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

1

H. Chan, Mohawk College 2

MAIN TOPICS

Angle ModulationFM EquipmentRadio-Wave PropagationOptical CommunicationsAntennas


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.


Comparison of FM or PM with AM

Advantages over AM:

better SNR, and more resistant to noiseefficient - class C amplifier can be used, and

less power is required to angle modulatecapture effect reduces mutual interferenceDisadvantages:

much wider bandwidth is requiredslightly more complex circuitry is needed


Frequency Shift Keying (FSK)

Carrier

Modulatingsignal

FSKsignal


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.


Frequency Modulation

Carrier

ModulatingSignal

FMsignal


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.


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: f = 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 + f = fc + kf em(t)


Maximum or Peak Frequency Deviation

If the modulating signal is a sine wave, i.e., em(t) = Emsin mt, then fs = fc + kfEmsin mt.

The peak or maximum frequency deviation:

= kf Em

The modulation index of an FM signal is:

mf = / fm

Note that mf can be greater than 1.


Relationship between FM and PM

For PM, phase deviation, = kpem, and the peak phase deviation, max = mp = mf.

Since frequency (in rad/s) is given by:

dtttordt

tdt )()(

)()(

the above equations suggest that FM can be

obtained by first integrating the modulating

signal, then applying it to a phase modulator.


Equation for FM Signal

If ec = Ec sin ct, and em = Em sin mt, then the equation for the FM signal is:

es = Ec sin (ct + mf sin mt)

This signal can be expressed as a series of sinusoids: es = Ec{Jo(mf) sin ct

- J1(mf)[sin (c - m)t - sin (c + m)t]

+ J2(mf)[sin (c - 2m)t + sin (c + 2m)t]

- J3(mf)[sin (c - 3m)t + sin (c + 3m)t] + … .


Bessel Functions

The J’s in the equation are known as Bessel functions of the first kind:

mf Jo J1 J2 J3 J4 J5 J6 . . .

0 1

0.5 .94 .24 .03

1 .77 .44 .11 .02

2.4 0.0 .52 .43 .20 .06 .02

5.5 0.0 -.34 -.12 .26 .40 .32 .19 . . .


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.


Graph of Bessel Functions


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.


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( + fm (max) )


Narrowband & Wideband FM

FM systems with a bandwidth < 15 kHz, are considered to be NBFM. A more restricted definition is that their mf < 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 mf are called WBFM.


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.


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.

= R1C = 75 s.


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.


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)

kHzL-R L+R

.05 15 23 38 53 6074

67

19 kHz PilotCarrier SCA

(optional)


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.


Block Diagram of FM Transmitter

FMModulator

Buffer

Pre-emphasis

Audio

FrequencyMultiplier(s)

Driver PowerAmp

Antenna


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.


Crosby AFC System

An LC oscillator operated as a VCO with automatic frequency control is known as the Crosby system.


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.


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.


Example of Indirect FM Generator

ArmstrongModulator


Block Diagram of FM Receiver


FM Receivers

FM receivers, like AM receivers, utilize the superheterodyne principle, but they operate at much higher frequencies (88 - 108 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.


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.


S-curve Characteristics of FM Detectors

fIF

fi

vo

Em


PLL FM Detector

PLL and quadrature detectors are commonly found in modern FM receivers.

PhaseDetector

LPFDemodulated

output

VCO

FM IFSignal


Quadrature Detector

Both the quadrature and the PLL detector are conveniently found as IC packages.


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.


Transverse Electromagnetic Waves

x

y

z

Electric Field

Magnetic FieldDirection of Propagation


Speed & Wavelength of em Waves

The speed of propagation and the wavelength () of an electromagnetic wave are given, respectively, by:

f

vand

cv

r

where c = 3x108 m/s, r = medium’s relative permittivity

or dielectric constant, and f = frequency of wave in Hz.


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 . For other media:

r

orZ

377

where = medium’s permeability, in H/m

and = medium’s permittivity in F/m


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:

24 r

PP t

D where Pt = total power in W, and

r = distance from antenna in m


Electric Field Strength and EIRP

The strength of a signal is more often given by its electric field intensity in V/m:

r

PE t30

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.


Path Loss

Free space path loss, Lfs, is given by:

Lfs (dB) = (32.44 + 20 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.


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.

NormalIncidentRay

ReflectedRay

i rConductor


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 :

1

2

1

2

2

1

sin

sin

r

r

n

n

1

2

n1<n2

where r = relative

permittivity of medium


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


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

IncreasingTilt

Earth


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.


Sky-Wave Propagation

From geometry (assuming flat earth):

d = 2hv tan i

From theory (secant law):

MUF = fc sec i

i

hv

d

F-Layer

Earth


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.


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:

RT hhd 17where d = max. distance in km

hT = height of the TX antenna in m

hR = height of the RX antenna in m


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.


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


Optical Fibre Link

InputSignal

Coder orConverter

LightSource

Source-to-fibreInterface

Fibre-to-lightInterface

LightDetector

Amplifier/ShaperDecoder

Output

Fibre-optic Cable

Transmitter

Receiver


Types Of Optical Fibre

Single-mode step-index fibre

Multimode step-index fibre

Multimode graded-index fibre

n1 coren2 cladding

no air

n2 cladding

n1 core

Variablen

no air

Lightray

Index porfile


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


Acceptance Cone & Numerical Aperture

n2 cladding

n2 cladding

n1 coreAcceptance

Cone

Acceptance angle, c, is the maximum angle in whichexternal light rays may strike the air/fibre interfaceand still propagate down the fibre with <10 dB loss.

22

21

1sin nnC Numerical aperture:NA = sin c = (n1

2 - n22)

C


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


Absorption Losses In Optic FibreL

oss

(dB

/km

)

1

00.7 0.8

Wavelength (m)0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7

2

3

4

5

6

Peaks causedby OH- ions

Infraredabsorption

Rayleigh scattering& ultravioletabsorption


Fibre Alignment Impairments

Axial displacement Gap displacement

Angular displacement Imperfect surface finish


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


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


Optical Transmitter Circuits

+VCC

Data Input

Enable

C1

R1 LED

Q1

R2

+HV

ILD

C2Enable

C1

R1

Q1

R2

R3

Data Input


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


Photodetector Circuit

+V R1

--

++

+ -Threshold adjust

Enable

Comparatorshaper

PIN orAPD

DataOut


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, Lm (dB) is:

Lm = Pr - Ps = Pt - M - Lsf - (DxLf) - Lc - Lfd - Ps 0where 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).


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


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, Rr. For a real antenna, losses in the antenna can be represented by a loss resistance, Rd. Its efficiency is then:

dr

r

T

r

RR

R

P

P


3-D Antenna Radiation Pattern


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


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.

4;

4

2

2R

effD

GA

r

EIRPP


Impedance and Polarization

A half-wave dipole in free space and centre-fed has a radiation resistance of about 70 .

At resonance, the antenna’s impedance will be completely resistive and its efficiency maximum. If its length is < /2, it becomes capacitive, and if > /2, it is inductive.

The polarization of a half-wave dipole is the same as the axis of the conductor.


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.


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.


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


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


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.


Inductive and Capacitive Loading

Inductive LoadingCapacitive

Loading


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.


Yagi-Uda Array

More commonly known as the Yagi array, it has one driven element, one reflector, and one or more directors.

Radiationpattern


Characteristics of Yagi array

unidirectional radiation pattern (one main lobe, some sidelobes and backlobes)

relatively narrow bandwidth since it is resonant3-element array has a gain of about 7 dBimore directors will increase gain and reduce the

beamwidth and feedpoint impedancea folded dipole is generally used for the driven

element to widen the bandwidth and increase the feedpoint impedance.


Log-Periodic Dipole Array (LPDA)


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, = L1/L2 = D1/D2 = L2/L3 = ….


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


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


Broadside Array

all /2 elements are fed in phase and spaced /2 with axis placed vertically, radiation would have a

narrow bidirectional horizontal pattern


End-Fire Array

dipole elements are fed 90o out of phase resulting in a narrow unidirectional radiation pattern off the end of the antenna


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.


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.


Parabolic Reflector

small horn antenna is placed at focus of parabolic “dish”

beamwidth, , and gain, G, are given by:

2

22

;70

D

GD

where = wavelength in m, D = dish’s diameter in m

= antenna efficiency


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.

Documents

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

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