Micowave and Millimeter Wave Technologies Modern UWB antennas and equipment Part 17 doc

MicrowaveandMillimeterWaveTechnologies:ModernUWBantennasandequipment472 T a In de 4. 4. 1 Le o p th e o n co n sp a th e co o di m si g at t to m e T h is s ke y sh o in fr e ov m e Fi g RF Pulse Jitte r PRF Supply Volta g Current cons u Weight Dimensions a ble VI. Paramete r the next sectio n scribed briefly. Magnetron b a 1 General consi d t us to remind b r p erational mode i e spectrum of RF n the parameters n ceptually in loc a a ce of signal par a e simplest case o rdinates of am p m ensions should g nal in the space t endin g the ma gn provide a precis e e asurement capa b h e most evident b s impl y to measu y issue to impl e o uld be provide d order to measu r e quenc y is next i v erall radar perf o e asured. g . 5. T y pical bloc k r g e min u mptio n , max @ 28 r s of W band sh o n some desi g n a sed radars – d d eration r iefl y that any m s used; (ii) each R oscillation depe n of external micr o a tin g the receive d a meters dependi n of non-cohere n p litude and time r be added. In tr u of si g nal para m n etron utilization e location of rad i b ilities of the rad a b ut comprehensi v re its parameter s e ment Doppler p d with correspon d r e its parameter s i mportant para m o rmance. At firs t k -dia g ram of ma g ns kHz V V A kg o rt pulse ma g netr approaches use d d esign approa c a g netron based r R F pulse is char a n ds stron g l y on a o wave circuits. O d si g nal as respe c ng of the radar m n t pulsed radar r espectivel y . For u ly coherent rad a m eters is known a in the radars re q i ated si g nal in s u a r. v e method to ens s . It is the only w p rocessin g . Thus d in g circuits to s a s like it is depict e m eter, whose me a t , it determines h g netron based ra d 19 ” on transmitter. d in the above c hes r adar is featured a cterized b y an a r a shape of modul a O n other hand, r a c t to the radiated m easurement cap a this space is t w Doppler radar t h a r systems the ex a a priori. Instead, q uire introductio n u ch space and ex t ure exact locatio n w a y to get phase i each ma g netro n a mple a small p o e d in Fig. 5. Th e a surement accur a h ow precisely a d ar. 3 3…30 18…32 14.1 25 ” , 5U unit mentioned rad a as follows: (i) a p r bitrar y phase; a n a tion volta g e as w a dar operation c o one in a corresp o a bilities. For exa m w o dimensional , h e phase and fre q act location of ra the above pecul i n of specific appr o t end its dimensi o n of the radiated i nformation, wh i n based Dopple r o rtion of radiated e ma g netron osc i a c y affects stron g target velocity c a rs are p ulsed n d (iii) w ell as o nsists o ndin g m ple in , with q uenc y a diated i arities o aches o ns, i.e. si g nal i ch is a r radar si g nal i llation g l y the c an be It does not require a great accuracy and may be implemented relatively easily. Practically in the most cases no measurements are required at all due to a specified magnetron frequency deviation does not exceed a portion of percent in the worst case. A different matter is a pulse-to-pulse frequency deviation. This parameter introduces both non-coherent (noise) and regular components (spurs) into Doppler signal processing (see Fig. 3). In part it determines the ability of the radar to resolve targets with different velocities and reflectivity in the same range bin, e.g. clouds in a strong rain or a moving target in presence of a much stronger reflection from a clutter. As it has been exposed above, the magnetron frequency should be measured with accuracy of about 10 -7 for a period of several hundred nanoseconds typically or even less, if a higher spatial resolution is required, in order to provide 70 dB spectral dynamical range for Ka band radar and the distance of 5 km. The indicated accuracy is on the edge of contemporary technical capabilities or beyond them, not even to mention the situation inherent to very recent time. Thus at the time being, achieving the maximal possible Doppler performance is the responsibility for the radar circuits, which should ensure as much as possible tight control of magnetron operational parameters – voltage, filament, loading etc, and, finally, its frequency stability. In the nearest future due to a dramatically fast progress in the development of data acquisition and processing hardware we expect that precise measurements of the parameters of the radiated pulse will be a basic method defining radar resolution and instrumentation capabilities. Some promising prospects concerned to this possibility will be discussed later (see Section 4.3.5 ). Below in this section we will try to analyze requirements to high performance magnetron based radar and discuss some methods to meet them. 4.2 Transmitter. 4.2.1 General consideration As mentioned above the modern requirements to the radar performance cannot be met otherwise than designing the magnetron environment to ensure as much as possible stability and safety of its operation. Therefore, the transmitter is probably the most valuable part of either magnetron based high performance radar. Before we will proceed to discuss some design approaches used in the transmitters, let us make a simple calculation in order to give an impression about how precisely its circuits should work. Assume that the aforementioned value of pulse-to-pulse frequency stability ߜ݂Ȁ݂ of 10 -7 should be provided. The variations of the amplitude of voltage pulse across magnetron should not exceed value given by the following expression: οܸ൑ ௙ ೚ೞ೎ ி ೡ೚೗೟ ȉቀ ఋ௙ ௙ ೚ೞ೎ ቁȉܴ ௗ (4) where ݂ ௢௦௖ is a magnetron oscillation frequency, ܨ ௩௢௟௧ – a magnetron oscillation frequency pushing factor, ܴ ௗ - a dynamical resistance of the magnetron in an operational point, i.e. the slope of its volt-ampere characteristic in this point. Let us take into consideration Ka band magnetron and suggest that the magnetron frequency pushing factor is of 500 kHz/A – a very respectable value, inherent to a highly stable coaxial magnetron rather than any other type, and a dynamical resistance of 300 Ohms – a typical value for devices with 10-100 kW peak power. Then the above expression gives an impressive value of about 2 V, or less than 200 ppm typically, for the required value of pulse-to-pulse amplitude instability of magnetron anode voltage! Note, that the indicated value should be ensured during the MagnetronBasedRadarSystemsforMillimeter WavelengthBand–ModernApproachesandProspects 473 T a In de 4. 4. 1 Le o p th e o n co n sp a th e co o di m si g at t to m e T h is s ke y sh o in fr e o v m e Fi g RF Pulse Jitte r PRF Supply Volta g Current cons u Weight Dimensions a ble VI. Paramete r the next sectio n scribed briefly. Magnetron b a 1 General consi d t us to remind b r p erational mode i e spectrum of RF n the parameters n ceptually in loc a a ce of signal par a e simplest case o rdinates of am p m ensio n s should g nal in the space t endin g the ma gn provide a precis e e asurement capa b h e most evident b s impl y to measu y issue to impl e o uld be provide d order to measu r e quenc y is next i v erall radar perf o e asured. g . 5. T y pical bloc k r g e min u mptio n , max @ 28 r s of W band sh o n some desi g n a sed radars – d d eration r iefl y that any m s used; (ii) each R oscillation depe n of external micr o a tin g the receive d a meters dependi n of non-cohere n p litude and time r be added. In tr u of si g nal para m n etron utilization e location of rad i b ilities of the rad a b ut comprehensi v re its parameter s e ment Doppler p d with correspon d r e its parameter s i mportant para m o rmance. At firs t k -dia g ram of ma g ns kHz V V A kg o rt pulse ma g netr approaches use d d esign approa c a g netron based r R F pulse is char a n ds stron g l y on a o wave circuits. O d si g nal as respe c ng of the radar m n t pulsed radar r espectivel y . For u l y coherent rad a m eters is known a in the radars re q i ated si g nal in s u a r. v e method to ens s . It is the only w p rocessin g . Thus d in g circuits to s a s like it is depict e m eter, whose me a t , it determines h g netron based ra d 19 ” on transmitter. d in the above c hes r adar is featured a cterized b y an a r a shape of modul a O n other hand, r a c t to the radiated m easurement cap a this space is t w Doppler radar t h a r s y stems the ex a a priori. Instead, q uire introductio n u ch space and ex t ure exact locatio n w a y to get phase i each ma g netro n a mple a small p o e d in Fig. 5. Th e a surement accur a h ow precisel y a d ar. 3 3…30 18…32 14.1 25 ” , 5U unit mentioned rad a as follows: (i) a p r bitrar y phase; a n a tion volta g e as w a dar operation c o one in a corresp o a bilities. For exa m w o dimensional , h e phase and fre q a ct location of r a the above pecul i n of specific appr o t end its dimensi o n of the radiated i nformation, wh i n based Dopple r o rtion of radiated e ma g netron osc i a c y affects stron g tar g et velocit y c a rs are p ulsed n d (iii) w ell as o nsists o ndin g m ple in , with q uenc y a diated i arities o aches o ns, i.e. si g nal i ch is a r radar si g nal i llation g l y the c an be It does not require a great accuracy and may be implemented relatively easily. Practically in the most cases no measurements are required at all due to a specified magnetron frequency deviation does not exceed a portion of percent in the worst case. A different matter is a pulse-to-pulse frequency deviation. This parameter introduces both non-coherent (noise) and regular components (spurs) into Doppler signal processing (see Fig. 3). In part it determines the ability of the radar to resolve targets with different velocities and reflectivity in the same range bin, e.g. clouds in a strong rain or a moving target in presence of a much stronger reflection from a clutter. As it has been exposed above, the magnetron frequency should be measured with accuracy of about 10 -7 for a period of several hundred nanoseconds typically or even less, if a higher spatial resolution is required, in order to provide 70 dB spectral dynamical range for Ka band radar and the distance of 5 km. The indicated accuracy is on the edge of contemporary technical capabilities or beyond them, not even to mention the situation inherent to very recent time. Thus at the time being, achieving the maximal possible Doppler performance is the responsibility for the radar circuits, which should ensure as much as possible tight control of magnetron operational parameters – voltage, filament, loading etc, and, finally, its frequency stability. In the nearest future due to a dramatically fast progress in the development of data acquisition and processing hardware we expect that precise measurements of the parameters of the radiated pulse will be a basic method defining radar resolution and instrumentation capabilities. Some promising prospects concerned to this possibility will be discussed later (see Section 4.3.5 ). Below in this section we will try to analyze requirements to high performance magnetron based radar and discuss some methods to meet them. 4.2 Transmitter. 4.2.1 General consideration As mentioned above the modern requirements to the radar performance cannot be met otherwise than designing the magnetron environment to ensure as much as possible stability and safety of its operation. Therefore, the transmitter is probably the most valuable part of either magnetron based high performance radar. Before we will proceed to discuss some design approaches used in the transmitters, let us make a simple calculation in order to give an impression about how precisely its circuits should work. Assume that the aforementioned value of pulse-to-pulse frequency stability ߜ݂Ȁ݂ of 10 -7 should be provided. The variations of the amplitude of voltage pulse across magnetron should not exceed value given by the following expression: οܸ൑ ௙ ೚ೞ೎ ி ೡ೚೗೟ ȉቀ ఋ௙ ௙ ೚ೞ೎ ቁȉܴ ௗ (4) where ݂ ௢௦௖ is a magnetron oscillation frequency, ܨ ௩௢௟௧ – a magnetron oscillation frequency pushing factor, ܴ ௗ - a dynamical resistance of the magnetron in an operational point, i.e. the slope of its volt-ampere characteristic in this point. Let us take into consideration Ka band magnetron and suggest that the magnetron frequency pushing factor is of 500 kHz/A – a very respectable value, inherent to a highly stable coaxial magnetron rather than any other type, and a dynamical resistance of 300 Ohms – a typical value for devices with 10-100 kW peak power. Then the above expression gives an impressive value of about 2 V, or less than 200 ppm typically, for the required value of pulse-to-pulse amplitude instability of magnetron anode voltage! Note, that the indicated value should be ensured during the MicrowaveandMillimeterWaveTechnologies:ModernUWBantennasandequipment474 in t m a N o or di a hi g co n th a m a Fi g tr a th e 4. 2 T h st a m a A (P W in h pr o ch a ut i ba ut i m o as s vo m u co m ut i fa c Fo D o de P W t erval of data ac c ay var y within t h o w, when a refer e other, it is possi b ag ram of a trans m g h volta g e pow e n troller. Let us l e a t it handles ot h ag netron operati o g . 6. Block-dia g r a a nsmitter with r e e ma g netron per f 2 .2 High voltage h e hi g h volta g e p a bilit y , i.e. Dopp l a tter of the hi g he s switchin g mod e W M) converter, c h erent hi g h effic i o vided b y such s a racteristics of P i lization. Our ex p sed radars dem o i lize operation in o de. Such appro a s ists maximizin g lta g e re g ulation l u ltiple to the p m pletel y the in f i lization of a pa r c tor corrector for r information, t h o ppler performa n veloped accordi n W M converter. F r c umulation for F o h e ran g e from ten s e nce point for th e b le to consider s o m itter is depicte d e r suppl y ; (ii) a e ave the latter u n h er units accordi n o nal mode as we l a m of ma g netron e mote control an d f ormance, thus w power supply ower suppl y det l er performance o s t priorit y under e power suppl y, c annot be altern a i enc y , small dim e s uppl y is lower g P WM converter m p erience to deve l o nstrates a benefi either peak curr e a ch as well as th e both re j ection o f l oop. Next, it is m p ulse repetition f luence of ripp l r ticular pre-re g ul AC powered s y s h e line of Ka b n ce (see Section ng strictl y to the r om our opinion , o urier processin g s millisecond up e ma g netron tra n o lutions enablin g d in Fig. 6. It inc l modulator; (iii) n it be y ond a mo r ng the procedur e l l as provides the transmitter. d dia g nostics ab i e would like to o ermines essentia l o f whole radar. T the developmen t , based on the u a ted to produce h e nsions, and li gh g enerall y tha n t h m ay be improve l op the hi g h vol t t of the followin g e nt or close to it m e usa g e of a freq u f the input volta g m andator y to s yn frequenc y of t h l es at PWM op ator is preferabl y tems is virtuall y b and meteorolo gi 0) is equipped above recomme n , such topology i g . As usual the d to several portio n n smitter design is g its consummati o l udes the follow i a filament po w r e detailed consi d e s ensurin g the m i lities. Other ab o utline their desi g l l y the short ter m T hus ensurin g it s t . u tilization of p u h i g h volta g e in m h t wei g ht. Howe v h at of linear re gu d to an extent a t a g e power sup p g rules. At first, P m ixed mode rat h u enc y compensat e g e ripples and th e n chronize PWM c h e radar, which erational freque n y . In this respec t compulsorily. i cal radar dem o with the hi g h n datio n s. A fl y ba i s the most suita d uration of this i n n s of second. indicated in so m o n . A simplified i n g essential uni t w er suppl y ; and deration, mentio m ost optimal a n o ve units affect d g n in more detail. m ma g netron fre q s maximal stabil i u lse width mod u m odern s y stems v er the volta g e s t u lators. On other a llowing its stan d p lies for the ma gn P WM converter s h er than in pure v e d hi g h volta g e d e overall stabilit y onverter at a fre q eliminates pra c n c y . And at la s t , the usa g e of a o nstrating a ver y volta g e power s ck topology is u s ble to the hi g h v n terval m e wa y block- t s: (i) a (iv) a n onl y n d safe d irectl y q uenc y i t y is a u lation due to t abilit y hand, d alone n etron s hould v olta g e d ivider y of the q uenc y c ticall y s t, the power y solid s uppl y s ed for v olta g e ap ra d is w i s w su p hi g Fi g hi g o p 4. 2 In hi g in c th e de de se n m i ch a m a m o m e dr a sh o co n p u N o m u p u m a w h Fi g plications with t h d ar s y stems or e v used. The essen t i thin a wide ran g w in g across the p p pl y volta g e. T h g h volta g e powe r g . 3 there is no r g h volta g e pow e p erational freque n 2 .3 Modulator this section we w g h volta g e mod c ludes circuits to e most cases a velopment. Sinc e viation of the p n sitivit y . Thus, b i nimized. Especi a a racterized b y a ag netrons requi r o dulation pulse t e an better! An o p a wn to ensure it s o uld be taken i n siderable thres h u lse throu g h the o tice that at low e u ch g reater as r e u lse duration an d ag netron perfor m h ile a pulse repet i g . 7. Waveforms o h e output powe r v en airborne DC t ial advance of s u g e of output po w p rimar y windin g h e above peculia r r suppl y in a m a e g ular spurious e r suppl y at the n c y of PWM con v w ill consider brie f ulators used in form the pulse w near-rectan g le s e the ma g netron p ulse shape fro m b oth transients a a ll y it is importa n rather short wid t r es a well cont r t o facilitate runn i p posite situation a s appropriatel y s i nto considerati o h old current to p r ma g netron ma y e r volta g es the p o e spect to anode p d hi g her pulse r m ance. Thus the i tion rate g reater o f volta g e pulse a r up to 1 kW an d powered radars u ch scheme is a w er as well as t h s of the hi g h v o r ities meet perfe c ag netron based t r components cau harmonics of b o v erter (folded). f l y some issues r hi g h performa n w ith a definite s h s hape of RF p u frequenc y depe n m the rectan g ul a a nd the distortio n t for the millim e t h of the output r ollable volta g e i n g oscillatio n ( O a ppears for the t r s hort duration. H o n there. It is d r oduce RF oscill a y be much lon ge o wer of back bo m p ower as indica t r epetition rate t h above issue sho u than several kil o a cross ma g netro n d volta g es up to 2 if an appropriat e stable operation h e abilit y to pro v o lta g e transform e c tl y actual opera t r ansmitters. As c sed b y ripples o f o th AC power l elated to the de v n ce radars. In g h ape across the m u lse is a tar g et n ds stron g l y on t h a r one results i n ns of flat part o e ter wavelengths pulse. On other h rate durin g th e O kress, 1961). In t h r ailin g ed g e. As u H owever, not onl y d ue to the ma gn a tion as usual. It e r than RF pulse m bardment of th e t ed in Fig. 7. Ev i h e stron g er the a u ld be alwa y s t a o hertz is required n and RF envelop e 2 0 kV for AC p o e step-up pre-re g with a capaciti v v ide the output v e r much g reater t ional conditions c an be easil y see n f the output vol t l ine frequenc y a n v elopment of up- t g eneral the mo d m a g netron termi n under the mo d h e applied volta g n a drop in the o f the pulse sho u ma g netrons, wh i h and the most t y e leadin g ed g e h is case faster d o u sual a less atte n y shape of RF en v n etrons have a means that the c as depicted in e ma g netron cat h i dentl y , the sho r a bove effect affe c a ken into consid e . e . o wered g ulator v e load v olta g e than a of the n form t a g e of n d the t o date d ulator n als. In d ulator g e, an y radar u ld be i ch are y pes of of the o es not n tion is v elope rather c urrent Fig. 7. h ode is r ter RF c ts the e ratio n MagnetronBasedRadarSystemsforMillimeter WavelengthBand–ModernApproachesandProspects 475 in t m a N o or di a hi g co n th a m a Fi g tr a th e 4. 2 T h st a m a A (P W in h pr o ch a ut i ba ut i m o as s vo m u co m ut i fa c Fo D o de P W t erval of data ac c ay var y within t h o w, when a refer e other, it is possi b ag ram of a trans m g h volta g e pow e n troller. Let us l e a t it handles ot h ag netron operati o g . 6. Block-dia g r a a nsmitter with r e e ma g netron per f 2 .2 High voltage h e hi g h volta g e p a bilit y , i.e. Dopp l a tter of the hi g he s switchin g mod e W M) converter, c h erent hi g h effic i o vided b y such s a racteristics of P i lization. Our ex p sed radars dem o i lize operation in o de. Such appro a s ists maximizin g lta g e re g ulation l u ltiple to the p m pletel y the in f i lization of a pa r c tor corrector for r information, t h o ppler performa n veloped accordi n W M converter. F r c umulation for F o h e ran g e from ten s e nce point for th e b le to consider s o m itter is depicte d e r suppl y ; (ii) a e ave the latter u n h er units accordi n o nal mode as we l a m of ma g netron e mote control an d f ormance, thus w power supply ower suppl y det l er performance o s t priorit y under e power suppl y, c annot be altern a i enc y , small dim e s uppl y is lower g P WM converter m p erience to deve l o nstrates a benefi either peak curr e a ch as well as th e both re j ection o f l oop. Next, it is m p ulse repetition f luence of ripp l r ticular pre-re g ul AC powered s y s h e line of Ka b n ce (see Section ng strictl y to the r om our opinion , o urier processin g s millisecond up e ma g netron tra n o lutions enablin g d in Fig. 6. It inc l modulator; (iii) n it be y ond a mo r ng the procedur e l l as provides the transmitter. d dia g nostics ab i e would like to o ermines essentia l o f whole radar. T the developmen t , based on the u a ted to produce h e nsions, and li gh g enerall y tha n t h m a y be improve l op the hi g h vol t t of the followin g e nt or close to it m e usa g e of a freq u f the input volta g m andator y to s yn frequenc y of t h l es at PWM op ator is preferabl y tems is virtuall y b and meteorolo gi 0) is equipped above recomme n , such topology i g . As usual the d to several portio n n smitter design is g its consummati o l udes the follow i a filament po w r e detailed consi d e s ensurin g the m i lities. Other ab o utline their desi g l l y the short ter m T hus ensurin g it s t . u tilization of p u h i g h volta g e in m h t wei g ht. Howe v h at of linear re gu d to an extent a t a g e power sup p g rules. At first, P m ixed mode rat h u enc y compensat e g e ripples and th e n chronize PWM c h e radar, which erational freque n y . In this respec t compulsorily. i cal radar dem o with the hi g h n datio n s. A fl y ba i s the most suita d uration of this i n n s of second. indicated in so m o n . A simplified i n g essential uni t w er suppl y ; and deration, mentio m ost optimal a n o ve units affect d g n in more detail. m ma g netron fre q s maximal stabil i u lse width mod u m odern s y stems v er the volta g e s t u lators. On other a llowin g its stan d p lies for the ma gn P WM converter s h er than in pure v e d hi g h volta g e d e overall stabilit y onverter at a fre q eliminates pra c n c y . And at la s t , the usa g e of a o nstratin g a ver y volta g e power s ck topology is u s ble to the hi g h v n terval m e wa y block- t s: (i) a (iv) a n onl y n d safe d irectl y q uenc y i t y is a u lation due to t abilit y hand, d alone n etron s hould v olta g e d ivider y of the q uenc y c ticall y s t, the power y solid s uppl y s ed for v olta g e ap ra d is w i s w su p hi g Fi g hi g o p 4. 2 In hi g in c th e de de se n m i ch a m a m o m e dr a sh o co n p u N o m u p u m a w h Fi g plications with t h d ar s y stems or e v used. The essen t i thin a wide ran g w in g across the p p pl y volta g e. T h g h volta g e powe r g . 3 there is no r g h volta g e pow e p erational freque n 2 .3 Modulator this section we w g h volta g e mod c ludes circuits to e most cases a velopment. Sinc e viation of the p n sitivit y . Thus, b i nimized. Especi a a racterized b y a ag netrons requi r o dulation pulse t e an better! An o p a wn to ensure it s o uld be taken i n siderable thres h u lse through the o tice that at low e u ch g reater as r e u lse duration an d ag netron perfor m h ile a pulse repet i g . 7. Waveforms o h e output powe r v en airborne DC t ial advance of s u g e of output po w p rimar y windin g h e above peculia r r suppl y in a m a e g ular spurious e r suppl y at the n cy of PWM con v w ill consider brie f ulators used in form the pulse w near-rectan g le s e the ma g netron p ulse shape fro m b oth transients a a ll y it is importa n rather short wid t r es a well cont r t o facilitate runn i p posite situation a s appropriatel y s i nto considerati o h old current to p r magnetron ma y e r volta g es the p o e spect to anode p d hi g her pulse r m ance. Thus the i tion rate g reater o f volta g e pulse a r up to 1 kW an d powered radars u ch scheme is a w er as well as t h s of the hi g h v o r ities meet perfe c ag netron based t r components cau harmonics of b o v erter (folded). f l y some issues r hi g h performa n w ith a definite s h s hape of RF p u frequenc y depe n m the rectan g ul a a nd the distortio n t for the millim e t h of the output r ollable voltage i n g oscillatio n ( O a ppears for the t r s hort duration. H o n there. It is d r oduce RF oscill a y be much longe o wer of back bo m p ower as indica t r epetition rate t h above issue sho u than several kil o a cross ma g netro n d volta g es up to 2 if an appropriat e stable operation h e abilit y to pro v o lta g e transform e c tl y actual opera t r ansmitters. As c sed b y ripples o f o th AC power l elated to the de v n ce radars. In g h ape across the m u lse is a tar g et n ds stron g l y on t h a r one results i n ns of flat part o e ter wavelengths pulse. On other h rate during th e O kress, 1961). In t h r ailin g ed g e. As u H owever, not onl y d ue to the ma gn a tion as usual. It e r than RF pulse m bardment of th e t ed in Fig. 7. Ev i h e stron g er the a u ld be alwa y s t a o hertz is required n and RF envelop e 2 0 kV for AC p o e step-up pre-re g with a capaciti v v ide the output v e r much g reater t ional conditions c an be easil y see n f the output vol t l ine frequenc y a n v elopment of up- t g eneral the mo d m a g netron termi n under the mo d h e applied volta g n a drop in the o f the pulse sho u ma g netrons, wh i h and the most t y e leading edge h is case faster d o u sual a less atte n y shape of RF en v n etrons have a means that the c as depicted in e ma g netron cat h i dentl y , the sho r a bove effect affe c a ken into consid e . e . o wered g ulator v e load v olta g e than a of the n form t a g e of n d the t o date d ulator n als. In d ulator g e, an y radar u ld be i ch are y pes of of the o es not n tion is v elope rather c urrent Fig. 7. h ode is r ter RF c ts the e ratio n MicrowaveandMillimeterWaveTechnologies:ModernUWBantennasandequipment476 N o de pa th e o u m o ut i A li n bo Fi g T h co n al s cu r in ut i (ii i br e o v pr o H o th e p u d u si g n o se n m a tr a ut i to all ra d m a A n w i fr o o w, when the es s si g n approaches rts: (i) ener gy st o e ma g netron wi t u tput and the m a o dulator t y pes b e i lized practicall y simplified block - n e arran g ed as a p th to store ener gy g . 8. Block dia g ra m h e utilization of n trolled devices l s o th y ristors or r rents and volta g g eneral; and d o i lization of the st e i ) practicall y co m e akdown due to v erall complexit y o babl y the lowes o wever there are e usa g e of a lu m u lse. Next, it is p r u ration. Further, t g nificant distorti o o nlinearit y of ma n sitivity from th a tchin g the imp e a ilin g ed g e of t h i lizin g modulato r be used in the hi mentioned disa d d ar performanc e ag netron based s y n other t y pe of h i i th partial discha r o m the line mo d s ential requirem e to meet them ca n o ra g e; (ii) a swit c t hin definite ti m ag netron; and (v i e in g in use, a lin e exclusivel y in m a - dia g ram of a lin p iece of cable or a y and to form th e m of line modul a line modulators l ike a hard tubes th y ratrons, whi c g es (MOSFET vs. o not require a e p-up transform e m plete cancellat i the total ener gy s and total cost o t as compared w i a number of seri o m ped element del a r acticall y imposs i t he inherent util i o ns of the mod u g netron volt-am p h e voltage rate a e dances of the d h e modulation p u r s equipped wit h g h performance r d vanta g es of the e only and cau s y stems. ig h volta g e mod u rg e (Sivan, 1994) d ulator is onl y a e nts to the mod u n be discussed. E c h or switches, w m e intervals onl y i ) protection an d e modulator and ag netron transm i e modulator is d a ssembled from l u e trailing edge of t a tor. provides the f o or MOSFETs ma y c h are rated fu n th y ristors comp a precise shape o e r to match the i m i on of the possi b s tored in the del a o f a transmitter i th other types. o us disadvanta ge ay line leads us t i ble to implemen i zation of the tr a u lator output p u p ere characterist i a cross the magn e d ela y line and t h u lse. Both nume h a transformer p r r adars. On other line modulators s e no problems u lator used to d r . The essential d a small part of e u lator circuits ha v E ither modulator w hich provides a y ; (iii) circuits to d decouplin g circ a modulator wi t i tters (Sivan, 199 4 d epicted schemat i u mp inductors a n t he pulse across m o llowin g advant a y be used in the h n damentall y to a rison); demonst r o f tri gg erin g pul m pedances of del b le ma g netron d ay line is limited . equipped with t e s of such type o f t o oscillations a p t smooth re g ulat i a nsformer result s u lse especiall y d u i c as well as po s e tron (Okress, 1 9 h e ma g netron d rical simulation r event us to rec o hands it should b are important t o in a variet y of r ive ma g netrons ifference of suc h e ner gy accumul a v e been outlined , contains the f ol l a ppl y in g volta g e match the mo d uits. From a var i t h partial discha r 4 ). i call y in Fig. 8. A n d capacitors is u m agnetron. ag es: (i) not onl y h i g h volta g e swi t work at much r ate a g reater eff i se; (ii) the man a y line and ma gn d ama g e in the c . All above resul t t he line modula t f modulator. The p pear at flat part in g of the outpu t s in the introduc t u e to there is a s sible appearanc e 9 61). It makes d urin g the leadi n and the experi e o mmend such ap p b e accepted that a o achieve the ma x simple and lo w is so called mo d h t y pe of the mo d a ted in an appr o , some l owin g across d ulator i et y of rg e are A dela y u sed as y full y t ch but hi g her i cienc y dator y n etron; c ase of t in the t or are first is of the t pulse t ion of stron g e of its ifficult ng a n d e nce of p roach a lmost x imum w cost d ulator d ulator o priate st o m a ut i p u A ba di s fr e is bi a m o a s to Fi g A p o di s co m or d re s ch o a p m o co m g e n pa co m pa w i o n 20 0 A in p o co n ad de o ra g e is used to f ag netron modul a i lization of matc h u lse than the line m variet y of varia n sic confi g uratio n s advanta g es pro v e quentl y than ot h a g reat advanta g a s power supplie o dulator desi g n. T s tack of transisto r drive each transi a) g . 9. Schemes of t h resistor or chok e o wer suppl y dur s tortions of ma g m ponent with f r d er to decrease s istor should be o ke is used inste a p ossibilit y to pro d o dulator suppl y v m ponent, the ou t n erall y , there ar e rameters takin g m ponent is bul k rts of the modul a i th inductive dec o n l y if retrievin g o f 0 2)), such schem e simplified block - Fi g 9.b The onl y o wer suppl y and n ceptuall y in thi s ditional resistor picted in Fig. 9. b f orm the output a tors exclusivel y . h in g circuitr y co n m odulator. n ts can be used t n s are depicted v ided b y each o h er confi g uration s g e for whichever s for the modula t T his issue is not s r s connected in s e stor independen t h e modulator wi t e ma y be used t o in g interval of p g netron pulse d u r equenc y depen d duration of the chosen small en a d the resistor, t h d uce the output v olta g e. Howeve t put pulse distor t e bi g constructi v into account it i ky and character i a tor. Thus we w o o uplin g in mod e f very short puls e e ma y appear to b - diagram of the m y capacitor ma y b stora g e capacito s case. Neverthe l to decrease the d b by a dash line. I pulse. A capaci t Due to such t y p n ceptuall y it pro v t o build the mo d i n Fig. 9. Let o f them. The fi r s . It is due to the hard tube base d t or tube are g ro u s o important for e ries. Actuall y , a g t l y on either the h b) t h partial discha r o decouple the s p ulse formation. u rin g its front a d ant impedance trailin g ed g e of ou g h, which res u h e efficienc y is b e pulse with ampl i r due to the puls t ions are hi g her i n v e dif f iculties to d i s under high p u i zed b y a rather o uld not like to r e rn hi g h perform a e s at a high repe t b e the most opti m m odulator with a b e used both as t r of the modula t l ess, as mention e d uration of the o I t is difficult evi d t or is used as th e e of the modula t v ides a much bet d ulator with par t us consider b r r st scheme is tr hi g h volta g e sw i d modulator. In t h u nded also, simp l a solid state swit g alvanic decoup l h i g h volta g e swit c rg e. s witch from the o In the case of r a nd flat part a r is in the pulse the output puls u lts in lower m o e tter fundament a i tude, which is h e formation net w n the case of the c d esi g n a char g i n u lsed volta g e co n lar g e wei g ht a s r ecommend the u a nce ma g netron t ition rate is req u m al choice. floatin g hi g h vo l t he output capac i t or. No decoupli n e d above if a hi gh utput pulse trail i d entl y to use a h a e ener gy stora g e t or does not req u ter shape of the o t ial dischar g e. S o r iefl y advanta ge aditionall y use d i tch is g rounded, h is case a filame n l if y in g considera b t ch, arran g ed us u l in g is required a n c h is g rounded o c) o utput of hi g h v r esistor utilizati o r e minimal due formation netw o e the resistance o dulator efficien c a ll y . In addition t h h i g her than the v a w ork includes a r e c hoke utilization ng choke with re q n dition. As usu a s compared wit h u tilization of mo d based radars. Pr o u ired (see (Beliko v l ta g e switch is d e i tor of the hi g h v ng circuits are re q h PRF is su gg es t i n g ed g e is requ i a rd tube in such in the u ire the o utput o me of e s and d more which n t and b l y the u all y as ny wa y r not. v olta g e on , the to no o rk. In of the cy . If a h ere is a lue of e active . Next, q uired a l such h other d ulator o babl y v et al, e picted v olta g e q uired t ed, an i red as circuit MagnetronBasedRadarSystemsforMillimeter WavelengthBand–ModernApproachesandProspects 477 N o de pa th e o u m o ut i A li n bo Fi g T h co n al s cu r in ut i (ii i br e o v pr o H o th e p u d u si g n o se n m a tr a ut i to all ra d m a A n w i fr o o w, when the es s si g n approaches rts: (i) ener gy st o e ma g netron wi t u tput and the m a o dulator t y pes b e i lized practicall y simplified block - n e arran g ed as a p th to store ener gy g . 8. Block dia g ra m h e utilization of n trolled devices l s o th y ristors or r rents and volta g g eneral; and d o i lization of the st e i ) practicall y co m e akdown due to v erall complexit y o babl y the lowes o wever there are e usa g e of a lu m u lse. Next, it is p r u ration. Further, t g nificant distorti o o nlinearit y of ma n sitivit y from t h a tchin g the imp e a ilin g ed g e of t h i lizin g modulato r be used in the hi mentioned disa d d ar performanc e ag netron based s y n other t y pe of h i i th partial discha r o m the line mo d s ential requirem e to meet them ca n o ra g e; (ii) a swit c t hin definite ti m ag netron; and (v i e in g in use, a lin e exclusivel y in m a - dia g ram of a lin p iece of cable or a y and to form th e m of line modul a line modulators l ike a hard tubes th y ratrons, whi c g es (MOSFET vs. o not require a e p-up transform e m plete cancellat i the total ener gy s and total cost o t as compared w i a number of seri o m ped element del a r acticall y imposs i t he inherent util i o ns of the mod u g netron volt-am p h e volta g e rate a e dances of the d h e modulation p u r s equipped wit h g h performance r d vanta g es of the e only and cau s y stems. ig h volta g e mod u rg e (Sivan, 1994) d ulator is onl y a e nts to the mod u n be discussed. E c h or switches, w m e intervals onl y i ) protection an d e modulator and ag netron transm i e modulator is d a ssembled from l u e trailin g ed g e of t a tor. provides the f o or MOSFETs ma y c h are rated fu n th y ristors comp a precise shape o e r to match the i m i on of the possi b s tored in the del a o f a transmitter i th other types. o us disadvanta ge ay line leads us t i ble to implemen i zation of the tr a u lator output p u p ere characterist i a cross the ma g n e d ela y line and t h u lse. Both nume h a transformer p r r adars. On other line modulators s e no problems u lator used to d r . The essential d a small part of e u lator circuits ha v E ither modulator w hich provides a y ; (iii) circuits to d decouplin g circ a modulator wi t i tters (Sivan, 199 4 d epicted schemat i u mp inductors a n t he pulse across m o llowin g advant a y be used in the h n damentall y to a rison); demonst r o f tri gg erin g pul m pedances of del b le ma g netron d ay line is limited . equipped with t e s of such t y pe o f t o oscillations a p t smooth re g ulat i a nsformer result s u lse especiall y d u i c as well as po s e tro n (Okress, 1 9 h e ma g netron d rical simulation r event us to rec o hands it should b are important t o in a variet y of r ive ma g netrons ifference of suc h e ner gy accumul a v e been outlined , contains the f ol l a ppl y in g volta g e match the mo d uits. From a var i t h partial discha r 4 ). i call y in Fig. 8. A n d capacitors is u m a g netron. ag es: (i) not onl y h i g h volta g e swi t work at much r ate a g reater eff i se; (ii) the man a y line and ma gn d ama g e in the c . All above resul t t he line modula t f modulator. The p pear at flat part in g of the outpu t s in the introduc t u e to there is a s sible appearanc e 9 61). It makes d urin g the leadi n and the experi e o mmend such ap p b e accepted that a o achieve the ma x simple and lo w is so called mo d h t y pe of the mo d a ted in an appr o , some l owin g across d ulator i et y of rg e are A dela y u sed as y full y t ch but hi g her i cienc y dator y n etron; c ase of t in the t or are first is of the t pulse t ion of stron g e of its ifficult ng a n d e nce of p roach a lmost x imum w cost d ulator d ulator o priate st o m a ut i p u A ba di s fr e is bi a m o a s to Fi g A p o di s co m or d re s ch o a p m o co m g e n pa co m pa wi o n 20 0 A in p o co n ad de o ra g e is used to f ag netron modul a i lization of matc h u lse than the line m variet y of varia n sic confi g uratio n s advanta g es pro v e quentl y than ot h a g reat advanta g a s power supplie o dulator desi g n. T s tack of transisto r drive each transi a) g . 9. Schemes of t h resistor or chok e o wer suppl y dur s tortions of ma g m ponent with f r d er to decrease s istor should be o ke is used inste a p ossibilit y to pro d o dulator suppl y v m ponent, the ou t n erall y , there ar e rameters takin g m ponent is bul k rts of the modul a i th inductive dec o n l y if retrievin g o f 0 2)), such schem e simplified block - Fi g 9.b The onl y o wer suppl y and n ceptuall y in thi s ditional resistor picted in Fig. 9. b f orm the output a tors exclusivel y . h in g circuitr y co n m odulator. n ts can be used t n s are depicted v ided b y each o h er confi g uration s g e for whichever s for the modula t T his issue is not s r s connected in s e stor independen t h e modulator wi t e ma y be used t o in g interval of p g netron pulse d u r equency depen d duration of the chosen small en a d the resistor, t h d uce the output v olta g e. Howeve t put pulse distor t e bi g constructi v into account it i ky and character i a tor. Thus we w o o upling in mode f very short puls e e ma y appear to b - diagram of the m y capacitor ma y b stora g e capacito s case. Neverthe l to decrease the d b by a dash line. I pulse. A capaci t Due to such t y p n ceptuall y it pro v t o build the mo d i n Fig. 9. Let o f them. The fi r s . It is due to the hard tube base d tor tube are gro u s o important for e ries. Actuall y , a g t l y on either the h b) t h partial discha r o decouple the s p ulse formation. u rin g its front a d ant impedance trailin g ed g e of ou g h, which res u h e efficienc y is b e pulse with ampl i r due to the puls t ions are hi g her i n v e dif f iculties to d i s under high p u i zed b y a rather o uld not like to r e rn high perform a e s at a high repe t b e the most opti m m odulator with a b e used both as t r of the modula t l ess, as mention e d uration of the o I t is difficult evi d t or is used as th e e of the modula t v ides a much bet d ulator with par t us consider b r r st scheme is tr hi g h volta g e sw i d modulator. In t h u nded also, simp l a solid state swit g alvanic decoup l h i g h volta g e swit c rg e. s witch from the o In the case of r a nd flat part a r is in the pulse the output puls u lts in lower m o e tter fundament a i tude, which is h e formation net w n the case of the c d esi g n a char g i n u lsed volta g e co n lar g e wei g ht a s r ecommend the u a nce magnetron t ition rate is req u m al choice. floatin g hi g h vo l t he output capac i t or. No decoupli n e d above if a hi gh utput pulse trail i d entl y to use a h a e ener gy stora g e t or does not req u ter shape of the o t ial dischar g e. S o r iefl y advanta ge aditionall y use d i tch is g rounded, h is case a filame n l ifying considera b t ch, arran g ed us u l in g is required a n c h is g rounded o c) o utput of hi g h v r esistor utilizati o r e minimal due formation netw o e the resistance o dulator efficien c a ll y . In addition t h h i g her than the v a w ork includes a r e c hoke utilization ng choke with re q n dition. As usu a s compared wit h u tilization of mo d based radars. Pr o u ired (see (Beliko v l ta g e switch is d e i tor of the hi g h v ng circuits are re q h PRF is su gg es t i n g ed g e is requ i a rd tube in such in the u ire the o utput o me of e s and d more which n t and b ly the u all y as ny wa y r not. v olta g e on , the to no o rk. In of the cy . If a h ere is a lue of e active . Next, q uired a l such h other d ulator o bably v et al, e picted v olta g e q uired t ed, an i red as circuit MicrowaveandMillimeterWaveTechnologies:ModernUWBantennasandequipment478 configuration. Instead, from our opinion, it is the most preferable scheme to utilize a solid state high voltage switch. The push-pull scheme of the modulator is depicted in Fig. 9, c. It provides the tightest control of the output pulse shape as well as the highest energy efficiency among the schemes discussed before. The expense for that is much more complexity in design as compared to previous solutions. It is one of the reasons why the modulators utilizing such approach may be found in a very limited number of radars despite its evident advantages. Let us now discuss some issues concerned to the selection of an appropriate electronic device to build the high voltage switch. According to modern tendencies solid state devices should be considered as first choice while designing any new electronic system. Two types of solid state devices, namely, MOSFETS, and IGBT may be used in magnetron modulators. Despite IGBT are superior generally to MOSFETs as respect to both maximal rated voltage and current, as well as efficiency provided, they are characterized by a lower switching speed and an attitude to a second-induced breakdown, which confines the overall reliability of the modulator. Thus, MOSFETs remain the only choice among solid state devices to use in the magnetron modulators intended for high performance millimeter wavelength radars featured by rather short operational pulse width. Hard tubes were historically the first devices used to build high voltage modulators in radars. They remain to be utilized widely until now despite a serious competition from solid state devices. A considerably greater robustness should be indicated as an essential reason. High voltage circuitry have a strong attitude to appearance of various local breakdowns, leakages etc, which are difficult to control and prevent especially for long-term unattended radar operation. Such phenomena stress the modulator parts greatly. Energy to destroy a hard tube is on orders higher than that for any solid state device. Next, the only tube can be used practically always to build any modulator. Instead the limitations for currently available powerful MOSFETs in the maximal rated voltage, makes inevitable utilizing a stack arranged from many transistors connected in series and, possibly, parallel in order to achieve the modulator parameter being enough to drive the most magnetrons. Utilization of pulse transformer allows in principle to minimize number of the transistors used but, as mentioned above, causes considerable pulse distortions. Certainly there is a well-known disadvantage of hard tubes, namely, a limited life time. We dare to claim that it may be considered as almost virtual at the time being. The situation is very similar to that mentioned above for the magnetrons. A current state of cathode manufacturing as well general state of vacuum technique makes expected lifetime of the modulator hard tubes of several ten thousand hours very realistic. These expectations were ascertained completely by our experience of utilization of hard tube based modulators in the line of meteorological radars (see Section 3). The above consideration allows us to make a conclusion that despite of a strong competition from solid state devices, partially MOSFETs, hard tubes are keeping their positions under development of modern magnetron based radars. The only issue may prevent using them in the modulators, namely, their commercial availability and assortment, which decreases actually generally at the time being due to, essentially, a shortage in demand from non-radar application. Certainly we do not mean a fantastic breakthrough in the development of high voltage, high power semiconductor devices, which makes all hard tubes obsolete at one bout! It should be noticed that the modulator may operates as either voltage or current source. The latter is conceptually better for any cross-field vacuum tube (Sivan, 1994). On other hand the introduction of the current mode in the modulator makes its design more complex especially in the case if a short pulse length is required. Our experience demonstrates that providing an appropriate stability of the high voltage power supply it is possible to achieve a great magnetron performance even if the much simpler voltage mode is utilized in the modulator. Nevertheless, the development of a modulator, operating in the current mode, remains the greatest challenge a designer faces from our opinion. 4.2.4 Magnetron filament and protection circuits As mentioned in Section 0, keeping an adequate condition of the cathode surface is the most important issue to prolong the magnetron lifetime. It depends on the following factors: (i) cathode temperature; (iii) vacuum condition inside the magnetron; (iii) electron back bombardment. The cathode temperature depends both of a filament power applied and the power dissipated on the cathode due to its back bombardment. This temperature should be kept within a rather narrow interval of several tens degrees typically. Thus, the filament power supply should ensure a very tight control of the magnetron filament power as well as provide a dedicated procedure to regulate it depending on the parameters of the magnetron operational mode. In the developed radars (see Section 0) the following proven principle is used. The filament power supply comprises of two parts, low and high side ones correspondingly. The low side part is simply PWM inverter equipped with either analog or digital controller. The high side includes a high voltage decoupling transformer, a rectifier, and a dedicated controller. It should be noticed that we consider that DC filament voltage should be used to supply magnetron filament due to in this case a possible alternating of the magnetron frequency is canceled. The controller is used to measure both filament voltage and current and to transfer the corresponding data to the low side in a digital form. An optical link is used for such communication. The above approach provides accurate and independent measurement and control of the magnetron filament parameters. The vacuum conditions inside the magnetrons depend not only on quality of manufacturing routine and materials it is made of. Electrical breakdowns affect them strongly. In general, it is considered that the magnetrons demonstrate a rather strong attitude to the development of breakdowns. They may cause in addition direct magnetron damage if no current limiting is provided by the modulator circuitry. It is important for the millimeter wavelengths magnetrons especially due to such magnetrons are characterized by a rather delicate structure of cavities. Hard tubes provide inherently such current limitation, which can be regulated moreover. MOSFETs are featured similarly, but it is difficult to regulate a limit value due to the high voltage switch consists always of several devices connected in series. As a result, this limit is defined by the maximal rated current of the transistors used. Certainly, there is a contradiction between the necessity both to limit the output current and to ensure a minimal settling time of the output voltage of the modulator. Our experience demonstrates that there is a simple way to prevent possible worsening of the magnetron parameters due to breakdowns without a noticeable degradation in the shape of its output pulse. Namely ferrite beads should be connected in series with the magnetron. This effect may be explained as follows. Breakdown in the magnetron appears usually in two stages. The first stage caused by field emission from a tip located somewhere inside magnetron. This stage runs very fast, typically during several nanoseconds. It cause negligible damages of the magnetron internal parts, but initiates developing the next stage MagnetronBasedRadarSystemsforMillimeter WavelengthBand–ModernApproachesandProspects 479 configuration. Instead, from our opinion, it is the most preferable scheme to utilize a solid state high voltage switch. The push-pull scheme of the modulator is depicted in Fig. 9, c. It provides the tightest control of the output pulse shape as well as the highest energy efficiency among the schemes discussed before. The expense for that is much more complexity in design as compared to previous solutions. It is one of the reasons why the modulators utilizing such approach may be found in a very limited number of radars despite its evident advantages. Let us now discuss some issues concerned to the selection of an appropriate electronic device to build the high voltage switch. According to modern tendencies solid state devices should be considered as first choice while designing any new electronic system. Two types of solid state devices, namely, MOSFETS, and IGBT may be used in magnetron modulators. Despite IGBT are superior generally to MOSFETs as respect to both maximal rated voltage and current, as well as efficiency provided, they are characterized by a lower switching speed and an attitude to a second-induced breakdown, which confines the overall reliability of the modulator. Thus, MOSFETs remain the only choice among solid state devices to use in the magnetron modulators intended for high performance millimeter wavelength radars featured by rather short operational pulse width. Hard tubes were historically the first devices used to build high voltage modulators in radars. They remain to be utilized widely until now despite a serious competition from solid state devices. A considerably greater robustness should be indicated as an essential reason. High voltage circuitry have a strong attitude to appearance of various local breakdowns, leakages etc, which are difficult to control and prevent especially for long-term unattended radar operation. Such phenomena stress the modulator parts greatly. Energy to destroy a hard tube is on orders higher than that for any solid state device. Next, the only tube can be used practically always to build any modulator. Instead the limitations for currently available powerful MOSFETs in the maximal rated voltage, makes inevitable utilizing a stack arranged from many transistors connected in series and, possibly, parallel in order to achieve the modulator parameter being enough to drive the most magnetrons. Utilization of pulse transformer allows in principle to minimize number of the transistors used but, as mentioned above, causes considerable pulse distortions. Certainly there is a well-known disadvantage of hard tubes, namely, a limited life time. We dare to claim that it may be considered as almost virtual at the time being. The situation is very similar to that mentioned above for the magnetrons. A current state of cathode manufacturing as well general state of vacuum technique makes expected lifetime of the modulator hard tubes of several ten thousand hours very realistic. These expectations were ascertained completely by our experience of utilization of hard tube based modulators in the line of meteorological radars (see Section 3). The above consideration allows us to make a conclusion that despite of a strong competition from solid state devices, partially MOSFETs, hard tubes are keeping their positions under development of modern magnetron based radars. The only issue may prevent using them in the modulators, namely, their commercial availability and assortment, which decreases actually generally at the time being due to, essentially, a shortage in demand from non-radar application. Certainly we do not mean a fantastic breakthrough in the development of high voltage, high power semiconductor devices, which makes all hard tubes obsolete at one bout! It should be noticed that the modulator may operates as either voltage or current source. The latter is conceptually better for any cross-field vacuum tube (Sivan, 1994). On other hand the introduction of the current mode in the modulator makes its design more complex especially in the case if a short pulse length is required. Our experience demonstrates that providing an appropriate stability of the high voltage power supply it is possible to achieve a great magnetron performance even if the much simpler voltage mode is utilized in the modulator. Nevertheless, the development of a modulator, operating in the current mode, remains the greatest challenge a designer faces from our opinion. 4.2.4 Magnetron filament and protection circuits As mentioned in Section 0, keeping an adequate condition of the cathode surface is the most important issue to prolong the magnetron lifetime. It depends on the following factors: (i) cathode temperature; (iii) vacuum condition inside the magnetron; (iii) electron back bombardment. The cathode temperature depends both of a filament power applied and the power dissipated on the cathode due to its back bombardment. This temperature should be kept within a rather narrow interval of several tens degrees typically. Thus, the filament power supply should ensure a very tight control of the magnetron filament power as well as provide a dedicated procedure to regulate it depending on the parameters of the magnetron operational mode. In the developed radars (see Section 0) the following proven principle is used. The filament power supply comprises of two parts, low and high side ones correspondingly. The low side part is simply PWM inverter equipped with either analog or digital controller. The high side includes a high voltage decoupling transformer, a rectifier, and a dedicated controller. It should be noticed that we consider that DC filament voltage should be used to supply magnetron filament due to in this case a possible alternating of the magnetron frequency is canceled. The controller is used to measure both filament voltage and current and to transfer the corresponding data to the low side in a digital form. An optical link is used for such communication. The above approach provides accurate and independent measurement and control of the magnetron filament parameters. The vacuum conditions inside the magnetrons depend not only on quality of manufacturing routine and materials it is made of. Electrical breakdowns affect them strongly. In general, it is considered that the magnetrons demonstrate a rather strong attitude to the development of breakdowns. They may cause in addition direct magnetron damage if no current limiting is provided by the modulator circuitry. It is important for the millimeter wavelengths magnetrons especially due to such magnetrons are characterized by a rather delicate structure of cavities. Hard tubes provide inherently such current limitation, which can be regulated moreover. MOSFETs are featured similarly, but it is difficult to regulate a limit value due to the high voltage switch consists always of several devices connected in series. As a result, this limit is defined by the maximal rated current of the transistors used. Certainly, there is a contradiction between the necessity both to limit the output current and to ensure a minimal settling time of the output voltage of the modulator. Our experience demonstrates that there is a simple way to prevent possible worsening of the magnetron parameters due to breakdowns without a noticeable degradation in the shape of its output pulse. Namely ferrite beads should be connected in series with the magnetron. This effect may be explained as follows. Breakdown in the magnetron appears usually in two stages. The first stage caused by field emission from a tip located somewhere inside magnetron. This stage runs very fast, typically during several nanoseconds. It cause negligible damages of the magnetron internal parts, but initiates developing the next stage MicrowaveandMillimeterWaveTechnologies:ModernUWBantennasandequipment480 brought by a local overheating, cooper sputtering, and further arcing. This stage requires much longer time to run but can cause significant degradation of the magnetron performance. The ferrite beads demonstrate rather high impedance under transient conditions. Thus, it leads to fast decrease in the voltage across magnetron during the first stage of the breakdown and prevents developing of the second stage. Moreover the magnetron keeps producing RF oscillations as usual. There are some more benefits from the utilization of ferrite beads. They restrict the change rate of magnetron current resulting in a great reduction of electromagnetic interferences and an increase to overall stability of the transmitter operation. In addition such beads may improve the output pulse shape weakening the influence of stray circuit inductance and capacitance on the pulse front formation. It should be noticed that the usage of ferrite beads is practically mandatory if MOSFET based high voltage switch is used in the modulator. Such devices are characterized by rather low rated    capability. According to a numerical simulation and experimental investigations, an excess in this value is an essential reason of transistors damage during transients in the magnetron. At last, it should be noticed that there are evidences that influence of the cathode back bombardment is not limited by thermal effects only as mentioned above. Change in the cathode surface condition has been fixed even after single pulses. Probably it is due to a very high value of instantaneous power of the back bombardment, which is about 10 % of anode power at a nominal anode voltages and much more – up to 30 % - at lower voltages. In order to minimize effect of back bombardment it is necessity to keep the duration of the modulation pulse trailing edge as shorter as possible (see Section 0) 4.3 Receiver 4.3.1 General consideration The principles to develop receivers for the magnetron based radars are the same as for any other systems. Nevertheless, we would like to discuss below some design approaches, which have proven their efficiency in a number of radars developed in the Institute of Radio Astronomy of National Academy of Sciences of Ukraine. A simplified block-diagram of a full-featured double channel receiver used in the most modern modifications of Ka band magnetron based meteorological radars (see Section 0) is depicted in Fig. 12. It may be considered as typical and reflecting the essential design approaches. At first, double frequency conversion is used in the receiver. It makes the receiver design more complex apparently, but simplifies greatly the design of local oscillators as well as allows utilizing a simple filtration to reject image frequency. The local oscillator(s) should meet the following requirements: (i) cover wide enough frequency range – typical values of possible frequency variation are 500 MHz for Ka band and 700…1000 MHz for W and G band magnetrons respectively; (ii) provide frequency tuning with a relatively small step; and (iii) ensure a low phase noise with a fast roll-off beyond the band of         , which corresponds to the unambiguous frequency range of Doppler processing. Fi g T h pr o fi x os c If q fr e N e p u ev i pr o In s ill u fr e di s (ii ) p h pi c N e s ys di r m u g . 10. Block-dia gr h e latter is requ i o cessin g . In the c x ed frequenc y , w h c illator with fre q q ualit y of Doppl e e quenc y conversi o e xt point we wo u u lse to the data i dent wa y is to u o vides the hi g h e s tead a si g nal, l e u strated b y Fi g . e quenc y onl y . T h s advanta g es: (i) a ) as usual the rec e h ase instabilit y . S c k a sample of th e e vertheless it w a s tems, which us e r ect comparison u ch more desi g n r am of receiver f o i red to avoid a n c ase of double fr e h ich makes its de q uenc y multiplie r e r processin g pr o o n simplifies its d u ld like to discus acquisition uni u se a separate d o e st possible perf o e aked throu g h t h 11. In this case h is wa y is certa i a n additional ph a e iver circuitr y o p S everal radars h a e transmitted pu l a s worse on se v e a separate dow n is hardl y possi b details tha n the m o r ma g netron bas e n increase in th e e quenc y convers i si g n relativel y si m r provides enou g o vided b y a rada r d esi gn si g nifican t s is a methods t o t to implement o wnconvertin g c h o rmance but re q h e receiver prote c newly introduc e i nl y much simp l a se j itter ma y be i p erates in a deep a ve been develo p l se up. The y de m v eral decibels a s n convertin g cha n b le due to the s y m ethod of sampl i e d Doppler rada r e noise floor of i on the first local m ple. As usual a g h performance l e r is not an issue, t l y (Volkov et al, o couple a sampl e coherence-on-r e h annel as depict e q uires additional c tion circuitr y , m e d circuits oper a l er but characte r i ntroduced b y a n saturation, whic h p ed b y utilizin g m onstrated a soli d s compared wi t n nel. It should b e y stems in consi d i n g transmitted p r . coherence-on-r e oscillator opera t dielectricall y sta b e vel for the mos t the utilization of 2007) e of the transmit t e ceiver processi n e d in Fig. 10. Th i hardware to b e m a y be used in t h a te at the inter m r ized b y the fol l n antenna enviro n h ma y cause ad d the above appr o d Doppler perfor m t h the performa n e noticed actuall y d eration distin gu p ulse. e ceiver t es at a b ilized t cases. sin g le t ed RF ng . An i s wa y e used. h e wa y m ediate l owin g n ment; d itional o ach to m ance. n ce of y that a u ish in [...]... (1994) Microwave Tube Transmitters London: Chapman&Hill 488 Microwave and Millimeter Wave Technologies: Modern UWB antennas and equipment Skolnik, (2008) Radar Handbook, Third Edition New York: McGraw-Hill Tsimring (2007) Electron Beams and Microwave Vacuum Electronics Hoboken, NJ: John Wiley & Sons, Inc Usikov (1972) Investigationgs in Microwave Electronics made in IRE AS Ukraine (in Russian) Electronic... the voltag across magnet ge tron drops rather slowly (see Fig 7) and duration of this r pro ocess depends strongly on variou parameters Th magnetron is g us he generally charact terized by attitude to produ oscillations at rather low anod voltages y uce t de 484 Microwave and Millimeter Wave Technologies: Modern UWB antennas and equipment Bandwidth, MHz Maximum peak power @ 400 nsek pulse duration, W... sampled transmitter signal, digitized by an ADC Due to using a digital quadrature phase detector, the sign of the frequency offset can be measured, and the result of the measurements is 486 Microwave and Millimeter Wave Technologies: Modern UWB antennas and equipment practically independent on the amplitude of the sampled signal This loop ensures a much better accuracy, which can be as precise as ± 50... than the m uch method of sampli transmitted p ing pulse 482 Microwave and Millimeter Wave Technologies: Modern UWB antennas and equipment Fig 11 Utilization o leakage signal to implement coh g of herence-on-receiv technique ver 4.3 Receiver pro 3.2 otection circuitry y As known the mag s gnetron is charac cterized by a rath high both pea and average p her ak power, wh hereas maximal r rated input power... accuracy may be as lower as 1 kHz that corresponds to relative accuracy of 3·10-8 and 10-8 for Ka and W band magnetrons respectively The indicated value is on about order better than the best frequency stability achieved (see Section 0) resulting in the Magnetron Based Radar Systems for Millimeter Wavelength Band – Modern Approaches and Prospects 487 corresponding improvement of Doppler processing quality... Systems for Millimeter Wavelength Band – Modern Approaches and Prospects 485 elements Despite such approach has proven its usefulness, there were evident disadvantages At first, the attenuation introduced depends on temperature and frequency rather strongly Next, every time when the switch is replaced, the corresponding P-i-N diode driver circuits should be re-adjusted in order to keep a standardized... for Millimeter Wavelength Band – Modern Approaches and Prospects a) Fig 12 Design of high power P-i-N switch for receive protection g er 483 b) uch ows us to build a absorptive sw an witch as well as to improve its ma aximal Su approach allo ted er r rat average powe due to the incident wave power dissipated on a termination but not on the diode The par e rameters of some switches used in Ka band rad... a fruitful discussions and comprehensive assistance 7 References Barker et al (2005) Modern Microwave and Millimeter- Wave Power Electronics Wiley-IEEE Press Belikov et all (2002) 95 GHz, 2 kW magnetron transmitter of 20 ns pulses Proceedings of German Radar Symposium-2002, (стр 571-574) Bonn Brown (1999) Technical and Military Imperatives: A Radar History of World War 2 New York: Taylor & Francis Gritsaenko,... of the magnetron operation and introducing sophisticated signal processing algorithms make possible to expect even higher performance level of the magnetron based radars, especially for that operating within short millimeter wavelength bands in the nearest future 6 Acknowledgments I am appreciating vastly to Prof D M Vavriv and Dr V D Naumenko for a fruitful discussions and comprehensive assistance... 94 and 36 GHz radar system for remote sensing applications Proceedings of International Geoscience and Remote Sensing Symposium, 5, стр 25962598 Hamburg Li, Hua; Illingworth, A J.; Eastment, J (1994) A simple method of Dopplerizing a pulsed magnetron radar Microwave Journal , 37 (4), 226-235 Okress (1961) Crossed-field Microwave Devices (Т 1,2) New York & London: Academic Press Sivan (1994) Microwave . internal parts, but initiates developing the next stage Microwave and Millimeter Wave Technologies: Modern UWB antennas and equipment 480 brought by a local overheating, cooper sputtering, and further. Microwave and Millimeter Wave Technologies: Modern UWB antennas and equipment 488 Skolnik, (2008). Radar Handbook, Third Edition. New York: McGraw-Hill. Tsimring. (2007). Electron Beams and Microwave. sign of the frequency offset can be measured, and the result of the measurements is Microwave and Millimeter Wave Technologies: Modern UWB antennas and equipment 486 practically independent on the