Radar Transmitter-4
October 12, 2022 | Author: Anonymous | Category: N/A
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Description
Simplified Radar Block Diagram
Antenna Antenn a
Target
Waveguide
Transmitter Duplexer
Master clock
Modulator
Receiver
Signal processor (computer)
Display
Key Components of a Radar System •
Transmitter • Electroni Electronicc device used to generate the microwave microwa ve EM energy transmitted by the
•
Receiver radar • Electroni Electronicc device used to detect the microwave pulse that is reflected by the area being imaged by the radar Antenna • Electroni Electronicc component through which microwave microwa ve pulses are transmitted and received
•
CW radars Target speed Measurements Doppler sift
Range Measurements Frequency-modulation (!M)
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 Sift Small" lo#$po#er versions of %W Doppler radars are used as& Speed sensors (police sensors (police radar) 'eicle detectors for traffic control roximity fues in fues in rockets, bombs, and projectiles. pr ojectiles. *n tese applications& The range to the target is usually small
The loss in sensitivity because of the use of a single antenna is acceptable . &"'C unnple*er +oppler transceiver, which packs a transmitter, ferrite circulator, and mi*er into a single module.
An +$,and Doppler transceiver echanical tuning coarsely sets fre!uency, whereas fine tuning and "#C can be provided by modulating the operating voltage. ($.%. "rmy photo.)
" unn oscillator is the ba basic sic transmitter, transmitter, which is coupled to a single antenna through the circulator. Transmitter Transmitter power reflected back from the antenna port acts as the local oscillator into the single s ingle balanced mi*er (an adjustable screw allows intentional standing'wave ratio (%W-) mismatch to force an ade!uate level of return signal). The addition of an antenna, fre!uency meter, and a direct'current (+C) power source completes the radar.
lock diagram for a simple single' antenna CW +oppler radar based on a +oppler transceiver. %W Radar& #-r-t ulsed radar '/ess comple* ' /ow cost ' /ower /ower operating voltage, and in some cases (high pow power) er) uses two antennas (Wastes in area)
ulsed radar Te pulsed radar transmitter& enerates powerful pulses of 0 energy at precise intervals
1igh'powerr microwave oscillator (magnetron) 1igh'powe (m agnetron) icrowave amplifier (klystron), supplied by a low'power -# source Modulator&
roperly'timed, high'amplitude, high'amplitude, rectangular rectangular pulse 2 .ig$po#er oscillator %witches the oscillator on and off 2 Micro#ave po#er amplifier "ctivates the amplifier amplifier just before the arrival of an electromagnetic electromagnetic pulse from a preceding stage or a fre!uency'generation source.
*n Amplifiers, 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 ha3ards because of the e*tremely high voltage involved. The modulator pulse may be more than 455 67 in high'power radar transmitters.
Radar transmitters produce& 7oltages, currents, and radiation ha3ards that are e*tremely dangerous 7oltages, to personnel. %afety precautions must always be strictly observed when working in or around a radar transmitter
Common Features of Radar Transmitter • t is usually large fraction of radar system • !igh cost • "arge si#e • !eavy • Re$uires significant efforts • t re$uires a ma%or share of system prime power and maintenance& because Radars are re$uired to generate so much power output • Most people prefer to 'eep away from it
Range / o#er Relation
-8 ∝ 9 " 9 T R
Detection Range
Transmitter o#er
A T
Aperture area Scanning time (te time allo#ed to scan te re0uired solid angle of coverage #ic limits o# long te signal in eac direction can ,e collected and integrated to improve S12)
/ A Trade off .uge / %ostly Antenna
2o sense
Tiny inexpensive Transmitter
Dou,ling te Tiny part
%utting te uge part in alf
Reduce te total system cost Reasona,le ,alance (according to te application) minimiing te total cost
Target carrying self$ screening 3ammer
:
- ∝
r 9 "r j 9 " j
r / Ar are still te driving factors Balanced System Design Results in Significant Tr Transmitter ansmitter o#er Max Radar erformance pused te antenna aperture A and te transmitter po#er to max afforda,le values
%ommon Micro#ave %omponents of Radar Transmitters
2 Wave 4uide %omponents 2 .ig po#er Micro#ave 4enerations 5scillators (Magnetron) Amplifiers 2 Modulators
Wave uide Concepts and features 2
ipe through which waves propagate
2
Can have various cross sections ; -ectangular ; Circular
2
; 0lliptical Can be rigid or fle*ible
2
Waveguides Wave guides have very low loss
2
1igh ower
Z
X
a Y b
Waveguide can handle power levels far in e*cess of coa*ial line ratings. Waveguide ecause there is no center conductor, waveguide is much less susc susceptible eptible to shock and vibration during shipping and installation. 7&m, which corresponds to a power range of 45 Wat 813 and 455 kW at 85 13. +iscontinuities and irregularities in the waveguide may impose a security factor of 8 or more. #urthermore, losses in copper walls are of the order of 5.5> d&m at 813 and 5.?@ d&m at 8513 (@).
T045 ode ode with lowest cutoff fre!uency is dominant mode 2%ingle mode propagation is highly desirable to reduce dispersion 2This occurs between cutoff fre!uency for T045 mode and twice that fre!uency
Circular Waveguide
Waveguide Wa veguide components commonly used in -adars
Wave guide Te Tee e
Hybrid Tee The hybrid coupler is used some applications, namely, • i*ers • odulators • =solated power power splitters since the iisolation solation between its input ports may be independent of the value of the two two balanced impedance loads.
ort 8
ort 4
ort :
ort >
Mecanical S#itces Direct s microwave power from one transmission line to another or turns microwave power on and off. 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 'apered absorber( usually consistin& of a carbon% impre&nated dielectric material that absorbs the microwave power
A.: ; 4:.8 13
13? ' 45
handles ?@ watts
watt>55
Important specifications: specifications:
• SW !or S""# $ower%handlin& dlin& capability • $ower%han
(ave (a ve guide coupler
Coa)ial and microstrip coupler
Hi&h power Hi&h directivity
Wide band $oor directivity
limited in )W
*imited power
+ is not critical for sampling microwave power + is e*tremely important for a return loss measurement, to measure the small power reflected from the mismatch.
Coa)ial coupler
+uple*er %irculator Circulator route microwave signals signals from one port of the device to anotherB 4.
owe owerr ent enter erin ing g por portt 4 is is dir direc ecte ted do out ut of of the the circ circul ulat ator or at at por portt :. :.
:.
" signal signal en enter tering ing port port : is routed routed to leav leave e the the circul circulato atorr at at port port > and does does not get back into port 4.
>.
" signal signal ent enteri ering ng port port > does does not not g get et into into port port :, but goes goes out out throu through gh port port 4.
, The % matri* of an ideal circulator is
+
"
-S /
0
0 "
"
0
0
0
"
0
The important specifications of a circulator* nsertion loss1 'he 'he los losss o off si&n si&nal al as it trav travel elss in in the the ri ri&h &htt dir direc ecti tion on !typically 0.2 d)# +irectivity
'he loss in the si&nal as it travel in the wron& direction (Typically +0d)#
3irculator enable the use of one antenna for both transmitter and receiver of communication system.
-eceiver
Transmitter
1igh =solation ath
-eceiver
Transmitter
/ow /oss ath
T#o possi,le metods of acieving ig ig output po#er in micro#ave system
!igh power tube "ow power !igh power tube semiconductor amplifier precise oscillator
oscillator
T67S 5! M*%R5WA'7 T8B7S Tu,es
Advantages
%ommon Applications -adars Communications jammers
Traveling wave tube (TWT) Traveling amplifier
Wide bandwidth
6lystron amplifier
1igh gain D high η
-adar medical applications
agnetron oscillator
low'cost
-adars +omestic cooking industrial heating
yrotron oscillator
1igh average power =n band (>5;>55 13)
of materials -adar lasma heating in controlled thermonuclear fusion research
.ig o#er R! 4eneration ulsed 5scillator System
recise lo# po#er source
9 Amplifiers
(8sually) Magnetron Many stages (eac #it its o#n po#er supplies and control)
All stages must ,e sta,le
*mportant features could not ,e provided using Magnetron %omplexity and cost
2 %oded pulsed 2 !re0uency agility
%om,ining and arraying
5scillators 'e 'ersus rsus Amplifiers *ssues of Selection (1) Accuracy and Stability of Carrier Frequency : Magnetron fre0uency is affected ,y& ; Tu Tu, , #arm #armup up drift ; using
; Temperature drift
; ; ulling
: *n Amplifiers ; !re0uency depends on te lo# po#er crystal oscillator ; !re0uency can ,e canged instantaneously ,y electronic s#itcing (faster tan mecanical tuner) (2) Coherence - Amplifier ,ased transmitter& %oerent R! and *! pps (-C duty cycle) and provides > W of average R! po#erpo# er• E F 2 Te C$ to ?$ s pulse duration provides C>$ to $m range resolution Magnetron !eatures
.ig peak po#er Guite small and Simple lo# cost ulsed magnetrons vary magnetrons vary from a 4'in>, 4'kW peak'power to several megawatts peak and several kW average power %W magnetrons have magnetrons have been made up to :@ kW for industrial heating. %table enough for T= operation "utomatic fre!uency control ("#C) is typically used to keep the receiver tuned
to the transmitter
Magnetron !eatures %ont-
Tuner 1igh'power magnetrons can be mechanically tuned over a > to C percent fre!uency percent fre!uency range routinely, and in some cases as much as :@ percent. +otary Tunin$ The rotary'tuned (Espin'tunedE) magnetron was developed around =FG5. " 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 fre!uency. ( 13 for power amplifiers but #0Ts dominate at higher fre!uencies 2oth are limited in fre!uency by transit time effects that are similar to those encountered by vacuum triodes 2
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