Transcript Principles of Electronic Communication Systems

Principles of Electronic
Communication Systems
Second Edition
Louis Frenzel
© 2002 The McGraw-Hill Companies
Principles of Electronic
Communication Systems
Second Edition
Chapter 13
Transmission Lines
©2003 The McGraw-Hill Companies
Transmission Lines
Transmission lines in communication carry telephone
signals, computer data in LANs, TV signals in cable
TV systems, and signals from a transmitter to an
antenna or from an antenna to a receiver.
Topics Covered in Chapter 13
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Transmission-Line Basics
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Standing Waves
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Transmission Lines as Circuit Elements
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The Smith Chart
Transmission-Line Basics
The two primary requirements of a transmission line
are:
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The line should introduce minimum attenuation to the
signal
The line should not radiate any of the signal as radio
energy
Types of Transmission Lines
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Parallel-wire line is made of two parallel conductors separated
by a space of ½ inch to several inches.
A variation of parallel line is the 300-Ω twin-lead where the
spacing between the wires is maintained by a continuous
plastic insulator.
The most widely used type of transmission line is the coaxial
cable, which consists of a solid center conductor surrounded
by a dielectric material, usually a plastic insulator such as
Teflon.
Coaxial cable comes in sizes from ¼ inch to several inches in
diameter.
Open Wire 300-Ω Twin Lead
Coaxial Cable
Balanced Versus Unbalanced Lines
Transmission lines can be balanced or unbalanced.
 A balanced line is one in which neither wire is
connected to ground.
 The signal on each wire is referenced to ground.
 In an unbalanced line, one conductor is connected to
ground.
 Open-wire line has a balanced configuration.
 Twisted-pair lines may be balanced or unbalanced.
Balanced Versus Unbalanced Lines
(Continued)
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Balanced-line wires offer significant protection from
noise pickup and cross talk.
Coaxial cable and twisted pair provide significant but
not complete protection from noise or cross talk.
Unshielded lines may pick up signals and cross talk
and can even radiate energy, resulting in an
undesirable loss of signal.
A device called a balun is used to convert from
balanced to unbalanced lines and vice versa.
Unbalanced Line
Wavelength of Cables
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The electrical length of conductors is typically short compared
to 1 wavelength of the frequency they carry.
A pair of current-carrying conductors is not considered to be a
transmission line unless it is at least 0.1 λ long at the signal
frequency.
Wavelength is the length or distance of one cycle of an AC
wave or the distance that an AC wave travels in the time
required for one cycle of the signal.
The distance represented by a wavelength in a given cable
depends on the type of cable.
Connectors
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Coaxial cables are designed not only to provide a
convenient way to attach and disconnect equipment
and cables but also to maintain the physical integrity
and electrical properties of the cable.
The most common types are the PL-259 or UHF,
BNC, F, SMA, and N-type connectors.
The PL-259, also referred to as a UHF connector, can
be used up to low UHF frequencies (less than 500
MHz.)
Connectors (Continued)
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BNC connectors are widely used on 0.25 inch coaxial cables
for attaching test equipment.
In BNC connectors the center conductor of the cable is
soldered or crimped to a male pin and the shield braid is
attached the body of the connector.
The least expensive coaxial connector is the F-type, which is
used for TV sets, VCRs, and cable TV.
The RCA phonograph connector is a coaxial connector which
is used primarily in audio equipment.
The best performing coaxial connector is the N-type, which is
used mainly on large coaxial cable at higher frequencies.
The F Connector
RCA Phonograph Connector
Characteristic Impedance
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When the length of transmission line is longer than
several wavelengths at the signal frequency, the two
parallel conductors of the transmission line appear as
a complex impedance.
An RF generator connected to a considerable length
of transmission line sees an impedance that is a
function of the inductance, resistance, and
capacitance in the circuit—the characteristic or surge
impedance (Z0).
Velocity Factor
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The speed of the signal in the transmission line is
slower than the speed of a signal in free space.
The velocity of propagation of a signal in a cable is
less than the velocity of propagation of light in free
space by a fraction called the velocity factor.
Velocity factors in transmission lines vary from
approximately 0.5 to 0.9.
The velocity factor of a coaxial cable is typically in
the 0.6 to 0.8 range.
Time Delay
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Because the velocity of propagation of a transmission
line is less than the velocity of propagation in free
space, any line will slow down or delay any signal
applied to it.
A signal applied at one end of a line appears some
time later at the other end of the line.
This is called the time delay or transit time.
A transmission line used specifically for the purpose
of achieving delay is called a delay line.
Transmission-Line Specifications
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Many coaxial cables are designated by an
alphanumeric code beginning with the letters RG or a
manufacturer’s part number.
Primary specifications are characteristic impedance
and attenuation.
Other important specifications are maximum
breakdown voltage rating, capacitance per foot,
velocity factor, and outside diameter in inches.
The attenuation is the amount of power lost per 100 ft
of cable expressed in decibels at 100 MHz.
Transmission-Line Specifications
(Continued)
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Attenuation is directly proportional to cable length
and increases with frequency.
A transmission line is a low-pass filter whose cutoff
frequency depends on distributed inductance and
capacitance along the line and on length.
It is important to use larger, low-loss cables for longer
runs despite cost and handling inconvenience.
A gain antenna can be used to offset cable loss.
Standing Waves
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When a signal is applied to a transmission line, it
appears at the other end of the line some time later
because of the propagation delay.
If the load on the line is an antenna, the signal is
converted into electromagnetic energy and radiated
into space.
If the load at the end of the line is an open or a short
or has an impedance other than the characteristic
impedance of the line, the signal is not fully absorbed
by the load.
Standing Waves (Continued)
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When a line is not terminated properly, some of the
energy is reflected and moves back up the line,
toward the generator.
This reflected voltage adds to the forward or incident
generator voltage and forms a composite voltage that
is distributed along the line.
The pattern of voltage and its related current
constitute what is called a standing wave.
By Definition…
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A matched transmission line is one terminated in a load that
has a resistive impedance equal to the characteristic impedance
of the line.
Alternating voltage (or current) at any point on a matched line
is a constant value and the line is said to be flat.
The power sent down the line toward the load is called
forward or incident power.
Power not absorbed by the load is called reflected power.
The magnitude of the standing waves on a transmission line is
determined by the ratio of the maximum current to the
minimum current along the line.
Transmission Line Terminated in Its
Characteristic Impedance
Transmission Lines as Circuit
Elements
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The standing wave conditions resulting from openand short-circuited loads must usually be avoided
when working with transmission lines.
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With quarter- and half-wavelength transmissions,
these open- and short-circuited loads can be used as
resonant or reactive circuits.
Resonant Circuits and Reactive
Components
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Shorted and open quarter wavelengths act like LC
tuned or resonant circuits at the reference frequency.
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If the line length is more or less than one-quarter
wavelength, the transmission line looks like a
capacitor or inductor at the reference frequency.
Stripline and Microstrip
Special transmission lines constructed with copper
patterns on a printed circuit board (PCB), called
microstrip or stripline , can be used as tuned circuits,
filters, phase shifters, reactive components, and
impedance-matching circuits at high frequencies.
Microstrip
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Microstrip is a flat conductor separated by an
insulating dielectric from a large conducting ground
plane.
The microstrip is usually a quarter or half wavelength
long.
The ground plane is the circuit common and this is
equivalent to an unbalanced line.
The characteristic impedance of microstrip is
dependent on its physical characteristics.
Microstrip
Stripline
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Stripline is a flat conductor sandwiched between two
ground planes.
It is more difficult to make than microstrip; however,
it does not radiate as microstrip does.
The length is one-quarter or one-half wavelength at
the desired operating frequency.
Shorted lines are more commonly used than open
lines.
Characteristic impedance is dependent on its physical
characteristics.
Stripline
The Smith Chart
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The mathematics required to design and analyze
transmission lines is complex, whether the line is a
physical cable connecting a transceiver to an antenna
or is being used as a filter or impedance-matching
network.
This is because the impedances involved are complex
ones, involving both resistive and reactive elements.
The impedances are in the familiar rectangular form,
R + jX.
The Smith Chart (Continued)
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Computations with complex numbers are long and timeconsuming.
Many calculations involve trigonometric relationships and
although no individual calculation is difficult, the sheer
volume of the calculations can lead to error.
The Smith Chart published in 1939, is a sophisticated graph
that permits visual solutions to transmission-line calculations.
Despite the availability of the computing options today, this
format provides a more or less standardized way of viewing
and solving transmission-line and related problems.
The Smith Chart