H. Chan; Mohawk College1 R. F. Systems EE731. H. Chan; Mohawk College2 Main Topics Transmission Line...

H. Chan; Mohawk College 1

R. F. Systems

EE731


Main Topics

• Transmission Line Characteristics

• Waveguides and Microwave Devices

• Cable Television Systems

Test #1 - Week #4 30 %Final Exam - Week #7 60 %TLM (Assignments) 10 %


Types of Transmission Lines

• Differential or balanced lines (where neither conductor is grounded): e.g. twin lead, twisted-cable pair, and shielded-cable pair.

• Single-ended or unbalanced lines (where one conductor is grounded): e.g. concentric or coaxial cable.

• Transmission lines for microwave use: e.g. striplines, microstrips, and waveguides.


Transmission Line Equivalent Circuit

R L R L

C G C G

L L

C C

“Lossy” Line Lossless Line

CjG

LjRZo

C

LZo

ZoZo


Notes on Transmission Line

• Characteristics of a line is determined by its primary electrical constants or distributed parameters: R (/m), L (H/m), C (F/m), and G (S/m).

• Characteristic impedance, Zo, is defined as the input impedance of an infinite line or that of a finite line terminated with a load impedance, ZL = Zo.


Formulas for Common Cables

D

d

D

d

d

DZ

dD

Cd

DL

r

o

2ln

120;

2ln

;2

ln

For parallel two-wire line:

For co-axial cable:

d

DZ

dD

Cd

DL

r

o ln60

;ln

2;ln

2

= or; = or; o = 4x10-7 H/m; o = 8.854 pF/m


Transmission-Line Wave Propagation

Electromagnetic waves travel at < c in a transmissionline because of the dielectric separating the conductors.The velocity of propagation is given by:

r

c

LCv

11m/s

Velocity factor, VF, is defined as:

rc

vVF

1


Time Delay & Attenuation•A signal will take time to travel down a transmission line. The amount of time delay is given by:

LCv

td 1

(usually in ns/ft or ns/m)

•For coaxial cable, rdt 016.1 ns/ft

•The phase shift coefficient, 2

radians/m

•Cable attenuation is expressed in dB/100 ft


Incident & Reflected Waves

• For an infinitely long line or a line terminated with a matched load, no incident power is reflected. The line is called a flat or nonresonant line.

• For a finite line with no matching termination, part or all of the incident voltage and current will be reflected.


Reflection Coefficient

The reflection coefficient is defined as:

i

r

i

r

I

Ior

E

E

It can also be shown that:

oL

oL

ZZ

ZZ

Note that when ZL = Zo, = 0; when ZL = 0, = -1;and when ZL = open circuit, = 1.


Standing WavesStanding Waves

Vmin = Ei - Er

With a mismatched line, the incident and reflectedwaves set up an interference pattern on the line known as a standing wave.The standing wave ratio is :

1

1

min

max

V

VSWR

Vmax = Ei + Er

Vol

tage


Other Formulas

When the load is purely resistive:(whichever gives an SWR > 1) L

o

o

L

R

Zor

Z

RSWR

Return Loss, RL = Fraction of power reflected= ||2, or -20 log || dBSo, Pr = ||2Pi

Mismatched Loss, ML = Fraction of powertransmitted/absorbed = 1 - ||2 or -10 log(1-||2) dBSo, Pt = Pi (1 - ||2) = Pi - Pr


Time-Domain Reflectometry

ZL

Pulse or StepGenerator

OscilloscopeTransmission Line

TDR is a practical technique for determining thelength of the line, the way it is terminated, and thetype and location of any impedance discontinuities.The distance to the discontinuity is: d = vt/2, wheret = elapsed time of returned reflection.

d


Typical TDR Waveform Displays

t

RL > Zo RL < Zo

ZL inductive ZL capacitive

ViVr

Vr

Vi


Transmission-Line Input Impedance

The input impedance at a distance l from the load is:

)tan(

)tan(

ljZZ

ljZZZZ

Lo

oLoi

When the load is a short circuit, Zi = jZo tan (l).

For 0 l < /4, shorted line is inductive.

For l = /4, shorted line = a parallel resonant circuit.

For /4 < l /2, shorted line is capacitive.


T-L Input Impedance (cont’d)

When the load is an open circuit, Zi = -jZo cot (l)

For 0 < l < /4, open circuited line is capacitive.

For l = /4, open-line = series resonant circuit.

For /4 < l < /2, open-line is inductive.

• A /4 line with characteristic impedance, Zo’, can

be used as a matching transformer between a resistive load, ZL, and a line with characteristic

impedance, Zo, by choosing: Loo ZZZ '


Transmission Line Summary

orl < /4 l > /4

is equivalent to:

l > /4or

l < /4

is equivalent to:

=

=

/4Zo

Zo’

ZL

/4-section Matching Transformer

l = /4


Substrate Lines

• Miniaturized microwave circuits use striplines and microstrips rather than coaxial cables as transmission lines for greater flexibility and compactness in design.

• The basic stripline structure consists of a flat conductor embedded in a dielectric material and sandwiched between two ground planes.


Basic Stripline Structure

Ground Planes

Centre Conductor Solid Dielectric

bW

t

r


Notes On Striplines

• When properly designed, the E and H fields of the signal are completely confined within the dielectric material between the two ground planes.

• The characteristic impedance of the stripline is a function of its line geometry, specifically, the t/b and w/b ratios, and the dielectric constant, r.

• Graphs, design formulas, or computer programs are available to determine w for a desired Zo, t, and b.


Microstrip

w t

b

Ground Plane

r (dielectric)

Circuit Line

Microstrip line employs a single ground plane, theconductor pattern on the top surface being open.Graphs, formulas or computer programs would be used to design the conductor line width. However, since the electromagnetic field is partly in the solid dielectric, andpartly in the air space, the effective relative permittivity, eff, has to be used in the design instead of r.


Stripline vs Microstrip

• Advantages of stripline:– signal is shielded from external interference– shielding prevents radiation loss r and mode of propagation are more

predictable for design

• Advantages of microstrip:– easier to fabricate, therefore less costly– easier to lay, repair/replace components


Microstrip Directional Coupler

/4

Top View Cross-sectional View

Conductor Lines

Dielectric

Ground Plane

1

2

3

4

Most of the power into port #1 will flow to port #3.Some of the power will be coupled to port #2 butonly a minute amount will go to port #4.


Coupler Applications

• Some common applications for couplers:– monitoring/measuring the power or frequency

at a point in the circuit– sampling the microwave energy for used in

automatic leveling circuits (ALC)– reflection measurements which indirectly yield

information on VSWR, ZL, return loss, etc.


Hybrid Ring Coupler

Input power at port#1 divides evenlybetween ports 2 & 4and none for port 3.

Similarly, input atport #2 will divideevenly between ports1 and 3 and none for port 4.One application: circulator.

1

23

4

3/4


Microstrip & Stripline Filters

/4IN

OUT

Side-coupled half-wave resonator band-pass filter

IN OUTLC C C

LL

Conventional low-pass filter

L


Microwave Radiation Hazards

• The fact that microwaves can be used for cooking purposes and in heating applications suggests that they have the potential for causing biological damage.

• An exposure limit of 1 mW/cm2 for a maximum of one hour duration for frequencies from 10 MHz to 300 GHz is generally considered safe.

• Avoid being in the direct path of a microwave beam coming out of an antenna or waveguide.


Waveguides

• Reasons for using waveguide rather than coaxial cable at microwave frequency:– easier to fabricate– no solid dielectric and I2R losses

• Waveguides do not support TEM waves inside because of boundary conditions.

• Waves travel zig-zag down the waveguide by bouncing from one side wall to the other.


E-Field Pattern of TE1 0 Mode

a

b

g/2

End View Side View

TEmn means there are m number of half-wave variationsof the transverse E-field along the “a” side and n numberof half-wave variations along the “b” side.The magnetic field (not shown) forms closed loops horizontally around the E-field


TE and TM Modes

• TEmn mode has the E-field entirely transverse, i.e. perpendicular, to the direction of propagation.

• TMmn mode has the H-field entirely transverse to the direction of propagation.

• All TEmn and TMmn modes are theoretically permissible except, in a rectangular waveguide, TMmo or TMon modes are not possible since the magnetic field must form a closed loop.

• In practice, only the dominant mode, TE10 is used.


Wavelength for TE & TM Modes

Any signal with c will not propagate down the waveguide. For air-filled waveguide, cutoff freq., fc = c/c

Guide wavelength: 22 /1/1 ffor

cc

g

TE10 is called the dominant mode since c = 2a is the longest wavelength of any mode.

22 //

2

bnamc

Cutoff wavelength:


Other Formulas for TE & TM Modes

Group velocity: 2/1 cg

g corcv

Phase velocity: 2/1 c

gp

corcv

Wave impedance:

2

2

/1

/1

coTM

c

oTE

ZZ

ZZ

Zo = 377 for air-filled waveguide


Circular/Cylindrical Waveguides

• Differences versus rectangular waveguides : c = 2r/Bmn where r = waveguide radius, and Bmn is

obtained from table of Bessel functions.

– All TEmn and TMmn modes are supported since m and n subscripts are defined differently.

– Dominant mode is TE11.

• Advantages: higher power-handling capacity, lower attenuation for a given cutoff wavelength.

• Disadvantages: larger and heavier.


Waveguide Terminations

Dissipative Vane

Side View End View

Short-circuit

Sliding Short-Circuit

g/2

Dissipative vane is coated with a thin film of metalwhich in turn has a thin dielectric coating forprotection. Its impedance is made equal to thewave impedance. The taper minimizes reflection.Sliding short-circuit functions like a shorted stubfor impedance matching purpose.


Attenuators

Resistive Flap

Sliding-vane Type

Rotary-vane Type

Max. attenuation when flapis fully inside. Slot for flapis chosen to be at a non-radiating position.

Max. attenuation when vane is at centre of guide and min.at the side-wall.

Atten.(dB) = 10 log (Pi/Po) = Pi (dBm)-Po(dBm)

Pi Po

Pi Po


Iris Reactors

=

=

=

Inductive iris; vanes are vertical

Capacitive iris; vanes are horizontal

Irises can be used as reactanceelements, filters or impedancematching devices.


Tuning Screws

A post or screw can also serve as a reactive element.When the screw is advanced partway into the wave-guide, it acts capacitive. When the screw is advancedall the way into the waveguide, it acts inductive. Inbetween the two positions, one can get a resonant LCcircuit.

PostTuning Screws


Waveguide T-Junctions

1

23

1 2

3

E-Plane Junction H-Plane Junction

Input power at port 2 will split equally between ports 1 and3 but the outputs will be antiphase for E-plane T and inphasefor H-plane T. Input power at ports 1 & 3 will combine andexit from port 1 provided the correct phasing is used.


Hybrid-T Junction

1

23

4

It combines E-plane and H-plane junctions.Pin at port 1 or 2 will divide between ports 3 and 4.Pin at port 3 or 4 will divide between ports 1 and 2.

To antenna

TerminationLoad

From TX

To RX


Hybrid-T Junction (cont’d)

• If input power of the same phase is applied simultaneously at ports 1 and 2, the combined power exits from port 4. If the input is out-of-phase, the output is at port 3.

• Applications:– Combining power from two transmitters.– TX and a RX sharing a common antenna.– Low noise mixer circuit.


Directional Coupler

P1 P2

P4Terminationg/4

P3

2-hole Coupler

Holes spaced g/4 allow waves travelling towardport 4 to combine. Waves travelling toward port 3,however, will cancel. Therefore, ideally P3 = 0.To broaden frequency response bandwidth, practicalcouplers would usually have multi holes.

P1 P2


Directional Coupler (cont’d)

Definitions:

Coupling Factor, 4

1log10)(P

PdBC

Directivity,

)(4

)(4log10)(rev

fwd

P

PdBD

Insertion Loss, (I.L.) = 10 log (P1/P2) in dB

where P4(fwd) = power out of aux. arm when power in mainarm is forward, and P4(rev) = power out of aux. arm when power in main arm is reversed.


Cavity Resonators

a

b L

Resonant wavelength for arectangular cavity:

222 )/()/()/(

2

Lpbnamr

L

rFor a cylindrical resonator:

22

2

Lp

rBmn

r


Cavity Resonators (cont’d)

• Energy is coupled into the cavity either through a small opening, by a coupling loop or a coupling probe. These methods of coupling also apply for waveguides

• Applications of resonators:– filters– absorption wavemeters– microwave tubes


Ferrite Components

• Ferrites are compounds of metallic oxides such as those of Fe, Zn, Mn, Mg, Co, Al, and Ni.

• They have magnetic properties similar to ferromagnetic metals and at the same time have high resistivity associated with dielectrics.

• Their magnetic properties can be controlled by means of an external magnetic field.

• They can be transparent, reflective, absorptive, or cause wave rotation depending on the H-field..


Examples of Ferrite Devices

Attenuator Isolator

DifferentialPhase Shifter

1

2

3

4

4-portCirculator


Notes On Ferrite Devices

• Differential phase shifter - is the phase shift between the two directions of propagation.

• Isolator - permits power flow in one direction only.• Circulator - power entering port 1 will go to port 2

only; power entering port 2 will go to port 3 only; etc.

• Most of the above are based on Faraday rotation.• Other usage: filters, resonators, and substrates.


Schottky Barrier Diode

Semi-conductor

LayerSubstrate

Contact

SiO2

Dielectric

MetalElectrode

MetalElectrode

It’s based on a simple metal-semiconductor interface.There is no p-n junction buta depletion region exists.Current is by majoritycarriers; therefore, very lowin capacitance.

Applications: detectors, mixers, and switches.


Varactor Diode

Circuit Symbol V

Cj Co

Junction Capacitance Characteristic

Varactors operate under reverse-bias conditions.The junction capacitance is:

mb

oj VV

KCC

)( where Vb = barrier potential

(0.55 to 0.7 for silicon)and K = constant (often = 1)


Equivalent Circuit for Varactor

Cj Rj

Rs

The series resistance, Rs, and diodecapacitance, Cj, determine thecutoff frequency:

jsc CRf

2

1

The diode quality factor for a given frequency, f, is:

f

fQ c


Varactor Applications

• Voltage-controlled oscillator (VCO) in AFC and PLL circuits

• Variable phase shifter

• Harmonic generator in frequency multiplier circuits

• Up or down converter circuits

• Parametric amplifier circuits - low noise


Parametric Amplifier Circuit

Pump signal (fp)

Inputsignal

(fs)L1

C1C2

L2

D1 L3C3

Signaltank (fs)

Idlertank (fi)

Nondegenerative mode:Upconversion - fi = fs + fp

Downconversion - fi = fs - fp

Power gain, G = fi /fs

Regenerative mode: negative resistance very low noise very high gainfp = fs + fi

Degenerative Mode: fp = 2fs


PIN Diode

P+

IN+

+VR RFC

C1C2

S1

D1

InOut

PIN as shunt switch

PIN diode has an intrinsic region between the P+

and N+ materials. It has a very high resistance inthe OFF mode and a very low resistance whenforward biased.


PIN Diode Applications

• To switch devices such as attenuators, filters, and amplifiers in and out of the circuit.

• Voltage-variable attenuator• Amplitude modulator• Transmit-receive (TR) switch• Phase shifter (with section of transmission

line)


Tunnel Diode

Symbol

Ls

CjRs-R

EquivalentCircuit

i

VVv

Ip

Vp

Characteristic CurveHeavy doping of the semiconductor material createsa very thin potential barrier in the depletion zonewhich leads to electron tunneling through the barrier.Note the negative resistance zone between Vp and Vv.

B CA


More Notes On Tunnel Diode

• Tunnel diodes can be used in monostable (A or C), bistable (between A and C), or astable (B) modes.

• These modes lead to switching, oscillation, and amplification applications.

• However, the power output levels of the tunnel diode are restricted to a few mW only.


Transferred Electron Devices

• TEDs are made of compound semiconductors such as GaAs.

• They exhibit periodic fluctuations of current due to negative resistance effects when a threshold voltage (about 3.4 V) is exceeded.

• The negative resistance effect is due to electrons being swept from a lower valley (more mobile) region to an upper valley (less mobile) region in the conduction band.


Gunn Diode

The Gunn diode is a transferred electron device thatcan be used in microwave oscillators or one-port reflection amplifiers. Its basic structure is shownbelow. N-, the active region, is sandwiched betweentwo heavily doped N+ regions. Electrons from the

N-

MetallicElectrode N+ Metallic

Electrode

cathode (K) drifts tothe anode (A) in bunchedformation called domains.Note that there is no p-njunction.

AK

l


Gunn Operating Modes

• Stable Amplification (SA) Mode: diode behaves as an amplifier due to negative resistance effect.

• Transit Time (TT) Mode: operating frequency, fo = vd / l where vd is the domain velocity, and l is the effective length. Output power < 2 W, and frequency is between 1 GHz to 18 GHz.

• Limited Space-Charge (LSA) Mode: requires a high-Q resonant cavity; operating frequency up to 100 GHz and pulsed output power > 100 W.


Gunn Diode Circuit and Applications

TuningScrew

Diode

ResonantCavity

Iris

VGunn diode applications: microwave source forreceiver local oscillator, police radars, andmicrowave communication links.

The resonant cavityis shocked excited bycurrent pulses fromthe Gunn diode andthe RF energy iscoupled via the iristo the waveguide.


Avalanche Transit-Time Devices

• If the reverse-bias potential exceeds a certain threshold, the diode breaks down.

• Energetic carriers collide with bound electrons to create more hole-electron pairs.

• This multiplies to cause a rapid increase in reverse current.

• The onset of avalanche current and its drift across the diode is out of phase with the applied voltage thus producing a negative resistance phenomenon.


IMPATT Diode

A single-drift structure of an IMPATT (impactavalanche transit time) diode is shown below:

P+ N N+- +

lDrift Region

AvalancheRegion

Operating frequency:l

vf d

2 where vd = drift

velocity


Notes On IMPATT Diode

• The current build-up and the transit time for the current pulse to cross the drift region cause a 180o phase delay between V and I; thus, negative R.

• IMPATT diodes typically operate in the 3 to 6 GHz region but higher frequencies are possible.

• They must operate in conjunction with an external high-Q resonant circuit.

• They have relatively high output power (>100 W pulsed) but are very noisy and not very efficient.


Microwave Transistors

• Silicon BJTs and GaAsFETs are most widely used.

• BJT useful for amplification up to about 6 MHz.• MesFET (metal semiconductor FET) and HEMT

(high electron mobility transistor) are operable beyond 60 GHz.

• FETs have higher input impedance, better efficiency and more frequency stable than BJTs.


SAW Devices

• Surface Acoustic Wave is an ultrasonic wave that traverses the polished surface of a piezoelectric substrate such as quartz and lithium niobate.

• Examples of SAW devices: filters, resonators, delay lines, and oscillators.

• Applications of SAW devices: mobile telephone, DBS receiver, pager, CATV converter, cordless phone, UHF radio, measuring equipment , etc.


SAW FilterInput Output

Absorber

Piezoelectricsubstrate

Combelectrode

ofCentre

frequency

v = propagationvelocity

Comb electrodes for exciting and receiving waves are metallicdeposit on a piezoelectric substrate.


SAW Resonator

• The frequency of the resonator depends upon the pitch between the teeth of the comb electrodes.

• One-port resonators have high Q factors and are primarily used as oscillators.

Input

Output

1-portresonator


Microwave Tubes

• Classical vacuum tubes have several factors which limit their upper operating frequency:– interelectrode capacitance & lead inductance– dielectric losses & skin effect– transit time

• Microwave tubes utilize resonant cavities and the interaction between the electric field, magnetic field and the electrons.

Heng Chan ; Mohawk College 69

Magnetrons

It consists of a cylindrical cathode surrounded by theanode with a number of resonant cavities.

WaveguideOutput

CouplingWindow

CathodeAnode

InteractionSpace

Cavity

It’s a crossed-fielddevice since the E-fieldis perpendicular to thedc magnetic field.At a critical voltagethe electrons from the cathode will just grazethe anode.


Magnetron Operation

• When an electron cloud sweeps past a cavity, it excites the latter to self oscillation which in turn causes the electrons to bunch up into a spoked wheel formation in the interaction space.

• The continuous exchange of energy between the electrons and the cavities sustains oscillations at microwave frequency.

• Electrons will eventually lose their energy and fall back into the cathode while new ones are emitted.


More Notes On Magnetrons

• Alternate cavities are strapped (i.e., shorted) so that adjacent resonators are 180o out of phase. This enables only the dominant -mode to operate.

• Frequency tuning is possible either mechanically (screw tuner) or electrically with voltage.

• Magnetrons are used as oscillators for radars, beacons, microwave ovens, etc.

• Peak output power is from a few MW at UHF and X-band to 10 kW at 100 GHz.


Klystrons

• Klystrons are linear-beam devices since the E-field is parallel to the static magnetic field.

• Their operation is based on velocity and density modulation with resonating cavities to create the bunching effect.

• They can be employed as oscillators or power amplifiers.


Two-Cavity Klystron

Filament

RF In RF OutControlGrid

Cathode

Anode BuncherCavity

CatcherCavity

Collector

Gap

DriftRegion

Effect of velocity modulation

vElectronBeam


Klystron Operation

• RF signal applied to the buncher cavity sets up an alternating field across the buncher gap.

• This field alternately accelerates and decelerates the electron beam causing electrons to bunch up in the drift region.

• When the electron bundles pass the catcher gap, they excite the catcher cavity into resonance.

• RF power is extracted from the catcher cavity by the coupling loop.


Multicavity Klystrons

• Gain can be increased by inserting intermediate cavities between the buncher and catcher cavity.

• Each additional cavity increases power gain by 15- to 20-dB.

• Synchronous tuned klystrons have high gain but very narrow bandwidth, e.g. 0.25 % of fo.

• Stagger tuned klystrons have wider bandwidth at the expense of gain.

• Can operate as oscillator by positive feedback.


Reflex Klystron

OutputAnode

Filament

Cathode RepellerCavity

Vr

ElectronBeam

Condition for oscillation requires electron transittime to be:

Tnt

4

3 where n = an integer andT = period of oscillation


Reflex Klystron Operation

• Electron beam is velocity modulated when passing though gridded gap of the cavity.

• Repeller decelerates and turns back electrons thus causing bunching.

• Electrons are collected on the cavity walls and output power can be extracted.

• Repeller voltage, Vr, can be used to vary output frequency and power.


Notes On Reflex Klystrons

• Only one cavity used.

• No external dc magnetic field required.

• Compact size.

• Can be used as an oscillator only.

• Low output power and low efficiency.

• Output frequency can be tuned by Vr , or by changing the dimensions of the cavity.


Travelling-Wave Tube

RF In RF Out

Collector

Helix

AttenuatorElectron Beam

The TWT is a linear beam device with the magneticfield running parallel to tube lengthwise.The helix is also known as a slow wave structure toslow down the RF field so that its velocity down thethe tube is close to the velocity of the electron beam.


TWT Operation

• As the RF wave travels along the helix, its positive and negative oscillations velocity modulate the electron beam causing the electrons to bunch up.

• The prolonged interaction between the RF wave and electron beam along the TWT results in exponential growth of the RF voltage.

• The amplified wave is then extracted at the output.• The attenuator prevents reflected waves that can

cause oscillations.


Notes On TWTs

• Since interaction between the RF field and the electron beam is over the entire length of the tube, the power gain achievable is very high (> 50 dB).

• As TWTs are nonresonant devices, they have wider bandwidths and lower NF than klystrons.

• TWTs operate from 0.3 to 50 GHz.• The Twystron tube is a combination of the TWT

and klystron. It gives better gain and BW over either the conventional TWT or klystron.


Master Antenna TV Systems

• For apartments and condos, a watered down form of cable TV, called MATV system can be used.

• The basic MATV system consists of a single broadband antenna mounted on the roof, broadband amplifiers, distribution cables, splitters, and subscriber outlets.

• It eliminates antennas cluttering the roof of the apartment building but reception is limited to local TV stations.


Cable TV Systems

• Today, the majority of homes receive cable TV where signals from antennas, satellites, studio, and other sources go to the headend first.

• The signals are processed, scrambled where necessary, and combined or frequency multiplexed onto a single cable for distribution.

• In addition to TV signals, cable also provide other services such as FM stations, pay TV, specialized programming, internet, distance education, etc.


Parts Of A CATV System

Headend

Satellite

TV StationsMicrowave Link

FM Radio Pro

cess

or

Com

bine

r Trunk Amplifier TrunkCable

DistributionAmps Feeder

Cable

Line Extender Amps

Drop CableSplitter

CableBox

TVSet


Signal Processing

•Heterodyne processing is used to translate each signalto a different frequency at the headend. This preventsinterference with local TV channels and allows satellitesignals to be converted to a lower frequency for the cable.

LORF

Amp

Mixer

IFAmp LO

Mixer

Input From OtherHeterodyne Processors

Combiner or Multiplexer

DirectionalCoupler


Cable TV Channels

• Low Band VHF: Ch. 2 to Ch. 6; 54 MHz to 88 MHz

• FM Channels: 88 MHz to 108 MHz

• Mid Band VHF: Ch. A1 to Ch. I; 108 MHz to 174 MHz

• High Band VHF: Ch. 7 to Ch. 13; 174 MHz to 216 MHz

• Super Band: Ch. J to Ch. W; 216 MHz to 300 MHz

• Hyper Band: Ch. AA to Ch. RR; 300 MHz to 408 MHz


Cable TV Spectrum

54 60fMHz66

VideoCarrier Audio

Carrier

1.25 MHz

4.5 MHz

Channel 2 Channel 3

Each TV channel occupies a bandwidth of 6 MHz.Audio info occupies a bandwidth of about 80 kHz.Video info occupies the rest of the channel.


Trunk Cable

• After amplification, the combined signals are sent to one or more trunk cables.

• Each trunk cable, constructed out of a large, low-loss coaxial cable, carries the signals to a series of distribution points. Booster amplifiers (max. 30-40) spaced at about 1 km intervals are usually required to restore the signal strength.

• Fibre-optic cables are now replacing coaxial cables as trunks since their losses are much lower.


Feeder & Drop Cables

• Feeder cables branch out from trunks to serve local neighbourhoods.

• A maximum of 2 line extender amplifiers are allowed per feed.

• Feeder cables are tapped at periodic locations for connection by co-ax drop cables to customer’s premises. Drop cables are limited in length to about 50 m.


Passive CATV Devices

• Splitters: They are used mainly for dividing RF energy equally to their output ports.

• Directional Couplers: They allow a portion of the RF energy in the main cable to be fed to a distribution or feeder cable.

• Taps: They are used to tap off RF energy from the feeder cable to the subscriber. They possess the combined features of the splitter and the directional coupler.


CATV Graphic Symbols

-3.5 dB

-3.5 dB

-7 dB

-7 dB

-7 dB

-7 dB

2-waysplitter

4-waysplitter

Tap output

Input Output

Directional Coupler

2-port tap

4-port tap

8-port tap

26

20

14


EqualizationThe differential in transmission loss through a length of co-axial cable between the lowest frequency of 50MHz and the upper frequency of 400 MHz is significant.Equalization must be applied at spaced distances of thecable to correct the “tilt” of the signal spectrum.

Equalizer

50 400MHz

50 400MHz

Incoming signal “tilt” Equalized output


Noise & Distortions

• In the CATV system, noise may be generated in amplifiers or picked up from external sources.

• Since a large number of channels are combined in the system, second and higher order intermodulation distortions can be a serious problem.

• All devices used in the CATV system must be impedance-matched to avoid reflections and echoes.


Amplifiers and AGC

• Since the resistance of co-ax cables varies with temperature and there are hundreds of km of cable, CATV amplifiers must have automatic gain control (AGC) to compensate for the variations in cable loss.

• Cascading lower-gain amplifiers would give the highest quality of transmission in terms of noise and intermodulation distortion for a given distance, but will incur higher initial & operating costs.


Elements of System Design

100’-1 dB

600’-6 dB

500’-5 dB

29 20 17

408 39 38.6 32.6 32 27 26.3Signallevel(dBmV)

10 12.6 10Drop input (dBmV) :

Tap insert loss (dB) : 0.4 0.6 0.7

Standard tap values are (in dB): 8, 11, 14, 17, 20, 23, 26, 29, 32.Tap insertion loss ranges from 0.4 dB to 2.8 dB.The desired signal level to the drop cable is about 10 dBmV.


Two-Way Amplifier

HPF HPF

LPF LPF

50-400MHz

50-400MHz

5-30MHz

5-30MHz

Amp

Amp

Two-way amplifiers permit the cable subscriber totransmit data (e.g. from a modem) to the headend.


Cable Modem

Click Web ProForums for tutorial on cable modems.


Cableless TV Systems

• Direct Broadcasting Satellites (DBS) enable consumers to receive multi-channel TV signals with a pizza-sized dish and a set-top box.

• Another alternative is to use a Multichannel Multipoint Distribution System (MMDS) where TV signals are received via a microwave beam at about 2.5 GHz.


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