RadarTransmitter4ppt

1、simplified radar block diagramtransmitterreceivermodulatormaster clocksignal processor (computer)duplexerwaveguidetargetantennadisplaykey components of a radar systemtransmitterelectronic device used to generate the microwave em energy transmitted by the radarreceiverelectronic device used to detect

2、 the microwave pulse that is reflected by the area being imaged by the radarantennaelectronic component through which microwave pulses are transmitted and receivedthe transmitted wave is varied and range is determined by observing the lag in time between this modulation and the corresponding modulat

3、ion of the received echoes.cw radarstarget speed measurementsrange measurementsdoppler shiftfrequency-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 thes

4、e 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

5、 photo.)doppler shiftm/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 rada

6、r 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 standin

7、g-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 in

8、tervalshigh-power microwave oscillator (magnetron)microwave amplifier (klystron), supplied by a low-power rf sourcemodulator:properly-timed, high-amplitude, rectangular pulse high-power oscillator switches the oscillator on and off microwave power amplifier activates the amplifier just before the ar

9、rival of an electromagnetic pulse from a preceding stage or a frequency-generation source. pulsed radarin 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 inv

10、olved.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

11、 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 ra

12、nge & power relationr4 p a trdetection rangeptransmitter poweraaperture 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 ante

13、nnatiny inexpensive transmitterno sensedoubling 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 jammerr2 pr arpj ajpr & ar are still the driving factorsbalanced sys

14、tem design results in significant transmitter powermax radar performance pushed the antenna aperture a and the transmitter power p to max affordable valuescommon microwave components of radar transmitters wave guide components high power microwave generationsoscillators (magnetron)amplifiers modulat

15、orswave guide concepts and featurespipe through which waves propagatecan have various cross sections rectangular circular ellipticalcan be rigid or flexiblewaveguides have very low losshigh poweryxzabwaveguide can handle power levels far in excess of coaxial line ratings.because there is no center c

16、onductor, 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, humidit

17、y, 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

18、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 modesingle mode propagation is highly desirable to reduce dispersion this occurs between cutoff frequency for te10 mode and twice that

19、frequencycircular waveguidewaveguide components commonly used in radarswave guide teehybrid 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 lo

20、ads. port 2port 3port 4port 1mechanical 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 d

21、evices section.waveguide terminationsimportant specifications: swr (or s11) power-handling capabilitytapered absorber, usually consisting of a carbon-impregnated dielectric material that absorbs the microwave powerghz7 - 10 watt300 8.2 12.4 ghz handles 75 watts d is not critical for sampling microwa

22、ve powerd is extremely important for a return loss measurement, to measure the small power reflected from the mismatch.high power high directivitylimited in bwwide band poor directivitylimited powerwave guide coupler coaxial and microstrip couplercoaxial couplercirculator circulator route microwave

23、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 por

24、t 1. duplexer123s = 0 0 11 0 00 1 0the s matrix of an ideal circulator isthe important specifications of a circulator:insertion loss:the loss of signal as it travels in the right direction (typically 0.5 db) directivitythe loss in the signal as it travel in the wrong direction (typically 20db) circu

25、lator enable the use of one antenna for both transmitter and receiver of communication system.low loss pathhigh isolation pathtransmittertransmitterreceiverreceivertwo possible methods of achieving high output power in microwave system low power semiconductor precise oscillatorhigh power tube amplif

26、ierhigh power tube oscillatortypes of microwave tubestubesadvantagescommon applicationstraveling wave tube (twt) amplifierwide bandwidthradars;communications;jammersklystron amplifierhigh gain & high hradar; medical applicationsmagnetron oscillatorlow-costradarsdomestic cooking;industrial heatin

27、gof materialsgyrotron oscillatorhigh average powerin band (30300 ghz)radar; plasma heating incontrolledthermonuclearfusion researchhigh power rf generationpulsed oscillator system (usually) magnetronprecise low power source + amplifierscomplexity and costmany stages (each with its own power supplies

28、 and control)all stages must be stable important features could not be provided using magnetron coded pulsed frequency agility combining and arrayingoscillators versus amplifiersissues of selection(1) accuracy and stability of carrier frequency magnetron frequency is affected by: tub warmup drift te

29、mperature 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- oscillat

30、or-based transmitter: manual tuning or an automatic frequency control (afc) to tune the lo to the correct frequency.(3) instabilitiesterms include frequency phase shift coho locking pulse timing pulse width pulse amplitude jitter amplifier chains: special considerations.1. timing.because modulator r

31、ise 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 match3. matchingimproved amplifier ratings are sometimes available if t

32、he 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 stability4. signal-to-noise ratio.output s/n cannot be better than that of the worst stage5- leveling. (to maintain constant power with frequency)6- sta

33、bility 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.

34、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 red

35、undant chain, and many combinations of switching are feasible.9- rf amplifiers.availability of suitable rf amplifier deviceslinear-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 th

36、e beam.crossed field tubes (magnetrons and cfas)the electric and magnetic fields are at right angles to each other.magnetron transmittersinvented 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

37、 mhz. 500-kw peak power (t t =1m ms) and 1000 pps, or (t =2m ms) and 500 pps (0.001 duty cycle) and provides 500 w of average rf power. h h = 40% the 1- to 2-m ms pulse duration provides 150- to 300-m range resolutionhigh peak powerquite small and simplelow costpulsed magnetrons vary from a 1-in3, 1

38、-kw peak-power to several megawatts peak and several kw average powercw 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 transmittermagnetron featuresmagnetron features co

39、nt.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, alt

40、ernately provides inductive and capacitive loadingof the cavities to raise and lower the frequency. (less average output power)electrons form rotating patternrf outputthe process begins with a low voltage being applied to the filament, which causes it to heat up.remember, in a magnetron tube, the fi

41、lament 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 f

42、orm 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 fro

43、m 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. magnetro

44、ns are not suitable if:1. precise frequency control is needed2. precise frequency jumping (within a pulse or within a pulse group) is required3. 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

45、 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. magnetron limitations5. coded or shaped pulses are required. a range of only a few decibels of pulse shaping is feasible wit

46、h 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

47、 signal bandwidth (coaxial magnetrons are somewhat better in this respect).common problems in magnetron1. 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 condition

48、s 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 inv

49、erse 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 produce

50、s 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 catho

51、de 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.

52、 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 va

53、ry significantly during warmup.7. pushing: the amount by which a magnetrons 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: th

54、e amount by which a magnetrons 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 ha

55、ve been obtained by improved handling procedures and proper operator training.amplifierscapability of rf amplifiersklystron amplifiershigh gainhigh-power capability 20 % tuning bandwidthtwo cavitytwo cavitymulti-cavity klystronelectron beamelectron gunintermediate cavitybeam collectormicrowave input

56、microwave outputin 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 w

57、aveguide, which transports them to the accelerator. the electrons are absorbed in the beam stop.twthigh bandwidth one octave (low-power (few kw) helix type)twt vs. klystronsimilarities:beam formation, focusing and collection are the sameinput and output rf coupling are similartwt uses a traveling wa

58、ve version of the discreet cavity interaction of the klystronlarge overlays in beam voltage, current and rf power outputdifferences: bandwidth klystron 1% waveguide twt 10% transmission line (helix) twt 1 - 3 octaves form factor more amenable to low-cost, light-weight ppm focusinghelix and contra-wo

59、und helix derived circuitscoupled-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 i

60、s 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 bandattractive for: lightweight systemsairborne