Download 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 15
Microwave Communication
©2003 The McGraw-Hill Companies
Microwave Communication
Microwaves are the ultra-high, super-high, and
extremely high frequencies directly above the lower
frequency ranges where most radio communication
now takes place and below the optical frequencies
that cover infrared, visible, and ultraviolet light.
Topics Covered in Chapter 15
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Microwave Concepts
Microwave Transistor Amplifiers
Waveguides and Cavity Resonators
Microwave Semiconductor Diodes
Microwave Tubes
Microwave Antennas
Microwave Applications
Microwave Frequencies and Bands
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The practical microwave region is generally
considered to extend from 1 to 30 GHz, although
frequencies could include up to 300 GHz.
Microwave signals in the 1- to 30-GHz have
wavelengths of 30 cm to 1 cm.
The microwave frequency spectrum is divided up into
groups of frequencies, or bands.
Frequencies above 40 GHz are referred to as
millimeter (mm) waves and those above 300 GHz are
in the submillimeter band.
Microwave Frequency Bands
Benefits of Microwaves
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A major approach to solving the problem of spectrum
crowding has been to move into higher frequency ranges.
Initially, the VHF and UHF bands were tapped, however,
today most new communication services are assigned to the
microwave region.
At higher frequencies there is a greater bandwidth available for
the transmission of information.
Wide bandwidths make it possible to use various multiplexing
techniques.
Transmission of high-speed binary information requires wide
bandwidths and these are easily transmitted on microwave
frequencies.
Disadvantages of Microwaves
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The higher the frequency, the more difficult it
becomes to analyze electronic circuits.
At microwave frequencies, conventional components
become difficult, if not impossible to implement.
Microwave signals, like light waves, travel in
perfectly straight lines and therefore communication
distance is limited to line-of-sight range.
Microwave signals penetrate the ionosphere, so
multiple-hop communication is not possible.
Microwave Communication
Systems
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Like any other communication system, a microwave
communication system uses transmitters, receivers,
and antennas.
The same modulation and multiplexing techniques
used at lower frequencies are also used in the
microwave range.
The RF part of the equipment, however, is physically
different because of the special circuits and
components that are used to implement the
components.
Transmitters
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Like any other transmitter, a microwave transmitter
starts with a carrier generator and a series of amps.
It also includes a modulator followed by more stages
of power amplification.
The final power amplifier applies the signal to the
transmission line and antenna.
A transmitter arrangement could have a mixer used to
up-convert an initial carrier signal with or without
modulation to the final microwave frequency.
Microwave Transmitter using Up
Conversion
Receivers
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Microwave receivers, like low-frequency receivers,
are the superheterodyne type.
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Their front ends are made up of microwave
components.
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Most receivers use double conversion.
Receiver Operation
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The antenna is connected to a tuned circuit, which could be a
cavity resonator or microstrip or stripline tuned circuit.
The signal is then applied to a special RF amplifier known as a
low-noise amplifier (LNA).
Another tuned circuit connects the amplifier input signal to the
mixer.
The local oscillator signal is applied to the mixer.
The mixer output is usually in the UHF or VHF range.
The remainder of the receiver is typical of other
superheterodynes.
Microwave Receiver
Transmission Lines
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Coaxial cable, most commonly used in lowerfrequency communication has very high attenuation
at microwave frequencies and conventional cable is
unsuitable for carrying microwave signals.
Special microwave coaxial that can be used on bands
L, S, and C is made of hard tubing and this low-loss
coaxial cable is known as hard line cable.
At higher microwave frequencies, a special hollow
rectangular or circular pipe called waveguide is used
for the transmission line.
Antennas
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At low microwave frequencies, standard antenna
types, including the simple dipole and quarter-wave
length vertical antennas, are still used.
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At these frequencies antennas are very small; for
example, a half-wave dipole at 2 GHz is about 3 in.
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At higher microwave frequencies, special antennas
are generally used.
Microwave Transistor Amplifiers
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Although vacuum and microwave tubes like the
klystron and magnetron are still used, most
microwave systems use transistor amplifiers.
Special geometries are used to make bipolar
transistors that provide voltage and power gain at
frequencies up to 10 GHz.
Microwave FET transistors have also been created.
Monolithic microwave integrated circuits (MMICs)
are widely used.
Microstrip Tuned Circuits
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At higher frequencies, standard techniques for
implementing lumped components such as coils and
capacitors are not possible.
At microwave frequencies, transmission lines,
specifically microstrip, are used.
Microstrip is preferred for reactive circuits at the
higher frequencies because it is simpler and less
expensive than stripline.
Stripline is used where shielding is necessary.
Microstrip Transmission Line used
for Reactive Circuit
Equivalent Circuits of Open and
Shorted Microstrip Lines
Microwave Transistors
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The primary differences between standard lowerfrequency transistors and microwave types are
internal geometry and packaging.
To reduce internal inductances and capacitances of
transistor elements, special chip configurations,
known as geometries are used.
Geometries permit the transistor to operate at higher
power levels and at the same time minimize
distributed and stray inductances and capacitances.
Microwave Transistors (Continued)
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The GaAs MESFET, a type of JFET, uses a Schottky
barrier junction and can operate at frequencies in
excess of 5 GHz.
A high electron mobility transistor (HEMT) is a
variant of the MESFET and extends the range beyond
20 GHz by adding an extra layer of semiconductor
material such as AlGaAs.
A popular device known as a heterojunction bipolar
transistor (HBT) is making even higher-frequency
amplification possible in discrete form and integrated
circuits.
Microwave Transistor: NPN Bipolar
Power
Small-Signal Amplifiers
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A small-signal microwave amplifier can be made up
of a single transistor or multiple transistors combined
with a biasing circuit and any microstrip circuits or
components as required.
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Most microwave amplifiers are of the tuned variety.
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Another type of small-signal microwave amplifier is
a multistage integrated circuit, a variety of MMIC.
Transistor Amplifiers
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A low-noise transistor with a gain of about 10 to 25
dB is typically used as a microwave amplifier.
Most microwave amplifiers are designed to have
input and output impedances of 50 Ω.
The transistor is biased into the linear region for class
A operation.
Radio frequency chokes (RFCs) and ferrite beads
(FB) are used to keep RF out of the supply and
prevent feedback which can cause oscillation and
circuit instability.
Monolithic Microwave Integrated
Circuit
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A common monolithic microwave integrated circuit
(MMIC) amplifier is one that incorporates two or
more stages of FET or bipolar transistors made on a
common chip to form a multistage amplifier.
The chip also incorporates resistors for biasing and
small bypass capacitors.
Physically, these devices look like transistors.
Another form of MMIC is the hybrid circuit, which
combines an amplifier IC connected to microstrip
circuits and discrete components.
Power Amplifiers
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A typical microwave power amplifier is designed
with microstrip lines which are used for impedance
matching and tuning.
Input and output impedances are 50 Ω.
Typical power-supply voltages are 12, 24, and 28
volts.
Most power amplifiers obtain their bias from
constant-current sources.
A single-stage FET power amplifier can achieve a
power output of 100 W in the high UHF region.
Constant-Current Bias Supply
Waveguides
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Most microwave energy transmission above 6 GHz is
handled by waveguides.
Waveguides are hollow metal conducting pipes
designed to carry and constrain the electromagnetic
waves of a microwave signal.
Most waveguides are rectangular.
Waveguides are made from copper, aluminum or
brass.
Often the insides of waveguides are plated with silver
to reduce resistance and transmission losses.
Waveguide
By Definition…
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The electric field is at a right angle to the direction of wave
propagation, so it is called a transverse field.
An interconnection of two sections of waveguide is called a
choke joint.
A T section, or T junction is used to split or combine two or
more sources of microwave power.
Waveguides and their fittings are precision-made so that the
dimensions match perfectly.
Directional couplers are used to facilitate the measurement of
microwave power in a waveguide and the SWR.
Choke Joint
Directional Coupler
Cavity Resonator
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A cavity resonator is a waveguide-like device that
acts like a high-Q parallel resonant circuit.
A simple cavity resonator can be formed with a short
piece of waveguide one-half wavelength long.
Energy is coupled into the cavity with a coaxial probe
at the center.
The internal walls of the cavity are often plated with
silver or some other low-loss material to ensure
minimum loss and maximum Q.
Cavity Resonator
Circulators
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A circulator is a three-port microwave device used for
coupling energy in only one direction around a closed
loop.
Microwave energy is applied to one port and passed
to another with minor attenuation, however the signal
will be greatly attenuated on its way to a third port.
The primary application of a circulator is a diplexer,
which allows a single antenna to be shared by a
transmitter and receiver.
Circulator Used as a Diplexer
Isolators
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Isolators are variations of circulators, but they have
one input and one output.
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They are configured like a circulator, but only ports 1
and 2 are used.
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Isolators are often used in situations where a
mismatch, or the lack of a proper load, could cause
reflection so large as to damage the source.
Small Signal Diodes
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Diodes used for signal detection and mixing are the
most common microwave semiconductor devices.
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Two types of microwave diodes are:
Point-contact diode
 Schottky barrier or hot-carrier diode
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Point-Contact Diode
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The oldest microwave semiconductor device is the
point-contact, also called a crystal diode.
A point-contact diode is a piece of semiconductor
material and a fine wire which makes contact with the
semiconductor material.
Point-contact diodes are ideal for small-signal
applications.
They are widely used in microwave mixers and
detectors and in microwave power measurement
equipment.
Point-Contact Diode
Hot Carrier Diodes
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For the most part, point-contact diodes have been
replaced by Schottky diodes, sometimes referred to as
hot carrier diodes.
Like the point-contact diode, the Schottky diode is
extremely small and therefore has a tiny junction
capacitance.
Schottky diodes are widely used in balanced
modulators and mixers.
They are also used as fast switches at microwave
frequencies.
Schottky Diode
Frequency-Multiplier Diodes
Microwave diodes designed primarily for frequencymultiplier service include:
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Varactor diodes
Step-recovery diodes
Varactor Diodes
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A varactor diode is basically a voltage variable
capacitor.
When a reverse bias is applied to the diode, it acts
like a capacitor.
A varactor is primarily used in microwave circuits as
a frequency multiplier.
Varactors are used in applications in which it is
difficult to generate microwave signals.
Varactor diodes are available for producing relatively
high power outputs at frequencies up to 100 GHz.
Varactor Frequency Multiplier
Step-Recovery Diodes
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A step-recovery diode or snap-off varactor is widely
used in microwave frequency-multiplier circuits.
A step-recovery diode is a PN-junction diode made
with gallium arsenide or silicon.
When it is forward-biased, it conducts like any diode,
but a charge is stored in the depletion layer.
When reverse bias is applied, the charge keeps the
diode on momentarily and then turns off abruptly.
This snap-off produces a high intensity pulse that is
rich in harmonics.
Oscillator Diodes
Three types of diodes other than the tunnel diode that
can oscillate due to negative resistance characteristics
are the:
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Gunn diode
IMPATT diode
TRAPATT diode
Gunn Diodes
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Gunn diodes, also called transferred-electron devices
(TEDs), are not diodes in the usual sense because
they do not have junctions.
A Gunn diode is a thin piece of N-type gallium
arsenide (GaAs) or indium phosphide (InP)
semiconductor which forms a special resistor when
voltage is applied to it.
The Gunn diode exhibits a negative-resistance
characteristic.
Gunn diodes oscillate at frequencies up to 50 GHz.
IMPATT and TRAPATT Diodes
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Two microwave diodes widely used as oscillators are
the IMPATT and TRAPATT diodes.
Both are PN-junction diodes made of silicon, GaAs,
or InP.
They are designed to operate with a high reverse bias
that causes them to avalanche or break down.
IMPATT diodes are available with power ratings up
to 25 W to frequencies as high as 30 GHz.
IMPATT are preferred over Gunn diodes if higher
power is required.
PIN Diodes
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A PIN diode is a special PN-junction diode with an I
(intrinsic) layer between the P and the N sections.
The P and N layers are usually silicon, although GaAs
is sometimes used and the I layer is a very lightly
doped N-type semiconductor.
PIN diodes are used as switches in microwave
circuits.
PIN diodes are widely used to switch sections of
quarter- or half-wavelength transmission lines to
provide varying phase shifts in a circuit.
Microwave Tubes
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Vacuum tubes are devices used for controlling a large
current with a small voltage to produce amplification,
oscillation, switching, and other operations.
Vacuum tubes are widely used in microwave
transmitters requiring high output power.
Special microwave tubes such as the klystron, the
magnetron, and the traveling-wave tube are widely
used for microwave power amplification.
Klystron
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A klystron is a microwave vacuum tube using cavity
resonators to produce velocity modulation of an
electron beam which produces amplification.
Klystrons are no longer widely used in most
microwave equipment.
Gunn diodes have replaced the smaller reflex
klystrons in signal-generating applications because
they are smaller and lower in cost.
The larger multicavity klystrons are being replaced by
traveling-wave tubes in high-power applications.
Magnetron
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A widely used microwave tube is the magnetron, a
combination of a simple diode vacuum tube with
built-in cavity resonators and an extremely powerful
permanent magnet.
Magnetrons are capable of developing extremely high
levels of microwave power.
When operated in a pulsed mode, magnetrons can
generate several megawatts of power.
An application for a continuous-wave magnetron is
for heating purposes in microwave ovens.
Magnetron Tube Oscillator
Traveling-Wave Tube
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One of the most versatile microwave RF power
amplifiers is the traveling-wave tube (TWT), which
can generate hundreds and even thousands of watts of
microwave power.
The main advantage of the TWT is an extremely wide
bandwidth.
Traveling-wave tubes can be made to amplify signals
in a range from UHF to hundreds of gigahertz.
A common application of TWTs is as power
amplifiers in satellite transponders.
Microwave Antennas
Because of the line-of-sight transmission of microwave
signals, highly directive, high gain antennas are
preferred because they do not waste the radiated
energy and because they provide an increase in gain,
which helps offset noise at microwave frequencies.
Low-Frequency Antennas
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At low microwave frequencies, less than 2 GHz,
standard antennas are commonly used, including the
dipole and its variations such as the bow-tie, the Yagi,
and the ground-plane antenna.
The corner reflector is a fat, wide-bandwidth, halfwave dipole fed with low-loss coaxial cable.
The overall gain of a corner reflector antenna is 10 to
15 dB.
Corner Reflector
Horn Antenna
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The problem with using a waveguide as a radiator is
that it provides a poor impedance match with free
space and this results in standing waves.
This mismatch can be offset by simply flaring the end
of the waveguide to create a horn antenna.
Horn antennas have excellent gain and directivity.
The gain and directivity of a horn are a direct
function of its dimensions; the most important
dimensions are length, aperture area, and flare angle.
Horn Antenna
Parabolic Antenna
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A parabolic reflector is a large dish-shaped structure
made of metal or screen mesh.
The energy radiated by the horn is pointed at the
reflector, which focuses the radiated energy into a
narrow beam and reflects it toward its destination.
Beamwidths of only a few degrees are typical with
parabolic reflectors.
Narrow beamwidths also represent extremely high
gains.
Helical Antennas
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A helical antenna, as its name suggests, is a wire
helix.
A center support is used to hold heavy wire or tubing
formed into a circular coil or helix.
The diameter of the helix is typically one-third
wavelength, and the spacing between turns is
approximately one-quarter wavelength.
The gain of a helical antenna is typically in the 12- to
20-dB range and beamwidths vary from
approximately 12 to 45 degrees.
Helical Antenna
Slot Antenna
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A slot antenna is a radiator made by cutting a halfwavelength slot in a conducting sheet of metal or
into the side or top of a waveguide.
The slot antenna has the same characteristics as a
standard dipole antenna, as long as the metal sheet is
very large compared to 1 λ at the operating frequency.
Slot antennas are widely used on high-speed aircraft
where the antenna can be integrated into the metallic
skin of the aircraft.
Slot Antennas on a Waveguide
Dielectric (Lens) Antennas
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Dielectric, or lens antennas use a special dielectric
material to collimate or focus the microwaves from a
source into a narrow beam.
An example of a lens antenna is one used in the
millimeter-wave range.
Lens antennas are usually made of polystyrene or
some other plastic, although other types of dielectric
can be used.
Their main use is in the millimeter range above 40
GHz.
Dielectric Lens Antenna Operation
Patch Antenna
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Patch antennas are made with microstrip on PCBs.
The antenna is a circular or rectangular area of copper
separated from the ground plane on the bottom of the
board by the PCBs insulating material.
Patch antennas are small, inexpensive, and easy to
construct.
Their bandwidth is directly related to the thickness of
the PCB material.
Their radiation pattern is circular in the direction
opposite to that of the ground plane.
Phased Arrays
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A phased array is an antenna system made up of a
large group of similar antennas on a common plane.
Patch antennas on a common PCB can be used, or
separate antennas like dipoles can be physically
mounted together in a plane.
Most phased arrays are used in radar systems, but
they are finding applications in some cell phone
communication systems and in satellites.
Microwave Applications
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Radar
Satellite
Wireless computer and personal area networks
Wireless broadband access to the Internet
Cell phones
Heating
Radar
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The electronic communication system known as radar
(radio detection and ranging) is based on the principle
that high-frequency RF signals are reflected by
conductive targets.
In a radar system, a signal is transmitted toward the
target and the reflected signal is picked up by a
receiver in the radar unit.
The radar unit can determine the distance to a target
(range), its direction (azimuth), and in some cases, its
elevation (distance above the horizon).