Presentation is loading. Please wait.

Presentation is loading. Please wait.

1. H. Chan, Mohawk College2 MAIN TOPICS *Angle Modulation *FM Equipment *Radio-Wave Propagation *Optical Communications *Antennas.

Similar presentations

Presentation on theme: "1. H. Chan, Mohawk College2 MAIN TOPICS *Angle Modulation *FM Equipment *Radio-Wave Propagation *Optical Communications *Antennas."— Presentation transcript:

1 1

2 H. Chan, Mohawk College2 MAIN TOPICS *Angle Modulation *FM Equipment *Radio-Wave Propagation *Optical Communications *Antennas

3 H. Chan, Mohawk College3 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.

4 H. Chan, Mohawk College4 Comparison of FM or PM with AM Advantages over AM: Àbetter SNR, and more resistant to noise Áefficient - class C amplifier can be used, and less power is required to angle modulate Âcapture effect reduces mutual interference Disadvantages: Àmuch wider bandwidth is required Áslightly more complex circuitry is needed

5 H. Chan, Mohawk College5 Frequency Shift Keying (FSK) Carrier Modulating signal FSK signal

6 H. Chan, Mohawk College6 FSK (contd).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 (1s and 0s) with slow speed modems.

7 H. Chan, Mohawk College7 Frequency Modulation Carrier Modulating Signal FM signal

8 H. Chan, Mohawk College8 Frequency Modulation (contd).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.

9 H. Chan, Mohawk College9 Frequency Deviation The amount by which the frequency of the FM signal varies with respect to its resting value (f c ) is known as frequency deviation: f = k f e m, where k f is a system constant, and e m is the instantaneous value of the modulating signal amplitude..Thus the frequency of the FM signal is: f s (t) = f c + f = f c + k f e m (t)

10 H. Chan, Mohawk College10 Maximum or Peak Frequency Deviation If the modulating signal is a sine wave, i.e., e m (t) = E m sin m t, then f s = f c + k f E m sin m t..The peak or maximum frequency deviation: = k f E m.The modulation index of an FM signal is: m f = / f m.Note that m f can be greater than 1.

11 H. Chan, Mohawk College11 Relationship between FM and PM For PM, phase deviation, = k p e m, and the peak phase deviation, max = m p = m f..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.

12 H. Chan, Mohawk College12 Equation for FM Signal If e c = E c sin c t, and e m = E m sin m t, then the equation for the FM signal is: e s = E c sin ( c t + m f sin m t) This signal can be expressed as a series of sinusoids: e s = E c {J o (m f ) sin c t - J 1 (m f )[sin ( c - m )t - sin ( c + m )t] + J 2 (m f )[sin ( c - 2 m )t + sin ( c + 2 m )t] - J 3 (m f )[sin ( c - 3 m )t + sin ( c + 3 m )t] + ….

13 H. Chan, Mohawk College13 Bessel Functions.The Js in the equation are known as Bessel functions of the first kind: m f J o J 1 J 2 J 3 J 4 J 5 J

14 H. Chan, Mohawk College14 Notes on Bessel Functions.Theoretically, there is an infinite number of side frequencies for any m f 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 m f = 2.4, 5.5, 8.65, etc. These points are useful for determining the deviation and the value of k f of an FM modulator system.

15 H. Chan, Mohawk College15 Graph of Bessel Functions

16 H. Chan, Mohawk College16 FM Side-Bands.Each (J) value in the table gives rise to a pair of side- frequencies..The higher the value of m f, the more pairs of significant side- frequencies will be generated.

17 H. Chan, Mohawk College17 Power and Bandwidth of FM Signal.Regardless of m f, 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 f m,where n is the number of pairs of side-frequencies. If m f > 6, a good estimate of the BW is given by Carsons rule:BW = 2( + f m (max) )

18 H. Chan, Mohawk College18 Narrowband & Wideband FM.FM systems with a bandwidth < 15 kHz, are considered to be NBFM. A more restricted definition is that their m f < 0.5. These systems are used for voice communication..Other FM systems, such as FM broadcasting and satellite TV, with wider BW and/or higher m f are called WBFM.

19 H. Chan, Mohawk College19 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, e m is pre- emphasized before frequency modulating e c.

20 H. Chan, Mohawk College20 Pre-emphasis circuit In FM broadcasting, the high frequency components are boosted by passing the modulating signal through a HPF with a 75 s time constant before modulation. = R 1 C = 75 s.

21 H. Chan, Mohawk College21 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.

22 H. Chan, Mohawk College22 FM Stereo Broadcasting: Baseband Spectra.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: L+R (mono) kHz L-RL+R kHz Pilot Carrier SCA (optional)

23 H. Chan, Mohawk College23 FM Stereo Broadcasting.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.

24 H. Chan, Mohawk College24 Block Diagram of FM Transmitter FM Modulator Buffer Pre-emphasis Audio Frequency Multiplier(s) Driver Power Amp Antenna

25 H. Chan, Mohawk College25 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.

26 H. Chan, Mohawk College26 Crosby AFC System.An LC oscillator operated as a VCO with automatic frequency control is known as the Crosby system.

27 H. Chan, Mohawk College27 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.

28 H. Chan, Mohawk College28 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 its 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.

29 H. Chan, Mohawk College29 Example of Indirect FM Generator Armstrong Modulator

30 H. Chan, Mohawk College30 Block Diagram of FM Receiver

31 H. Chan, Mohawk College31 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.

32 H. Chan, Mohawk College32 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.

33 H. Chan, Mohawk College33 S-curve Characteristics of FM Detectors f IF fifi vovo EmEm

34 H. Chan, Mohawk College34 PLL FM Detector.PLL and quadrature detectors are commonly found in modern FM receivers. Phase Detector LPF Demodulated output VCO FM IF Signal

35 H. Chan, Mohawk College35 Quadrature Detector.Both the quadrature and the PLL detector are conveniently found as IC packages.

36 H. Chan, Mohawk College36 Radio-Wave Propagation.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.

37 H. Chan, Mohawk College37 Transverse Electromagnetic Waves x y z Electric Field Magnetic Field Direction of Propagation

38 H. Chan, Mohawk College38 Speed & Wavelength of em Waves The speed of propagation and the wavelength ( ) of an electromagnetic wave are given, respectively, by: where c = 3x10 8 m/s, r = mediums relative permittivity or dielectric constant, and f = frequency of wave in Hz.

39 H. Chan, Mohawk College39 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, Z o = 377. For other media: where = mediums permeability, in H/m and = mediums permittivity in F/m

40 H. Chan, Mohawk College40 Power Density.The power density for em wave in free space is: P D = E 2 /Z o or H 2 Z o or EH in W/m 2.For an isotropic radiator, i.e. an antenna that radiates equally well in all directions and perfectly efficient, the power density is: where P t = total power in W, and r = distance from antenna in m

41 H. Chan, Mohawk College41 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 = P T G T where P T = total transmitter power, and G T = gain of transmitter antenna.

42 H. Chan, Mohawk College42 Path Loss.Free space path loss, L fs, is given by: L fs (dB) = ( log d +20 log f) - [G T (dBi) + G R (dBi)] = P T (dBm) - P R (dBm) or 10 log (P T /P R ) where d =distance between TX and RX in km, f = frequency in MHz, P T = transmitter power, and P R = received power in W.

43 H. Chan, Mohawk College43 Reflection Radio waves behave like light waves: They reflect from a surface where the angle of incidence, i = the angle of reflection, r. To minimize reflective losses, the surface should be an ideal conductor and smooth. Normal Incident Ray Reflected Ray i r Conductor

44 H. Chan, Mohawk College44 Refraction.Radio waves will bend or refract when they go from one medium with refractive index, n 1 to another with refractive index, n 2. The angles involved are given by : 1 2 n 1

45 H. Chan, Mohawk College45 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

46 H. Chan, Mohawk College46 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

47 H. Chan, Mohawk College47 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, f c..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.

48 H. Chan, Mohawk College48 Sky-Wave Propagation. From geometry (assuming flat earth): d = 2h v tan i. From theory (secant law): MUF = f c sec i i hvhv d F-Layer Earth

49 H. Chan, Mohawk College49 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.

50 H. Chan, Mohawk College50 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 h T = height of the TX antenna in m h R = height of the RX antenna in m

51 H. Chan, Mohawk College51 Space-Wave Propagation (contd).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.

52 H. Chan, Mohawk College52 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

53 H. Chan, Mohawk College53 Optical Fibre Link Input Signal Coder or Converter Light Source Source-to-fibre Interface Fibre-to-light Interface Light Detector Amplifier/Shaper Decoder Output Fibre-optic Cable Transmitter Receiver

54 H. Chan, Mohawk College54 Types Of Optical Fibre Single-mode step-index fibre Multimode step-index fibre Multimode graded-index fibre n 1 core n 2 cladding n o air n 2 cladding n 1 core Variable n n o air Light ray Index porfile

55 H. Chan, Mohawk College55 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

56 H. Chan, Mohawk College56 Acceptance Cone & Numerical Aperture n 2 cladding n 1 core Acceptance Cone Acceptance angle, c, 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 c = (n n 2 2 ) C

57 H. Chan, Mohawk College57 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

58 H. Chan, Mohawk College58 Absorption Losses In Optic Fibre Loss (dB/km) Wavelength ( m) Peaks caused by OH - ions Infrared absorption Rayleigh scattering & ultraviolet absorption

59 H. Chan, Mohawk College59 Fibre Alignment Impairments Axial displacementGap displacement Angular displacementImperfect surface finish

60 H. Chan, Mohawk College60 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

61 H. Chan, Mohawk College61 ILD versus LED.Advantages: –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

62 H. Chan, Mohawk College62 Optical Transmitter Circuits +V CC Data Input Enable C1C1 R1R1 LED Q1Q1 R2R2 +HV ILD C2C2 Enable C1C1 R1R1 Q1Q1 R2R2 R3R3 Data Input

63 H. Chan, Mohawk College63 Light Detectors.PIN Diodes –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

64 H. Chan, Mohawk College64 Photodetector Circuit +V R1R Threshold adjust Enable Comparator shaper PIN or APD Data Out

65 H. Chan, Mohawk College65 Bandwidth & Power Budget The maximum data rate R (Mbps) for a cable of given distance D (km) with a dispersion d ( s/km) is: R = 1/(5dD).Power or loss margin, L m (dB) is: L m = P r - P s = P t - M - L sf - (DxL f ) - L c - L fd - P s 0 where P r = received power (dBm), P s = receiver sensitivity(dBm), P t = Tx power (dBm), M = contingency loss allowance (dB), L sf = source-to-fibre loss (dB), L f = fibre loss (dB/km), L c = total connector/splice losses (dB), L fd = fibre-to-detector loss (dB).

66 H. Chan, Mohawk College66 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: Balanced Feedline Symbol /2

67 H. Chan, Mohawk College67 Half-Wave Dipole Typically, the physical length of a half-wave dipole is 0.95 of /2 in free space..Since power fed to the antenna is radiated into space, there is an equivalent radiation resistance, R r. For a real antenna, losses in the antenna can be represented by a loss resistance, R d. Its efficiency is then:

68 H. Chan, Mohawk College68 3-D Antenna Radiation Pattern

69 H. Chan, Mohawk College69 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 /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

70 H. Chan, Mohawk College70 EIRP and Effective Area.When power, P T, is applied to an antenna with a gain G T (with respect to an isotropic radiator), then the antenna is said to have an effective isotropic radiated power, EIRP = P T G T..The signal power delivered to a receiving antenna with a gain G R is P R = P D A eff where P D is the power density, and A eff is the effective area.

71 H. Chan, Mohawk College71 Impedance and Polarization A half-wave dipole in free space and centre-fed has a radiation resistance of about 70. At resonance, the antennas impedance will be completely resistive and its efficiency maximum. If its length is /2, it is inductive..The polarization of a half-wave dipole is the same as the axis of the conductor.

72 H. Chan, Mohawk College72 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.

73 H. Chan, Mohawk College73 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.

74 H. Chan, Mohawk College74 Monopole or Marconi Antenna Main characteristics: vertical and /4 good ground plane is required omnidirectional in the horizontal plane 3 dBd power gain impedance: about 36

75 H. Chan, Mohawk College75 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

76 H. Chan, Mohawk College76 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.

77 H. Chan, Mohawk College77 Inductive and Capacitive Loading Inductive LoadingCapacitive Loading

78 H. Chan, Mohawk College78 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.

79 H. Chan, Mohawk College79 Yagi-Uda Array.More commonly known as the Yagi array, it has one driven element, one reflector, and one or more directors. Radiation pattern

80 H. Chan, Mohawk College80 Characteristics of Yagi array Funidirectional radiation pattern (one main lobe, some sidelobes and backlobes) Frelatively narrow bandwidth since it is resonant F3-element array has a gain of about 7 dBi Fmore directors will increase gain and reduce the beamwidth and feedpoint impedance Fa folded dipole is generally used for the driven element to widen the bandwidth and increase the feedpoint impedance.

81 H. Chan, Mohawk College81 Log-Periodic Dipole Array (LPDA)

82 H. Chan, Mohawk College82 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 /2 of highest frequency, while longest element is at least /2 of lowest frequency reasonable gain, but lower than that of Yagi for the same number of elements design parameter, = L 1 /L 2 = D 1 /D 2 = L 2 /L 3 = ….

83 H. Chan, Mohawk College83 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

84 H. Chan, Mohawk College84 Collinear Array Fall elements lie along a straight line, fed in phase, and often mounted with main axis vertical Fresult in narrow radiation beam omnidirectional in the horizontal plane

85 H. Chan, Mohawk College85 Broadside Array all /2 elements are fed in phase and spaced /2 with axis placed vertically, radiation would have a narrow bidirectional horizontal pattern

86 H. Chan, Mohawk College86 End-Fire Array dipole elements are fed 90 o out of phase resulting in a narrow unidirectional radiation pattern off the end of the antenna

87 H. Chan, Mohawk College87 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 /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.

88 H. Chan, Mohawk College88 Plane and Corner Reflectors A plane reflector acts like a mirror and is normally placed /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.

89 H. Chan, Mohawk College89 Parabolic Reflector small horn antenna is placed at focus of parabolic dish beamwidth,, and gain, G, are given by: where = wavelength in m, D = dishs diameter in m = antenna efficiency

90 H. Chan, Mohawk College90 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.

Download ppt "1. H. Chan, Mohawk College2 MAIN TOPICS *Angle Modulation *FM Equipment *Radio-Wave Propagation *Optical Communications *Antennas."

Similar presentations

Ads by Google