stant   amplitude   signal   out   of   the   IF   amplifier,   inde- pendent  of  range. Logarithmic   IF   Amplifiers When  radars  are  operated  where  interference  and clutter  are  encountered  or  where  jamming  may  occur, techniques   can   be   used   to   adjust   the   receiver   sensi- tivity   as   the   interference   varies   in   intensity.   Digital processing   uses   a   threshold   criteria   similar   to   that where  an  operator  adjusts  the  sensitivity  to  the  point where  interference  is  just  barely  detectable.  This  is  the process  of  constant  false-alarm  rate  (CFAR)  when  the radar   receiver   makes   the   adjustments   automatically. Logarithmic  amplifiers  rely  on  the  CFAR  principle  to amplify   signals   that   exceed   a   certain   threshold   and provide  only  unity  gain  for  those  below  the  threshold. The   threshold   is   automatically   adjusted   to   maintain the   same   false-alarm   rate. Logarithmic  IF  amplifiers  are  widely  used  in  elec- tronic   attack   (EA)   receivers   and   monopulse   angle- tracking  channels.  They  are  also  used  in  digital  pulse compression   and   phase   detection. Detectors The  detector  in  a  basic  radar  receiver  converts  the IF  signal  into  a  video  signal  to  be  displayed  and/or processed   for   tracking.   There   are   many   forms   of detection,  and  they  vary  depending  on  the  type  of radar  and  method  of  coherency  used. Detectors   include   linear   (diode)   detectors,   log- arithmic   detectors,   and   phase-sensitive   detectors. LINEAR   (DIODE)   DETECTORS.—   The   sim- plest  form  of  detectors,  and  still  commonly  used,  are the  diode  detectors.  They  are  classified  as  linear  de- tectors  because  their  output  is  directly  proportional  to the  input. LOGARITHMIC   DETECTORS.—   The  loga- rithmic   detectors,   commonly   called   log  detectors, have  outputs  proportional  to  the  logarithm  of  the  IF envelope  input.  Logarithmic  detectors  are  fairly  com- mon   in   EA   receivers.   They   usually   have   multiple stages   where   the   overall   gain   varies   logarithmically. PHASE-SENSITIVE   DETECTORS.—   A  syn- chronous/coherent   detector   for   MTI   was   shown   in figure  2-11.  This  type  of  detector  has  several  possible configurations,  based  on  the  type  of  signal  desired  as an  output. A  phase  (0)  detector  is  one  form  of  synchron- ous/coherent  detector;  it  has  an  output  that  has  only phase   information.   A   synchronous   detector   has   both phase   and   amplitude   information   in   its   output.   A balanced   mixer   has   phase,   amplitude,   and   frequency information  in  its  output. Phase-sensitive   detectors   (0   detectors)   are   key elements   in   MTI   radars.   The   0 detector   detects   the phase-shift  information  caused  by  a  moving  target. The  detector  receives  inputs  of  the  IF  signal  and  the COHO  signal,  and  it  produces  a  video  signal  whose amplitude   and   polarity   varies   with   phase   differences only. The  0 detector  can  be  used  in  monopulse-tracking radar  receivers  to  determine  the  angular  error  of  the target   from   the   boresight   center.   Two   identical receiver  channels  are  required  to  provide  azimuth  and elevation  error  detection.  The  angle  channel  phase  is compared  to  the  range  channel  reference  phase  to  pro- duce  a  signal,  whose  amplitude  and  polarity  indicate the   amount   and   direction   of   phase   error,   which   is directly  related  to  the  angle  offset. A   synchronous   detector   can   provide   the   same information  as  a  phase  detector  in  monopulse  radars. The  difference  is  that  the  phase  comparison  deter- mines  the  polarity  of  the  output  signal  only,  while  the amplitude  comparison  determines  the  amount  of  error. A  balanced  mixer  is  more  suited  for  use  in  CW and  Doppler  signal  processing  since  frequency  infor- mation  is  also  contained  in  the  output  of  balanced mixers.  This  signal  is  then  fed  to  the  filter  bank  for Doppler   shift   detection. 2-27