Simplified Radar Block Diagram

Transmitter Receiver

ModulatorMaster clock

Signal processor

(computer)

Duplexer

Waveguide

Target

Antenna

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

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.

CW radars

Target speed Measurements

Range Measurements

Doppler shift Frequency-modulation (FM)

Small, low-power versions of CW Doppler radars are used as:Speed sensors (police radar)Vehicle detectors for traffic controlProximity 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 .

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.)

Doppler Shift

M/A-COM Gunnplexer Doppler transceiver, which packs a transmitter, ferrite circulator, and mixer into a single module.

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.

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).

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)

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.

Pulsed radar

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

R4 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

Tiny inexpensive TransmitterNo sense

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 self-screening Jammer R2

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– Circular– Elliptical

• Can be rigid or flexible• Waveguides have very low loss• High Power

Y

X

Z

a

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.

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 correspondsto 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 ModeMode 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 TeeThe 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 2

Port 3

Port 4

Port 1

Mechanical SwitchesDirect 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

Important specifications:

SWR (or S11) Power-handling capability

Tapered absorber, usually consisting of a carbon-impregnated dielectric material that absorbs the microwave power

GHz7 - 10

watt300

8.2 – 12.4 GHz handles 75 watts

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.

High power

High directivity

limited in BW

Wide band

Poor directivity

Limited power

Wave guide coupler Coaxial and microstrip coupler

Coaxial coupler

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.

Duplexer

12

3

[S] =

0 0 1

1 0 0

0 1 0

The S matrix of an ideal circulator is

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.

Low Loss PathHigh Isolation Path

Transmitter Transmitter

Receiver Receiver

Two possible methods of achieving high output power in microwave system

Low power semiconductor

precise oscillator

High power tube amplifier

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 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

(Usually) Magnetron

Precise low power source + Amplifiers

Complexity and cost

Many stages (each with its own power supplies and control)

All stages must be stable

Important features could not be provided using Magnetron

• 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 □ Temperature drift

□ Pushing □ 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- ReliabilityThe 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 IIThe 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 ( =1s) and 1000 pps, or (t =2s) and 500 pps (0.001 duty cycle) and provides 500 W of average RF power. • = 40%• The 1- to 2-s pulse duration provides 150- to 300-m range resolution

High peak powerQuite small and Simplelow 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 operationAutomatic frequency control (AFC) is typically used to keep the receiver tunedto the transmitter

Magnetron Features

Magnetron Features Cont.

Tuners High-power magnetrons can be mechanically tuned over a 5 to 10percent frequency range routinely, and in some cases as much as 25 percent.

Rotary Tuning The rotary-tuned ("spin-tuned") magnetron was developedaround I960. A slotted disk is suspended above the anode cavities as when rotated, alternately provides inductive and capacitive loadingof the cavities to raise and lower the frequency. (Less average output power)

Electrons form rotating pattern

RF output

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.

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.

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 S), and starting jitter limits their use at very short pulses (e.g., 0.1 S), especially at high power and lower frequency bands.

4. Coherence is required from pulse to pulse for second-time-around clutter cancellation, etc.

Magnetron Limitations

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. SparkingEspecially when a magnetron is first started, it is normal for anode-to-cathode 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 gainHigh-power capability~ 20 % tuning bandwidth

Two Cavity

Two Cavity

Multi-Cavity Klystron

Electron beam

Electron Gun

Intermediate cavity

Beam collector

Microwave input

Microwave output

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

• Low gain (~10 dB)• CFAs are generally used only in the one or two highest-

power 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 medium-power TWT that provides most of the chain gain.

• CFAs have also been used to boost the power output of previously existing radar systems.

Crossed-Field Amplifiers (CFAs.

High efficiencysmall sizeRelatively low-voltage operationCover from UHF to K band

Attractive for:• lightweight systems•airborne use

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 mid-1950s, 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

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 onlyin 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, nowasted heater power, and virtually no limit on operating life.

2. Device operation occurs at much lower voltages; therefore, power supplyvoltages 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 timebetween failures (MTBF) in comparison with tube-type transmitters. ModuleMTBFs 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

Solid stateDriver 10 W

Block diagram of CFA amplifier chain at 11 GHz for multi-megawatt system

TWT or klystronIntermediate amp

30 dB 10 kW

CFA+10 dB100 kW

CFA+10 dB1 MW

CFA+10 dB10 MW

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.

Pulse Modulator

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 modulatorIt 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