenna. H. Chan, Mohawk College

Path Loss Free space path loss, Lfs, is given by:
Lfs (dB) = ( log d +20 log f) - [GT (dBi) + GR (dBi)] = PT (dBm) - PR (dBm) or 10 log (PT/PR) where d =distance between TX and RX in km, f = frequency in MHz, PT = transmitter power, and PR = received power in W. H. Chan, Mohawk College

Reflection Radio waves behave like light waves:
They reflect from a surface where the angle of incidence, qi = the angle of reflection, qr . To minimize reflective losses, the surface should be an ideal conductor and smooth. Incident Ray Reflected Ray Normal qi qr Conductor H. Chan, Mohawk College

permittivity of medium
Refraction Radio waves will bend or refract when they go from one medium with refractive index, n1 to another with refractive index, n2. The angles involved are given by : q1 where r = relative permittivity of medium q2 n1<n2 H. Chan, Mohawk College

Direction of wave propagation
Diffraction Diffraction is the phenomenon which results in radio waves that normally travel in a straight line to bend around an obstacle. Direction of wave propagation Obstacle H. Chan, Mohawk College

Ground-Wave Propagation
At frequencies up to about 2 MHz, the most important method of propagation is by ground waves which are vertically polarized. They follow the curvature of the earth to propagate far beyond the horizon. Relatively high power is required. Direction of wave travel Increasing Tilt Earth H. Chan, Mohawk College

Ionospheric Propagation
HF radio waves are returned from the F-layer of the ionosphere by a form of refraction. The highest frequency that is returned to earth in the vertical direction is called the critical frequency, fc. The highest frequency that returns to earth over a given path is called the maximum usable frequency (MUF). Because of the general instability of the ionosphere, the optimum working frequency (OWF) = 0.85 MUF, is used instead. H. Chan, Mohawk College

Sky-Wave Propagation From geometry (assuming flat earth):
d = 2hv tan qi From theory (secant law): MUF = fc sec qi F-Layer qi hv Earth d H. Chan, Mohawk College

Sky-wave Propagation: Pros & Cons
Sky-wave propagation allows communication over great distances with simple equipment and reasonable power levels : 100 W to a few kW. However, HF communication via the ionosphere is noisy and uncertain. It is also prone to phase shifting and frequency-selective fading. For instance, the phase shift and signal attenuation may be different for the upper and lower sidebands of the same signal. Data transmission is restricted to very low rates. H. Chan, Mohawk College

Space-Wave Propagation
Most terrestrial communications in the VHF or higher frequency range use direct, line-of-sight, or tropospheric radio waves. The approximate maximum distance of communication is given by: where d = max. distance in km hT = height of the TX antenna in m hR = height of the RX antenna in m H. Chan, Mohawk College

Space-Wave Propagation (cont’d)
The radio horizon is greater than the optical horizon by about one third due to refraction of the atmosphere. Reflections from a relatively smooth surface, such as a body of water, could result in partial cancellation of the direct signal - a phenomenon known as fading. Also, large objects, such as buildings and hills, could cause multipath distortion from many reflections. H. Chan, Mohawk College

Optical Fibre Communications
Advantages over metallic/coaxial cable: much wider bandwidth and practically interference-free lower loss and light weight more resistive to environmental effects safer and easier to install almost impossible to tap into a fibre cable potentially lower in cost over the long term Disadvantages: higher initial cost in installation & more expensive to repair/maintain H. Chan, Mohawk College

Optical Fibre Link Transmitter Input Signal Coder or Converter Light
Source Source-to-fibre Interface Fibre-optic Cable Output Fibre-to-light Interface Light Detector Amplifier/Shaper Decoder Receiver H. Chan, Mohawk College

Types Of Optical Fibre Light ray n1 core n2 cladding
Single-mode step-index fibre no air n1 core n2 cladding Multimode step-index fibre no air Variable n Multimode graded-index fibre Index porfile H. Chan, Mohawk College

Comparison Of Optical Fibres
Single-mode step-index fibre: minimum signal dispersion; higher TX rate possible difficult to couple light into fibre; highly directive light source (e.g. laser) required; expensive to manufacture Multimode step-index fibres: inexpensive; easy to couple light into fibre result in higher signal distortion; lower TX rate Multimode graded-index fibre: intermediate between the other two types of fibres H. Chan, Mohawk College

Acceptance Cone & Numerical Aperture
n2 cladding qC n1 core n2 cladding Acceptance angle, qc, is the maximum angle in which external light rays may strike the air/fibre interface and still propagate down the fibre with <10 dB loss. Numerical aperture: NA = sin qc = (n12 - n22) H. Chan, Mohawk College

Losses In Optical Fibre Cables
The predominant losses in optic fibres are: absorption losses due to impurities in the fibre material material or Rayleigh scattering losses due to microscopic irregularities in the fibre chromatic or wavelength dispersion because of the use of a non-monochromatic source radiation losses caused by bends and kinks in the fibre modal dispersion or pulse spreading due to rays taking different paths down the fibre coupling losses caused by misalignment & imperfect surface finishes H. Chan, Mohawk College

Absorption Losses In Optic Fibre
6 Rayleigh scattering & ultraviolet absorption 5 4 Loss (dB/km) 3 Peaks caused by OH- ions Infrared absorption 2 1 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Wavelength (mm) H. Chan, Mohawk College

Fibre Alignment Impairments
Axial displacement Gap displacement Angular displacement Imperfect surface finish H. Chan, Mohawk College

Light Sources Light-Emitting Diodes (LED)
made from material such as AlGaAs or GaAsP light is emitted when electrons and holes recombine either surface emitting or edge emitting Injection Laser Diodes (ILD) similar in construction as LED except ends are highly polished to reflect photons back & forth H. Chan, Mohawk College

more focussed radiation pattern; smaller fibre much higher radiant power; longer span faster ON, OFF time; higher bit rates possible monochromatic light; reduces dispersion Disadvantages: much more expensive higher temperature; shorter lifespan H. Chan, Mohawk College

Optical Transmitter Circuits
+VCC C1 R2 Data Input Q1 +HV R1 LED Enable C1 R3 Q1 Data Input R1 C2 Enable R2 ILD H. Chan, Mohawk College

Light Detectors PIN Diodes Avalanche Photodiodes (APD)
photons are absorbed in the intrinsic layer sufficient energy is added to generate carriers in the depletion layer for current to flow through the device Avalanche Photodiodes (APD) photogenerated electrons are accelerated by relatively large reverse voltage and collide with other atoms to produce more free electrons avalanche multiplication effect makes APD more sensitive but also more noisy than PIN diodes H. Chan, Mohawk College

Photodetector Circuit
+V R1 Comparator shaper Data Out - - PIN or APD + + - Enable + Threshold adjust H. Chan, Mohawk College

Bandwidth & Power Budget
The maximum data rate R (Mbps) for a cable of given distance D (km) with a dispersion d (ms/km) is: R = 1/(5dD) Power or loss margin, Lm (dB) is: Lm = Pr - Ps = Pt - M - Lsf - (DxLf) - Lc - Lfd - Ps  0 where Pr = received power (dBm), Ps = receiver sensitivity(dBm), Pt = Tx power (dBm), M = contingency loss allowance (dB), Lsf = source-to-fibre loss (dB), Lf = fibre loss (dB/km), Lc = total connector/splice losses (dB), Lfd = fibre-to-detector loss (dB). H. Chan, Mohawk College

Simple Antennas An isotropic radiator would radiate all electrical power supplied to it equally in all directions. It is merely a theoretical concept but is useful as a reference for other antennas. A more practical antenna is the half-wave dipole: /2 Symbol Balanced Feedline H. Chan, Mohawk College

Half-Wave Dipole Typically, the physical length of a half-wave dipole is 0.95 of l/2 in free space. Since power fed to the antenna is radiated into space, there is an equivalent radiation resistance, Rr. For a real antenna, losses in the antenna can be represented by a loss resistance, Rd. Its efficiency is then: H. Chan, Mohawk College

H. Chan, Mohawk College

Gain and Directivity Antennas are designed to focus their radiation into lobes or beams thus providing gain in selected directions at the expense of energy reductions in others. The ideal l/2 dipole has a gain of 2.14 dBi (i.e. dB with respect to an isotropic radiator) Directivity is the gain calculated assuming a lossless antenna H. Chan, Mohawk College

EIRP and Effective Area
When power, PT, is applied to an antenna with a gain GT (with respect to an isotropic radiator), then the antenna is said to have an effective isotropic radiated power, EIRP = PTGT. The signal power delivered to a receiving antenna with a gain GR is PR = PDAeff where PD is the power density, and Aeff is the effective area. H. Chan, Mohawk College

Impedance and Polarization
A half-wave dipole in free space and centre-fed has a radiation resistance of about 70 W. At resonance, the antenna’s impedance will be completely resistive and its efficiency maximum. If its length is < l/2, it becomes capacitive, and if > l/2, it is inductive. The polarization of a half-wave dipole is the same as the axis of the conductor. H. Chan, Mohawk College

Ground Effects Ground effects on antenna pattern and resistance are complex and significant for heights less than one wavelength. This is particularly true for antennas operating at HF range and below. Generally, a horizontally polarized antenna is affected more by near ground reflections than a vertically polarized antenna. H. Chan, Mohawk College

Folded Dipole Often used - alone or with other elements - for TV and FM broadcast receiving antennas because it has a wider bandwidth and four times the feedpoint resistance of a single dipole. H. Chan, Mohawk College

Monopole or Marconi Antenna
Main characteristics: vertical and l/4 good ground plane is required omnidirectional in the horizontal plane 3 dBd power gain impedance: about 36W H. Chan, Mohawk College

Loop Antennas Main characteristics: very small dimensions
bidirectional greatest sensitivity in the plane of the loop very wide bandwidth efficient as RX antenna with single or multi-turn loop H. Chan, Mohawk College

Antenna Matching Antennas should be matched to their feedline for maximum power transfer efficiency by using an LC matching network. A simple but effective technique for matching a short vertical antenna to a feedline is to increase its electrical length by adding an inductance at its base. This inductance, called a loading coil, cancels the capacitive effect of the antenna. Another method is to use capacitive loading. H. Chan, Mohawk College

Antenna Arrays Antenna elements can be combined in an array to increase gain and desired radiation pattern. Arrays can be classified as broadside or end-fire, according to their direction of maximum radiation. In a phased array, all elements are fed or driven; i.e. they are connected to the feedline. Some arrays have only one driven element with several parasitic elements which act to absorb and reradiate power radiated from the driven element. H. Chan, Mohawk College

Yagi-Uda Array More commonly known as the Yagi array, it has one driven element, one reflector, and one or more directors. Radiation pattern H. Chan, Mohawk College

Characteristics of Yagi array
unidirectional radiation pattern (one main lobe, some sidelobes and backlobes) relatively narrow bandwidth since it is resonant 3-element array has a gain of about 7 dBi more directors will increase gain and reduce the beamwidth and feedpoint impedance a folded dipole is generally used for the driven element to widen the bandwidth and increase the feedpoint impedance. H. Chan, Mohawk College

Log-Periodic Dipole Array (LPDA)
H. Chan, Mohawk College

Characteristics of Log-Periodic Dipole Array
feedpoint impedance is a periodic function of operating frequency unidirectional radiation and wide bandwidth shortest element is less than or equal to l/2 of highest frequency, while longest element is at least l/2 of lowest frequency reasonable gain, but lower than that of Yagi for the same number of elements design parameter, t = L1/L2 = D1/D2 = L2/L3 = …. H. Chan, Mohawk College

Turnstile Array omnidirectional radiation in the horizontal plane, with horizontal polarization gain of about 3 dB less than that of a single dipole often used for FM broadcast RX and TX H. Chan, Mohawk College

Collinear Array all elements lie along a straight line, fed in phase, and often mounted with main axis vertical result in narrow radiation beam omnidirectional in the horizontal plane H. Chan, Mohawk College

Broadside Array all l/2 elements are fed in phase and spaced l/2
with axis placed vertically, radiation would have a narrow bidirectional horizontal pattern H. Chan, Mohawk College

End-Fire Array dipole elements are fed 90o out of phase resulting in a narrow unidirectional radiation pattern off the end of the antenna H. Chan, Mohawk College

Non-resonant Antennas
Monopole and dipole antennas are classified as resonant type since they operate efficiently only at frequencies that make their elements close to l/2. Non-resonant antennas do not use dipoles and are usually terminated with a matching load resistor. They have a broader bandwidth and a radiation pattern that has only one or two main lobes. Examples of non-resonant antennas are long-wire antennas, vee antennas, and rhombic antennas. H. Chan, Mohawk College

Plane and Corner Reflectors
A plane reflector acts like a mirror and is normally placed l/4 from the antenna, such as a collinear array, resulting in a directional radiation pattern. The plane does not have to be solid. It is often made of wire mesh, metal rods or tubes to reduce wind loading. Corner reflectors produce a sharper pattern. They are often combined with Yagi arrays in UHF television antennas. H. Chan, Mohawk College

where l = wavelength in m, D = dish’s diameter in m
Parabolic Reflector small horn antenna is placed at focus of parabolic “dish” beamwidth, q, and gain, G, are given by: where l = wavelength in m, D = dish’s diameter in m h = antenna efficiency H. Chan, Mohawk College

Hog-horn Antenna The hog-horn antenna, often used for terrestrial microwave links, integrates the feed horn and a parabolic reflecting surface to provide an obstruction-free path for the incoming and outgoing signals. H. Chan, Mohawk College

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