switched on, it supplies 115 volts ac and 28 volts dc to the  omega  receiver. ANTENNA   COUPLER.—   The   AS-2623/ ARN-99(V) antenna coupler has two loop antennas mounted  at  90-degree  angles  to  each  other  and  at 45-degree  angles  to  the  aircraft  centerline.  These antennae  are  directional,  and  the  proper  antenna selection is based upon location to the station being received, relative to the heading to the aircraft. One of the four antenna lobes is selected (A+, B+, A-, or B-)   to   give   the   receiver-converter   the   maximum signal   strength   from   the   desired   omega   ground station. Once the omega has been synchronized, the antenna  selection  process  is  automatically  controlled by the central computer. RECEIVER-CONVERTER.—   The OR-90/ ARN-99(V)   receiver-converter   consists   of   five sections.  These  sections  are  the  receiver  section, correlator  and  digital  converter  section,  computer communication section, discrete storage section, and the  power  supply  section. Receiver Section.— The receiver section consists of the antenna switching matrices, RF amplifiers, IF amplifiers,  and  a  precision  frequency  generator. The antenna switching matrices sum and phase shift  the  incoming  signals  to  provide  an  antenna configuration that will be best oriented to a specific omega station. These circuits also enables test signals to  be  injected  into  the  omega  system.  There  are  three of these matrices in this section, one for each of the operating   frequencies. The  RF  amplifiers  remove  the  IF  image  and provide  attenuation  to  remove  signals  far  from  the operating  frequency.  There  are  three  of  these  circuits, one  for  each  frequency,  with  the  only  difference between them being the tuning of the bandpass filters and  the  notch  filters.  The  heterodyne  mixers  are identical for all three. The  local  oscillator  frequencies  produce  a  1.33 kHz IF signal. Each frequency used has its own IF amplifier  circuit;  these  circuits  are  identical  for  all Figure 2-15.-Omega power control panel. three  frequencies.   The limiters in the circuit control the dynamic signal level in the amplifier, preventing saturation of the linear filters. The precision frequency generator generates the precision frequency signals required for operation of the system. The generator consists of a 10.608 MHz crystal oscillator and counters. The counters divide the oscillator frequencies to provide a 13.6 kHz RF test signal, a 1.133 kHz IF reference signal, a 14.733 kHz  local  oscillator  signal,  a  176.8  kHz  receiver- computer input/output clock signal, a 11.333 kHz RF test and local oscillator signal, and a 10.2 kHz RF test and  local  oscillator  signal. Correlator and Digital Converter Section.— This section converts the phase of the IF signals into digital form. The three channels use identical phase converters. The  phase  of  the  IF  signal  is  the navigation   information   needed   by   the   central computer. Computer  Communication  Section.—   The receiver-converter  operation  is  computer  controlled and  cannot  be  operated  manually.  This  section provides  a  means  of  communication  between  the receiver-converter  and  the  central  computer.  This section  receives  data  requests  from  the  computer  and sends the desired data to the computer. Discrete   Storage   Section.—   This   section provides a means of storing and controlling antenna switching and test signal gating commands from the central  computer  for  use  in  the  receiver-converter. The discrete storage consist of control line drivers and a decoder circuit. It acts as an interface between the communication  section  and  the  receiver  sections. Power  Supply  Section.—  The  power  supply generates regulated and unregulated dc voltages for the  system.  The  power  supply  also  provides  for short-circuit   protection   and   for   overvoltage protection.  The  short-circuit  protection  is  for  the three regulators, (+16, +5, and -16 Vdc regulators). A short in any of these will cause the regulator to be clamped  to  ground,  and  the  power  supply  will  need  to be reset. The overvoltage protection is for the +5 Vdc circuit. When the output of the +5 Vdc exceeds the breakdown voltage of the Zener, a relay is energized that  removes  the  input  power.  When  this  occurs, system  power  needs  to  be  cycled  to  reset  the protection   circuits. 2-17