Transcript Radar Transmitter

Simplified Radar Block Diagram
Antenna
Target
Waveguide
Transmitter
Duplexer
Modulator
Master
clock
Receiver
Signal
processor
(computer)
Display
Key Components of a Radar System
•
•
•
Transmitter
• Electronic device used to generate the
microwave EM energy transmitted by the
radar
Receiver
• Electronic device used to detect the
microwave pulse that is reflected by the area
being imaged by the radar
Antenna
• Electronic component through which
microwave pulses are transmitted and
received
CW radars
Target speed
Measurements
Doppler shift
Range
Measurements
Frequency-modulation (FM)
The transmitted wave is varied and range is
determined by observing the lag in time between
this modulation and the corresponding
modulation of the received echoes.
Doppler Shift
Small, low-power versions of CW Doppler radars are used as:
Speed sensors (police radar)
Vehicle detectors for traffic control
Proximity fuzes in rockets, bombs, and projectiles.
In these applications:
The range to the target is usually small
The loss in sensitivity because of the use of a single antenna is acceptable .
M/A-COM Gunnplexer Doppler transceiver,
which packs a transmitter, ferrite circulator,
and mixer into a single module.
An X-band Doppler transceiver
Mechanical tuning coarsely sets frequency,
whereas fine tuning and AFC can be
provided by modulating the operating
voltage. (U.S. Army photo.)
A Gunn oscillator is the basic transmitter, which is coupled to a single
antenna through the circulator. Transmitter power reflected back from
the antenna port acts as the local oscillator into the single balanced
mixer (an adjustable screw allows intentional standing-wave ratio
(SWR) mismatch to force an adequate level of return signal).
The addition of an antenna, frequency meter, and a direct-current (DC)
power source completes the radar.
Block diagram for a
simple single-antenna
CW Doppler radar
based on a Doppler
transceiver.
CW Radar: w.r.t Pulsed radar
-Less complex - Low cost - Lower operating voltage, and in some cases (high power)
uses two antennas (Wastes in area)
Pulsed radar
The pulsed radar transmitter:
Generates powerful pulses of EM energy at precise intervals
High-power microwave oscillator (magnetron)
Microwave amplifier (klystron), supplied by a low-power RF source
Modulator:
Properly-timed, high-amplitude, rectangular pulse
• High-power oscillator
Switches the oscillator on and off
• Microwave power amplifier
Activates the amplifier just before the arrival of an electromagnetic
pulse from a preceding stage or a frequency-generation source.
In Amplifiers, the modulator pulse is supplied to the cathode of the
power tube and the plate is at ground potential to shield personnel from
shock hazards because of the extremely high voltage involved.
The modulator pulse may be more than 100 KV in high-power radar
transmitters.
Radar transmitters produce:
Voltages, currents, and radiation hazards that are extremely dangerous
to personnel. Safety precautions must always be strictly observed when
working in or around a radar transmitter
Common Features of Radar Transmitter
• It is usually large fraction of radar system
• High cost
• Large size
• Heavy
• Requires significant efforts
• It requires a major share of system prime power
and maintenance, because Radars are required to
generate so much power output
• Most people prefer to keep away from it
Range & Power Relation
4
R
 P×A×T
R
Detection Range
P
Transmitter Power
A
Aperture area
T
Scanning time (the time allowed to scan the required
solid angle of coverage which limits how long the signal in
each direction can be collected and integrated to improve S/N)
P & A Trade off
Huge & Costly Antenna
No sense
Tiny inexpensive Transmitter
Doubling the Tiny part
Cutting the huge part in half
Reduce the total system cost
Reasonable balance (according to the
application) minimizing the total cost
Target carrying selfscreening Jammer
2
R

Pr × Ar
Pj × Aj
Pr & Ar are still the driving factors
Balanced System Design Results
in Significant Transmitter Power
Max Radar Performance pushed the antenna aperture A
and the transmitter power P to max affordable values
Common Microwave Components of Radar
Transmitters
• Wave Guide Components
• High power Microwave Generations
Oscillators (Magnetron)
Amplifiers
• Modulators
Wave Guide Concepts and features
•
•
•
•
•
Pipe through which waves propagate
Can have various cross sections
– Rectangular
X
– Circular
– Elliptical
Can be rigid or flexible
a
Waveguides have very low loss
High Power
Z
b
Waveguide can handle power levels far in excess of coaxial line ratings.
Because there is no center conductor, waveguide is much less susceptible
to shock and vibration during shipping and installation. No center conductor
means no insulators and consequently lower loss.
Y
Metallic waveguides can transport a significant power. Its value depends
on the medium filling the guide, surface quality, humidity, pressure,
possible temperature elevation, and frequency. If the guide is filled with
dry air, the electric field may not go beyond 3 MV/m, which corresponds
to a power range of 10 MWat 4GHz and 100 kW at 40 GHz.
Discontinuities and irregularities in the waveguide may impose a security
factor of 4 or more. Furthermore, losses in copper walls are of the order of
0.03 dB/m at 4GHz and 0.75 dB/m at 40GHz (5).
TE10 Mode
Mode with lowest cutoff frequency is dominant mode
•Single mode propagation is
highly desirable to reduce
dispersion
•This occurs between cutoff
frequency for TE10 mode and
twice that frequency
Circular Waveguide
Waveguide components commonly used in Radars
Wave guide Tee
Hybrid Tee
The hybrid coupler is used some applications, namely,
 Mixers
 Modulators
 Isolated power splitters since the isolation between its input
ports may be independent of the value of the two balanced
impedance loads.
Port 4
Port 1
Port 2
Port 3
Mechanical Switches
Direct s microwave power from one transmission line to another or turns
microwave power on and off. Switches can be mechanically or
electronically. Here we discuss some types of mechanical switchs.
Electronically switches will be introduced in active devices section.
Waveguide Terminations
Tapered absorber, usually consisting of a carbonimpregnated dielectric material that absorbs the
microwave power
8.2 – 12.4 GHz
handles 75 watts
GHz7 - 10
watt300
Important specifications:
 SWR (or S11)
 Power-handling capability
Wave guide coupler
Coaxial and microstrip coupler
High power
Wide band
High directivity
Poor directivity
limited in BW
Limited power
D is not critical for sampling microwave
power
D is extremely important for a return
loss measurement, to measure the
small power reflected from the
mismatch.
Coaxial coupler
Duplexer
Circulator
Circulator route microwave signals from one port of the device to
another:
1.
Power entering port 1 is directed out of the circulator at port 2.
2.
A signal entering port 2 is routed to leave the circulator at port 3 and
does not get back into port 1.
3.
A signal entering port 3 does not get into port 2, but goes out through
port 1.
3
The S matrix
of an ideal
circulator is
2
1
[S] =
0
0 1
1
0
0
0
1
0
The important specifications of a circulator:
Insertion loss: The loss of signal as it travels in the right direction
(typically 0.5 dB)
Directivity
The loss in the signal as it travel in the wrong direction
(Typically 20dB)
Circulator enable the use of one antenna for both transmitter and receiver
of communication system.
Receiver
Receiver
Transmitter
Transmitter
High Isolation Path
Low Loss Path
Two possible methods of achieving high output
power in microwave system
Low power
High power tube
semiconductor
amplifier
precise oscillator
High power
tube
oscillator
TYPES OF MICROWAVE TUBES
Tubes
Advantages
Common
Applications
Traveling wave tube (TWT)
amplifier
Wide bandwidth
Radars;
Communications;
jammers
Klystron amplifier
High gain & high h
Radar; medical
applications
Magnetron oscillator
low-cost
Radars
Domestic cooking;
industrial heating
of materials
Gyrotron oscillator
High average power
In band (30–300
GHz)
Radar; Plasma
heating in
controlled
thermonuclear
fusion research
High Power RF Generation
Pulsed Oscillator System
Precise low power source
+
Amplifiers
(Usually) Magnetron
Many stages (each with its own
power supplies and control)
All stages must be stable
Important features could not be
provided using Magnetron
Complexity
and cost
• Coded pulsed
• Frequency agility
• Combining and arraying
Oscillators Versus Amplifiers
Issues of Selection
(1) Accuracy and Stability of Carrier Frequency
■ Magnetron frequency is affected by:
□ Tub warmup drift
□ Pushing
□ Temperature drift
□ Pulling
■ In Amplifiers
□ Frequency depends on the low power crystal oscillator.
Frequency can be changed instantaneously by electronic switching
(faster than mechanical tuner)
(2) Coherence
- Amplifier based transmitter:
Coherent RF and IF LO are generated with precision
- Oscillator-based transmitter:
Manual tuning or an automatic frequency control (AFC) to tune the
LO to the correct frequency.
(3) Instabilities Terms include – frequency – phase shift – coho locking –
pulse timing – pulse width – pulse amplitude – jitter
Amplifier Chains: Special Considerations.
1. Timing.
•
Because modulator rise times differ, triggers to each amplifier
stage must usually be separately adjusted to provide proper
synchronization without excessive wasted beam energy.
2. Isolation.
•
Each intermediate stage of a chain must see proper load
match
3. Matching
•
Improved amplifier ratings are sometimes available if the tube
is guaranteed to see a good match.
•
CFAs and traveling wave tubes (TWTs) generally require that
wide band matching (than BW of operation) for stability
4. Signal-to-Noise Ratio.
•
Output S/N cannot be better than that of the worst stage
5- Leveling. (to maintain constant power with frequency)
6- Stability Budgets.
Each stage must have better stability than the overall requirement
on the transmitter, since the contributions of all stages may add.
Such stability budgets are usually required for pulse-to-pulse
variations, for intra-pulse variations, and sometimes for phase
linearity.
7. RF Leakage.
Keeping the chain from oscillating requires leakage, from the
output to the input, to be below certain level.
8- Reliability
The complexity of transmitter amplifier chains often makes it
difficult to achieve the desired reliability. Solutions usually involve
the use of redundant stages or a whole redundant chain, and many
combinations of switching are feasible.
9- RF Amplifiers.
availability of suitable RF amplifier devices
linear-beam tubes (Klystrons & TWTs )
direction of the dc Electric field that accelerates the beam coincides
with the axis of the Magnetic field that focuses and confines the
beam.
Crossed field tubes (magnetrons and CFAs)
The electric and magnetic fields are at right angles to each other.
MAGNETRON TRANSMITTERS
Invented during World War II
The 5J26, magnetron based , has been used in search radars for over 40 years
• operates at L- band
• mechanically tunable from 1250 to 1350 MHz.
• 500-kW peak power (t =1ms) and 1000 pps, or (t =2ms) and 500 pps
(0.001 duty cycle) and provides 500 W of average RF power.
• h = 40%
• The 1- to 2-ms pulse duration provides 150- to 300-m range resolution
Magnetron Features
High peak power
Quite small and Simple
low cost
Pulsed magnetrons vary from a 1-in3, 1-kW peak-power to several megawatts
peak and several kW average power
CW magnetrons have been made up to 25 kW for industrial heating.
Stable enough for MTI operation
Automatic frequency control (AFC) is typically used to keep the receiver tuned
to the transmitter
Magnetron Features Cont.
Tuners
High-power magnetrons can be mechanically tuned over a 5 to 10
percent frequency range routinely, and in some cases as much as 25
percent.
Rotary Tuning
The rotary-tuned ("spin-tuned") magnetron was developed
around I960. A slotted disk is suspended above the anode cavities as
when rotated, alternately provides inductive and capacitive loading
of the cavities to raise and lower the frequency. (Less average
output power)
The process begins with a low voltage being applied to
the filament, which causes it to heat up.
Remember, in a magnetron tube, the filament is also the
cathode. The temperature rise causes increased
molecular activity within the cathode, to the extent that it
begins to "boil off" or emit electrons. Electrons leaving
the surface of a heated filament wire might be compared
to molecules that leave the surface of boiling water in
the form of steam. Unlike steam, though, the electrons
do not evaporate. They float, or hover, just off the
surface of the cathode, waiting for some momentum.
Electrons, being negative charges, are strongly repelled
by other negative charges. So this floating cloud of
electrons would be repelled away from a negatively
charged cathode.
RF outp
The lectrons encounter the powerful magnetic field of
two permanent magnets . These are positioned so that
their magnetic fields are applied parallel to the cathode.
The effect of the magnetic fields tends to deflect the
speeding electrons away from the anode.
Electrons form rotating pattern
Magnetron Limitations
Magnetrons are not suitable if:
1. Precise frequency control is needed
2. Precise frequency jumping (within a pulse or within a pulse group) is
required
3. The best possible stability is required. not stable enough to be
suitable for very long pulses (e.g., 100 mS), and starting jitter limits
their use at very short pulses (e.g., 0.1 mS), especially at high power
and lower frequency bands.
4. Coherence is required from pulse to pulse for second-time-around
clutter cancellation, etc.
5. Coded or shaped pulses are required. A range of only a few
decibels of pulse shaping is feasible with a magnetron, and even
then frequency pushing may prevent obtaining the desired
benefits.
6. Lowest possible spurious power levels are required.
Magnetrons cannot provide a very pure spectrum but instead
produce considerable electromagnetic interference (EMI) across
a bandwidth much wider than their signal bandwidth (coaxial
magnetrons are somewhat better in this respect).
Common Problems in Magnetron
1. Sparking
Especially when a magnetron is first started, it is normal for anode-tocathode arcing to occur on a small percentage of the pulses.
2. Moding: If other possible operating-mode conditions exist too close to
the normal-mode current level, stable operation is difficult to achieve.
Starting in the proper mode requires the proper rate of rise of
magnetron cathode voltage, within limits that depend on the tube
starting time and the closeness of other modes.
3. Noise rings: Excessive inverse voltage following the pulse, or even a
small forward "postpulse" of voltage applied to the magnetron, may
make it produce sufficient noise to interfere with short-range
target echoes. The term noise ring is used because this noise occurs
at a constant delay after the transmitted pulse and produces a circle
on a plan position indicator (PPI). This can also occur if the pulse
voltage on the magnetron does not fall fast enough after the pulse.
4. Spurious RF output: In addition to their desired output power,
magnetrons generate significant amounts of spurious noise.
5. RF leakage out of the cathode stem: Typically, an S-band tube may
radiate significant VHF and UHF energy as well as fundamental and
harmonics out of its cathode stem. This effect varies greatly among different
magnetrons, and when it occurs, it also varies greatly with lead
arrangements, filament voltage, magnetic field, etc. Although it is preferable
to eliminate cathode stem leakage within the tube, it has sometimes been
successfully trapped, absorbed, or tolerated outside the tube.
6. Drift: Magnetron frequency varies with ambient temperature according to
the temperature coefficient of its cavities, and it may also vary significantly
during warmup.
7. Pushing: The amount by which a magnetron's frequency varies
with changes in anode current is called its pushing figure and the
resulting pulse-to-pulse and intra-pulse frequency changes must be
kept within system requirements by proper modulator design.
8. Pulling: The amount by which a magnetron's frequency varies as
the phase of a mismatched load is varied is called its pulling figure.
9. Life: Although some magnetrons have short wear-out life, many
others have short life because of miss-handling by inexperienced
personnel. Dramatic increases in average life have been obtained by
improved handling procedures and proper operator training.
Amplifiers
Capability of RF Amplifiers
Klystron Amplifiers
High gain
High-power capability
~ 20 % tuning bandwidth
Two Cavity
Two Cavity
Multi-Cavity Klystron
Microwave
input
Electron
beam
Microwave
output
Beam
collector
Electron
Gun
Intermediate cavity
In a klystron:
•The electron gun produces a flow of electrons.
•The bunching cavities regulate the speed of the electrons
so that they arrive in bunches at the output cavity.
•The bunches of electrons excite microwaves in the output
cavity of the klystron.
•The microwaves flow into the waveguide, which transports
them to the accelerator.
•The electrons are absorbed in the beam stop.
TWT
High bandwidth ~ one octave (low-power (few KW) helix type)
TWT vs. Klystron
Similarities:
• Beam formation, focusing and collection are the same
• Input and output rf coupling are similar
• TWT uses a traveling wave version of the discreet cavity
interaction of the klystron
• Large overlays in beam voltage, current and rf power output
Differences:
•
•
•
•
Bandwidth
Klystron ≈ 1%
Waveguide TWT ≈ 10%
Transmission Line (Helix) TWT ≈ 1 - 3 octaves
• Form factor more amenable to low-cost, light-weight PPM
focusing
Helix and contra-wound helix derived circuits
Coupled-cavity circuit
Crossed-Field Amplifiers (CFAs.
High efficiency
small size
Relatively low-voltage operation
Cover from UHF to K band
Attractive for:
• lightweight systems
•airborne use
• Low gain (~10 dB)
• CFAs are generally used only in the one or two highestpower stages of an amplifier chain, where they may
offer an advantage in efficiency, operating voltage, size,
and/or weight compared with linear-beam tubes.
• The output-stage CFA is usually preceded by a mediumpower TWT that provides most of the chain gain.
• CFAs have also been used to boost the power output of
previously existing radar systems.
If Prequired < Pavailable of a single tube
Combine the RF Power
of More tubes
Very
Complex
This Makes Solid State
Transmitter Practical
Combining and Arraying
It is often necessary to use more than one RF tube or solid-state device to
produce the required radar transmitter RF power output. Since the mid1950s, two or more microwave tubes have often been used to achieve more
total power output than can be obtained from a single tube. Since about
1960, there has been interest in using more than one RF device, especially if
it can then be solid-state, to provide increased system reliability from the
greatly lowered probability of multiple failures.
Combiners Include:
Magic T
Multi-branch Wilkenson
P1 the output power of the first tube
P2 the output power of the second tube
q the angle between the two combined outputs
Ways of Combining Power
Common way of operating two
identical devices in parallel.
(Magic-T as a splitter and Combiner)
The two outputs are recombined only
in space but the devices are still
effectively operating in parallel.
(Magic-T as a splitter)
Two whole chains operating in parallel; but the
greater the number of items that are included in
each of the two paths, the more chance exists for
phase differences to occur between the two
paths as a function of frequency, temperature, or
component tolerances.
Therefore, combining chains is more difficult than combining single stages and is usually avoided.
Solid State Amplifiers (SSAs)
Compared with tubes, solid-state devices offer many advantages:
1. No hot cathodes are required; therefore, there is no warmup delay, no
wasted heater power, and virtually no limit on operating life.
2. Device operation occurs at much lower voltages; therefore, power supply
voltages are on the order of volts rather than kilovolts. This avoids the need
for large spacings, oil filling, or encapsulation, thus saving size and weight
and leading to higher reliability of the power supplies as well as of the
microwave power amplifiers themselves.
3. Transmitters designed with solid-state devices exhibit improved mean time
between failures (MTBF) in comparison with tube-type transmitters. Module
MTBFs greater than 100,000 h have been measured.
4. No pulse modulator is required. Solid-state microwave devices
for radar generally operate Class-C, which is self-pulsing as the RF
drive is turned on and off.
5. Graceful degradation of system performance occurs when
modules fail. This results because a large number of solid-state
devices must be combined to provide the power for a radar
transmitter, and they are easily combined in ways that degrade
gracefully when individual units fail.
6. Extremely wide bandwidth can be realized. While high-power
microwave radar tubes can achieve 10 to 20 percent bandwidth,
solid-state transmitter modules can achieve up to 50 percent
bandwidth or more with good efficiency.
7. Flexibility can be realized for phased array applications. For
phased array systems, an active transceiver module can be
associated with every antenna element. RF distribution losses that
normally occur in a tube-powered system between a point-source
tube amplifier and the face of the array are thus eliminated.
Single SSA module
•Broad bandwidth, low power, moderate gain, low noise, low efficiency
devices
•Small size, low cost manufacturing process
•Ideal for use as drivers for high power sources
•Two basic transistor types BJTs and FETs
•Both are used at 3 GHz for power amplifiers but FETs dominate at higher
frequencies
•Both are limited in frequency by transit time effects that are similar to those
encountered by vacuum triodes
•New materials GaAs and GaN produce higher mobility carriers and higher
breakdown voltage to extend the performance envelop of solid state
amplifiers
Block diagram of CFA amplifier chain at 11 GHz for multi-megawatt system
Solid state
Driver 10 W
TWT or klystron
Intermediate amp
30 dB 10 kW
CFA
+10 dB
100 kW
CFA
+10 dB
1 MW
CFA
+10 dB
10 MW
Pulse Modulator
Most radar oscillators operate at pulse voltages between 5 and 20
kilovolts. They require currents of several amperes during the actual pulse
which places severe requirements on the modulator. The function of the
high-vacuum tube modulator is to act as a switch to turn a pulse ON and
OFF at the transmitter in response to a control signal. The best device for
this purpose is one which requires the least signal power for control and
allows the transfer of power from the transmitter power source to the
oscillator with the least loss. The pulse modulator circuits discussed in this
section are typical pulse modulators used in radar equipment.
GAS-FILLED TUBES
In some tubes, the air is removed and replaced
with an inert gas at a reduced pressure. The
gases used include mercury vapor, neon,
argon, and nitrogen.
They are capable of carrying much more
current than high-vacuum tubes, and they tend
to maintain a constant IR drop across their
terminals within a limited range of currents.
The electron stream from the hot cathode
encounters gas molecules on its way to the
plate (Ionization)
If the plate voltage is very low, the gas-filled
diode acts almost like an ordinary diode
except that the electron stream is slowed to a
certain extent by the gas molecules.
Increase plate voltage (Ionization POINT ) FIRING POTENTIAL
The value of the plate voltage at which ionization stops is called the
DEIONIZATION POTENTIAL, or EXTINCTION POTENTIAL
Thyratron gas-tube modulator
It consists of a power source (Ebb), a circuit
for storing energy (L2, C2, C3, C4, and C5), a
circuit for discharging the storage circuit (V2),
and a pulse transformer (T1). In addition this
circuit has a damping diode (V1) to prevent
reverse-polarity signals from being applied to
the plate of V2 which could cause V2 to
breakdown. With no trigger pulse applied, the
pfn charges through T1, the pfn, and the
charging coil L1 to the potential of Ebb. When
a trigger pulse is applied to the grid of V2, the
tube ionizes causing the pulse-forming
network to discharge through V2 and the
primary of T1. As the voltage across the pfn
falls below the ionization point of V2, the tube
shuts off. Because of the inductive properties
of the pfn, the positive discharge voltage has a
tendency to swing negative.
This negative overshoot is prevented from damaging the thyratron and affecting the output of the
circuit by V1, R1, R2, and C1. This is a damping circuit and provides a path for the overshoot
transient through V1. It is dissipated by R1 and R2 with C1 acting as a high-frequency bypass to
ground, preserving the sharp leading and trailing edges of the pulse. The hydrogen thyratron
modulator is the most common radar modulator