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EKT 467 – RADAR ENGINEERING

PART 2

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Chapter Outlines

.

3.1 Radar Antenna

3.2 Radar Transmitter

3.3 Radar Receiver

3.4 Displays

3.5 Duplexers

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3.2 Radar Transmitter

• Based on a power amplifier (klystron) or power oscillators (magnetron).

• RF power sources, exciter & driver amplifiers.

• RF conversion efficiency :– The ratio of RF power output available from the device to the dc power input to the electron stream.

• Transmitter system efficiency:– The ratio of the RF power available from the transmitter to the total power needed to operate the transmitter.

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Common Microwave Components in Radar Transmitter

– Wave Guide Components

– High power Microwave Generations

• Oscillators (Magnetron)

• Amplifiers (Klystron)

– Modulators

– Waveform generators

Provide sufficient energy to detect a target, be

easily modulated to produce desired waveform,

generate stable signal, high efficiency and

reliability, long life, easy to maintain.

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Cont’d…

• Maximum efficiency: Most high power

sources operate saturated (completely on

or off).

• The life of RF power sources > thousand

hours.

• Different RF power sources available for

radar applications.

– Klystron, twt, solid state transistor,

twystron, amplitron, magnetron,

gyrotron.

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Cont’d…

• RF Power Sources

– Linear beam tubes

– Solid state

– Crossed field tubes

– Others

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Cont’d…

• Oscillators VS amplifiers

– Amplifier is preferred over oscillator

– Amplifier:• The signal to be transmitted is precisely generated at a low power level and then amplified to achieve the required power to be radiated from the antenna.

• Being able to provide stable waveforms, coded or frequency modulated pulse compression waveforms, frequency agility as well as combining and arraying.

– Oscillator:• Has less flexibility, noisier than linear beam amplifier.

• Each pulse transmitted, the phase different from the previous one.

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Cont’d…

• Klystron:

– Excellent radar tube

– Good efficiency, high gain.

– Wide bandwidth : 8 to 10 % relative bandwidth.

– Capable of high average and peak power.

– Stable operation.

– Operated frequencies : UHF to millimetre

wavelengths.

– In airborne military aircraft > 10 kW.

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Cont’d…

three cavity klystron amplifier

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Cont’d…

• Travelling Wave Tube (TWT)– Slightly less power, less gain, less efficiency.

– Capable of wide bandwidth.

– Conceived with a helix as the slow wave RF structure.

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Cont’d…• Solid state transistor amplifier

– Wider bandwidth, low voltage.

– Ease maintenance.

– Generate long pulse and employ pulse compression: adequate efficiency.

• Magnetron:– Smaller size, utilizes lower voltage.

– Limited average power.

– Poor noise and stability characteristic.

• Gyrotron:– Produce very high power.

– Require large magnetic fields.

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Cont’d…

• Solid states over tubes:

– No hot cathodes are required

• no warm up delay, no wasted heater power

– Transistors operate at much lower power

– Improved meantime between failures

(MTBF).

– Ability to demonstrate wide bandwidth.

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3.3 Radar Receiver

• To extract the weak echo signals that appeared at the antenna terminals, amplify them to a level that could be displayed to a radar operator

• Modern radar : applied matched filter to maximize peak-signal-to-mean-noise-ratio and discriminate the unwanted signal (diff. waveform).

• Separating moving target from stationary clutter echoes by recognizing the Doppler frequency shift of the moving target.

• External interference and hostile electronic countermeasures are kept from interfering with the detection of the target.

• Superheterodyne receiver.

– Low noise amplifier, mixer, receiver dynamic range, flicker noise, oscillator noise and detector

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Radar receiver configuration

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Cont’d…

Noise and Dynamic Range

• Rx generates internal noise that masks weak signals being received from the radar transmission : noise temperature and noise figure.

• Dynamic range: ratio of the maximum input signal power to the minimum input signal power that receiver can handle without degradation on performance.

• Vital performance characteristic– Dynamic range and susceptibility to overload

– Instantaneous bandwidth and tuning range

– Phase and amplitude stability

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Cont’d…

Typical example/illustration

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Cont’d…

• Noise figure

– A measure of the noise produced by a

practical receiver compared to the noise

of an ideal receiver.

– Noise factor

Fn = Nout / (kT0BnG)

= (Sin/Nin) /(Sout/Nout)

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Cont’d…

• Noise Temperature– The temperature at the input of the network that

accounts for the additional noise at the output.

Fn = 1 + Te/T0

Te = T0 (Fn-1)

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Cont’d…

• System noise temperature

Ts = Ta + Te

= (Fs – 1) T0

Where Fs = system noise figure and Ta is antenna temperature

For cascade networks:

Te = T1 + T2/G1 + T3/(G1G2) + …

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Cont’d…

• Digital Receivers

– A single A/D converter is used to digitize the

received signal.

– Digital signal processing is used to perform

the downconversion to I (in-phase) and Q

(quadrature-phase) baseband signals

– Significant different compared to analogue

• Utilize a direct digital synthesizer (DDS) as the

local oscillator.

• Utilize direct bandpass sampling at IF before

detection, with all subsequent processing being

done digitally.

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Cont’d…

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Cont’d…

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Terms:

• SNR– The ratio of the signal level to the noise (dB)

• Instantaneous bandwidth– Resulting bandwidth set by the combination of RF, IF, video and digital filtering that occurs within the receiver.

• Phase uniformity– The change of insertion phase of the limiter with amplitude is less of a concern for modern radar systems that operate primarily in the linear operating region.

• Recovery time– A measure of how quickly the limiter returns to linear operation after the limiting signal is removed.

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Revision: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 MWatt at 4GHz and 100 kW at 40 GHz.

Discontinuities and irregularities in the waveguide may impose a security factor

of 4 or more.

Losses in copper walls are of the order of 0.03 dB/m at 4GHz and 0.75 dB/m at

40GHz (5).

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PART 2