Local  Oscillators Most  radar  receivers  use  a  30-  or  60-MHz  inter- mediate   frequency.   A   highly   important   factor   in receiver  operation  is  the  tracking  stability  of  the  local oscillator,   which   generates   the   frequency   that   beats with   the   incoming   signal   to   produce   the   IF.   For example,   if   the   local   oscillator   frequency   is   3000 MHz,  a  frequency  shift  of  as  much  as  0.1  percent would  be  a  3-MHz  frequency  shift.  This  is  equal  to the   bandwidth   of   most   receivers   and   would   cause   a considerable  loss  in  gain.  Bandwidth  is  the  inverse  of the  pulsewidth,  with  a  wider  bandwidth  for  narrow pulses. In   receivers   that   use   crystal   mixers,   the   power required  of  the  local  oscillator  is  small,  only  20  to  50 milliwatts   in   the   4000-MHz   region.   Because   of   the very   loose   coupling,   only   about   1   milliwatt   actually reaches   the   crystal. Another  requirement  of  a  local  oscillator  is  that  it must  be  tunable  over  a  range  of  several  megahertz  to compensate   for   changes   in   both   the   transmitted   fre- quencies  and  its  own  frequency.  It  is  desirable  that  the local  oscillator  have  the  capability  of  being  tuned  by varying   its   voltage. Because   the   reflex   klystron   meets   these   re- quirements,   it   is   used   as   a   local   oscillator   in   some radar  receivers.  As  the  local  oscillator  in  a  microwave receiver,  a  reflex  klystron  need  not  supply  large amounts  of  power,  but  it  should  oscillate  at  a  fre- quency  that  is  relatively  stable  and  easily  controlled. The  need  for  a  wide  electronic  tuning  range  sug- gests   the   use   of   a   voltage   mode   of   a   high   order. However,  if  a  mode  of  an  excessively  high  order  is selected,  the  power  available  will  be  too  small  for local  oscillator  applications,  and  a  compromise  be- tween   wide   range   and   power   is   necessary.   Also,   the use  of  a  very-  high-order  mode  is  undesirable  because the  noise  output  of  a  reflex  klystron  is  essentially  the same  for  all  voltage  modes.  Thus,  the  closer  coupling to   the   mixer   required   with   high-order,   low-power modes  increases  the  receiver  noise  figure.  Usually,  the 1-3/4-  or  2-3/4-voltage  mode  is  found  suitable.  Since the  modes  are  not  symmetrical,  the  point  of  operation is  usually  a  little  below  the  resonant  frequency  of  the cavity.  This  makes  possible  tuning  above  the  operat- ing  frequency  to  a  greater  degree  than  if  the  precise resonant   frequency   is   used. In  practice,  the  reflex  klystron  is  used  with  an automatic  frequency-control  circuit.  Since  the  repeller voltage   is   effective   in   making   small   changes   in   fre- quency,  the  AFC  circuit  is  used  to  control  the  repeller voltage   to   maintain   the   correct   intermediate   fre- quency.  It  should  be  noted  that  the  coarse  frequency of  oscillation  is  determined  by  the  dimensions  of  the cavity,  and  there  is,  on  most  reflex  klystrons,  a  coarse frequency   adjustment,   which   varies   the   cavity   size. Reflex  klystrons  are  also  used  as  drivers  for  RF power   amplifier   klystrons.   When   they   are   used   as drivers,  the  frequency  and  the  amplitude  stability  are much  more  critical.  Any  variation  in  driver  frequency is  reproduced  in  the  power  amplifier  output  and,  thus, on   the   target   echo   signal.   This   frequency-modulation (FM)  variation  can  result  in  degraded  Doppler  track- ing  and  velocity  computations.  If  the  FM  deviation  is large   enough   or   if   the   driver   is   not   operated   at   the peak  of  a  mode,  then  amplitude  variations  will  occur. This  amplitude  modulation  (AM)  may  be  very  small in  magnitude  on  the  driver  signal,  but  after  a  gain  of 30  dB  or  more  in  the  power  amplifier,  the  magnitude can   be   considerable. Both  FM  and  AM  signals  are  undesirable  and  are classified  as  noise.  Therefore,  extra  care  in  tuning  and maintenance  of  the  power  supplies  is  required  to  mini- mize  FM  and  AM  noise  generation. Frequency    Synthesizers Local   oscillator   configurations   vary   considerably, depending  on  the  requirements  of  the  individual  radar system  (the  alternate  system  is  shown  by  dotted  lines in  figure  2-11).  MTI,  pulse-Doppler,  and  CW-Doppler radars   require   close   control   over   the   phase   and   the frequency   of   the   local   oscillator   to   provide   coherent detection.   Pulse-compression   radar   receivers   also require  close  frequency  and  phase  control  for  their form   of   coherent   detection.   A   frequency   synthesizer system   (instead   of   STALO)   is   becoming   more   com- 2-23