R. F. Systems EE731 Main Topics Transmission Line

1
R. F. Systems

• EE731

2
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
3
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.

4
Transmission Line Equivalent Circuit
L
L
L
L
R
R
Zo
Zo
C
C
C
C
G
G
Lossless Line
Lossy Line
5
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.

6
Formulas for Common Cables
For parallel two-wire line
D
d
m momr e eoer mo 4px10-7 H/m eo 8.854
pF/m
For co-axial cable
D
d
7
Transmission-Line Wave Propagation
Electromagnetic waves travel at lt c in a
transmission line because of the dielectric
separating the conductors. The velocity of
propagation is given by
m/s
Velocity factor, VF, is defined as
8
Time Delay Attenuation

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

(usually in ns/ft or ns/m)

• For coaxial cable,

ns/ft

• The phase shift coefficient,

radians/m

• Cable attenuation is expressed in dB/100 ft

9
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.

10
Reflection Coefficient
The reflection coefficient is defined as
It can also be shown that
Note that when ZL Zo, ? 0 when ZL 0, ?
-1 and when ZL open circuit, ? 1.
11
Standing Waves
Vmax Ei Er
Voltage
Vmin Ei - Er
l 2
With a mismatched line, the incident and
reflected waves set up an interference pattern on
the line known as a standing wave. The standing
wave ratio is
12
Other Formulas
When the load is purely resistive (whichever
gives an SWR gt 1)
Return Loss, RL Fraction of power reflected
?2, or -20 log ? dB So, Pr ?2Pi
Mismatched Loss, ML Fraction of
power transmitted/absorbed 1 - ?2 or -10
log(1-?2) dB So, Pt Pi (1 - ?2) Pi - Pr
13
Time-Domain Reflectometry
d
ZL
Transmission Line
Oscilloscope
Pulse or Step Generator
TDR is a practical technique for determining
the length of the line, the way it is terminated,
and the type and location of any impedance
discontinuities. The distance to the
discontinuity is d vt/2, where t elapsed
time of returned reflection.
14
Typical TDR Waveform Displays
Vr
Vr
Vi
Vi
t
RL lt Zo
RL gt Zo
ZL capacitive
ZL inductive
15
Transmission-Line Input Impedance
The input impedance at a distance l from the load
is
When the load is a short circuit, Zi jZo tan
(?l).
For 0 ? l lt ?/4, shorted line is inductive.
For l ?/4, shorted line a parallel resonant
circuit.
For ?/4 lt l ? ?/2, shorted line is capacitive.
16
T-L Input Impedance (contd)

• When the load is an open circuit, Zi -jZo cot
  (?l)
• For 0 lt l lt ?/4, open circuited line is
  capacitive.
• For l ?/4, open-line series resonant circuit.
• For ?/4 lt l lt ?/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

17
Transmission Line Summary
or
is equivalent to
l gt ?/4
l lt ?/4
is equivalent to
or
l gt ?/4
l lt ?/4
?/4
Zo
ZL

Zo
l ?/4
?/4-section Matching Transformer

18
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.

19
Basic Stripline Structure
Ground Planes
W
b
t
er
Solid Dielectric
Centre Conductor
20
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.

21
Microstrip
w
Circuit Line
t
b
?r (dielectric)
Ground Plane
Microstrip line employs a single ground plane,
the conductor 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, and partly in the
air space, the effective relative permittivity,
?eff, has to be used in the design instead of ?r.
22
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

23
Microstrip Directional Coupler
2
4
Conductor Lines
?/4
Dielectric
Ground Plane
Top View
Cross-sectional View
1
3
Most of the power into port 1 will flow to port
3. Some of the power will be coupled to port 2
but only a minute amount will go to port 4.
24
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.

25
Hybrid Ring Coupler
Input power at port 1 divides evenly between
ports 2 4 and none for port 3.
3l/4
4
1
l/4
l/4
Similarly, input at port 2 will divide evenly
between ports 1 and 3 and none for port 4.
l/4
3
2
One application circulator.
26
Microstrip Stripline Filters
?/4
IN
OUT
Side-coupled half-wave resonator band-pass filter
IN
OUT
L
L
L
L
C
C
C
Conventional low-pass filter
27
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.

28
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.

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E-Field Pattern of TE1 0 Mode
b
a
?g/2
End View
Side View
TEmn means there are m number of half-wave
variations of the transverse E-field along the
a side and n number of half-wave variations
along the b side.
The magnetic field (not shown) forms closed loops
horizontally around the E-field
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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.

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Wavelength for TE TM Modes
Cutoff wavelength

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

• TE10 is called the dominant mode since lc 2a
• is the longest wavelength of any mode.

Guide wavelength
32
Other Formulas for TE TM Modes
Group velocity
Phase velocity
Wave impedance
Zo 377 W for air-filled waveguide
33
Circular/Cylindrical Waveguides

• Differences versus rectangular waveguides
• lc 2pr/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.

34
Waveguide Terminations
lg/2
Dissipative Vane
Short-circuit
Sliding Short-Circuit
Side View
End View
Dissipative vane is coated with a thin film of
metal which in turn has a thin dielectric coating
for protection. Its impedance is made equal to
the wave impedance. The taper minimizes
reflection.
Sliding short-circuit functions like a shorted
stub for impedance matching purpose.
35
Attenuators
Resistive Flap
Max. attenuation when flap is fully inside. Slot
for flap is chosen to be at a non- radiating
position.
Pi
Po
Rotary-vane Type
Atten.(dB) 10 log (Pi/Po) Pi (dBm)-Po(dBm)
Max. attenuation when vane is at centre of guide
and min. at the side-wall.
Pi
Po
Sliding-vane Type
36
Iris Reactors
Inductive iris vanes are vertical

Capacitive iris vanes are horizontal

Irises can be used as reactance elements, filters
or impedance matching devices.

37
Tuning Screws
Tuning Screws
Post
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 advanced all the way into the waveguide,
it acts inductive. In between the two positions,
one can get a resonant LC circuit.
38
Waveguide T-Junctions
2
3
3
1
2
1
E-Plane Junction
H-Plane Junction
Input power at port 2 will split equally between
ports 1 and 3 but the outputs will be antiphase
for E-plane T and inphase for H-plane T. Input
power at ports 1 3 will combine and exit from
port 1 provided the correct phasing is used.
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Hybrid-T Junction
To RX
To antenna
2
3
1
4
Termination Load
From TX
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.
40
Hybrid-T Junction (contd)

• 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.

41
Directional Coupler
lg/4
P4
Termination
P3
P2
P1
P1
P2
2-hole Coupler
Holes spaced lg/4 allow waves travelling
toward port 4 to combine. Waves travelling
toward port 3, however, will cancel. Therefore,
ideally P3 0.
To broaden frequency response bandwidth,
practical couplers would usually have multi holes.
42
Directional Coupler (contd)
Definitions
Coupling Factor,
Directivity,
where P4(fwd) power out of aux. arm when power
in main arm is forward, and P4(rev) power out
of aux. arm when power in main arm is reversed.
Insertion Loss, (I.L.) 10 log (P1/P2) in dB
43
Cavity Resonators
Resonant wavelength for a rectangular cavity
b
L
a
For a cylindrical resonator
r
L
44
Cavity Resonators (contd)

• 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

45
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..

46
Examples of Ferrite Devices
Isolator
Attenuator
2
q
3
1
Differential Phase Shifter
4-port Circulator
4
47
Notes On Ferrite Devices

• Differential phase shifter - q 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.

48
Schottky Barrier Diode
Metal Electrode
Its based on a simple metal- semiconductor
interface. There is no p-n junction but a
depletion region exists. Current is by
majority carriers therefore, very low in
capacitance.
Contact
Semi- conductor Layer
SiO2 Dielectric
Substrate
Metal Electrode
Applications detectors, mixers, and switches.
49
Varactor Diode
Cj
Co
Circuit Symbol
V
Junction Capacitance Characteristic
Varactors operate under reverse-bias
conditions. The junction capacitance is
where Vb barrier potential (0.55 to 0.7 for
silicon) and K constant (often 1)
50
Equivalent Circuit for Varactor
The series resistance, Rs, and diode capacitance,
Cj, determine the cutoff frequency
Cj
Rj
Rs
The diode quality factor for a given frequency,
f, is
51
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

52
Parametric Amplifier Circuit
Degenerative Mode fp 2fs
Pump signal (fp)
Nondegenerative mode
L2
Upconversion - fi fs fp Downconversion - fi
fs - fp Power gain, G fi /fs
C2
C1
Input signal (fs)

• Regenerative mode
• negative resistance
• very low noise
• very high gain
• fp fs fi

C3
L3
D1
L1
Idler tank (fi)
Signal tank (fs)
53
PIN Diode
S1
RFC
R
V
P
C2
C1
I
In
Out
N
D1
PIN as shunt switch
PIN diode has an intrinsic region between the
P and N materials. It has a very high
resistance in the OFF mode and a very low
resistance when forward biased.
54
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)

55
Tunnel Diode
i
Ls
Ip
B
Cj
-R
A
C
Rs
V
Equivalent Circuit
Symbol
Vv
Vp
Characteristic Curve
Heavy doping of the semiconductor material
creates a very thin potential barrier in the
depletion zone which leads to electron tunneling
through the barrier. Note the negative resistance
zone between Vp and Vv.
56
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.

57
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.

58
Gunn Diode
The Gunn diode is a transferred electron device
that can be used in microwave oscillators or
one-port reflection amplifiers. Its basic
structure is shown below. N-, the active region,
is sandwiched between two heavily doped N
regions. Electrons from the
cathode (K) drifts to the anode (A) in
bunched formation called domains. Note that there
is no p-n junction.
l
N-
A
K
Metallic Electrode
Metallic Electrode
N
59
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 lt 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 gt 100 W.

60
Gunn Diode Circuit and Applications
Tuning Screw
The resonant cavity is shocked excited by current
pulses from the Gunn diode and the RF energy
is coupled via the iris to the waveguide.
Resonant Cavity
Iris
Diode
V
Gunn diode applications microwave source
for receiver local oscillator, police radars,
and microwave communication links.
61
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.

62
IMPATT Diode
A single-drift structure of an IMPATT
(impact avalanche transit time) diode is shown
below
-

P
N
N
l
Avalanche Region
Drift Region
Operating frequency
where vd drift velocity
63
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 (gt100 W
  pulsed) but are very noisy and not very
  efficient.

64
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.

65
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.

66
SAW Filter
Input
Output
l
Centre frequency
v propagation velocity
Comb electrode
Absorber
Piezoelectric substrate
Comb electrodes for exciting and receiving waves
are metallic deposit on a piezoelectric substrate.
67
SAW Resonator
Input
1-port resonator
Output

• 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.

68
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.

69
Magnetrons
It consists of a cylindrical cathode surrounded
by the anode with a number of resonant cavities.
Its a crossed-field device since the E-field is
perpendicular to the dc magnetic field.
Interaction Space
Waveguide Output
At a critical voltage the electrons from the
cathode will just graze the anode.
Cavity
Coupling Window
Anode
Cathode
70
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.

71
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 p-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.

72
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.

73
Two-Cavity Klystron
RF In
RF Out
Control Grid
Gap
Filament
Collector
Drift Region
Cathode
Buncher Cavity
Catcher Cavity
Anode
v
Electron Beam
Effect of velocity modulation
74
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.

75
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.

76
Reflex Klystron
Output
Anode
Cavity
Cathode
Repeller
Filament
Electron Beam
Vr
Condition for oscillation requires electron
transit time to be
where n an integer and T period of oscillation
77
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.

78
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.

79
Travelling-Wave Tube
RF In
RF Out
Helix
Collector
Attenuator
Electron Beam
The TWT is a linear beam device with the
magnetic field running parallel to tube
lengthwise. The helix is also known as a slow
wave structure to slow down the RF field so that
its velocity down the the tube is close to the
velocity of the electron beam.
80
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.

81
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 (gt
  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.

82
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.

83
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.

84
Parts Of A CATV System
Trunk Amplifier
Satellite
Trunk Cable
Microwave Link
Processor
Combiner
TV Stations
FM Radio
Headend
Distribution Amps
Feeder Cable
Splitter
Drop Cable
Cable Box
TV Set
Line Extender Amps
85
Signal Processing
Directional Coupler
Input From Other Heterodyne Processors
Mixer
Mixer
RF Amp
IF Amp
LO
LO
Combiner or Multiplexer

• Heterodyne processing is used to translate each
  signal
• to a different frequency at the headend. This
  prevents
• interference with local TV channels and allows
  satellite
• signals to be converted to a lower frequency for
  the cable.

86
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

87
Cable TV Spectrum
1.25 MHz
4.5 MHz
Video Carrier
Audio Carrier
f MHz
54
60
66
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.
88
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.

89
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 customers
  premises. Drop cables are limited in length to
  about 50 m.

90
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.

91
CATV Graphic Symbols
-3.5 dB
Input
Output
2-way splitter
-3.5 dB
Tap output
-7 dB
Directional Coupler
-7 dB
2-port tap
26
-7 dB
4-port tap
20
4-way splitter
-7 dB
8-port tap
14
92
Equalization
The differential in transmission loss through a
length of co-axial cable between the lowest
frequency of 50 MHz and the upper frequency of
400 MHz is significant. Equalization must be
applied at spaced distances of the cable to
correct the tilt of the signal spectrum.
Equalizer
400
50
400
50
MHz
MHz
Incoming signal tilt
Equalized output
93
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.

94
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.

95
Elements of System Design
Signal level (dBmV)
40
8
39
38.6
32.6
32
27
26.3
100
600
500
29
20
17
-1 dB
-6 dB
-5 dB
10
12.6
Drop input (dBmV)
10
0.7
0.4
0.6
Tap insert loss (dB)
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.
96
Two-Way Amplifier
50-400 MHz
50-400 MHz
Amp
HPF
HPF
LPF
LPF
5-30 MHz
5-30 MHz
Amp
Two-way amplifiers permit the cable subscriber
to transmit data (e.g. from a modem) to the
headend.
97
Cable Modem
Click Web ProForums for tutorial on cable modems.
98
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.

99
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