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Traveling-Wave    Tubes
RECEIVER   SYSTEM   FUNCTIONAL   AREAS

Fire Controlman Volume 02-Fire Control Radar Fundamentals
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The   operating   frequency   of   a   TWT   oscillator   is determined  by  the  pitch  of  the  tube’s  helix.  The  os- cillator  can  be  tuned,  within  limits,  by  adjusting  the operating   potentials   of   the   tube. The   electron   beam   passing   through   the   helix   in- duces  an  electromagnetic  field  in  the  helix.  Although initially  weak,  this  field  will  cause  bunching  of  suc- ceeding  portions  of  the  electron  beam. With  the  proper  potentials  applied,  the  bunches  of electrons   reinforce   the   signal   on   the   helix.   This,   in turn,  increases  the  bunching  of  succeeding  portions  of the  electron  beam.  The  signal  on  the  helix  is  sustained and   amplified   by   this   positive   feedback,   resulting from  the  exchange  of  energy  between  the  electron beam  and  the  helix. MODULATORS Transmitter   tubes   all   have   one   common   require- ment—a   source   of   high   voltage—whether   they   are operated   in   a   CW   or   pulsed   mode.   A   fixed   high- voltage  supply  for  CW  operation  is  relatively  simple compared  to  a  pulsed  high-voltage  supply.  A  pulsed high-voltage  power  supply  is  called  a   modulator, which  means  to  shape,  as  well  as  to  control.  Modu- lators  are  also  called  pulsers  or keyers,  depending   on their   circuit   function. The   power   supply   provides   the   source   of   high voltage  to  charge  the  storage  device.  The  storage  de- vice  can  be  capacitive  or  inductive,  or  a  combination of  capacitance  and  inductance  that  stores  electrical energy   in   the   form   of   an   electrostatic   or   electro- magnetic  field.  Electrostatic  storage  is  the  most  com- mon  method  used. The   storage   device   is   charged   through   the   rela- tively   high   impedance   of   the   charge   path   while   the switch   is   open.   The   load   is   the   high-power   tube, which  is  usually  connected  by  a  step-up  pulse  trans- former.  The  pulse  transformer  allows  the  dc  to  charge the  storage  device. When  the  switch  is  closed,  a  low-impedance  dis- charge  path  is  provided  for  the  storage  device  through the  pulse  transformer.  The  charge  time  is  long  com- pared  to  the  discharge  time  because  of  the  differences in   impedances.   The   switch   is   normally   an   electronic element,   such   as   a   thyratron   or   a   silicon-controlled rectifier   (SCR),   since   mechanical   switching   is   im- practical  at  the  pulse-repetition  frequencies  used  in most   radars. There  are  many  different  types  of  pulse  modula- tors   in   use   today.   The   most   common   type   found   in fire-control  applications  is  the  line  modulator.  The basic  elements  of  a  line  modulator  are  shown  in  figure 2-10. Figure 2-10.—Basic elements of a modulator. 2-20






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