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RT  CHASSIS  (1A16).  Continued
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TM-11-5820-890-30-5 Ground Non-Icom Radio Sets Consisting of: Receiver-Transmitter Radio RT-1439/VRC (NSN 5896-01-195-0827) Manual
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RECEIVE SIGNAL PATH.  Continued


TM 11-5820-890-30-5 2-7. BASlC RT SIGNAL TYPES. There  are  five  basic  signal  types  used  in  the  RT: Analog Digital Control Power RF Analog  signals  include  the  audio  and  analog  data  signals. They  can  vary  greatly  in  signal  level,  shape,  and frequency. Digital signals include the timing clocks and digital data signals. Clock frequencies vary. The clocks are used to synchronize the serial digital data streams between modules. Within the RT they are typically at logic 0 and logic 1 levels. RT I/O digital signals use the ±5 V logic levels as required by MlL-STD-188-114. Logic 0 is 5 V dc. Logic 1 is -5 V dc. The ECCM fill device uses 0 V for logic 1 and -6.75 V for logic 0. The logic levels in the RT are as follows: 6.75  V  dc  Logic:  Logic  0  =  -0.7  to  2.0  V  dc, Logic 1   =  4.7  to  7.5  V  dc Negative  Logic: Logic 1 =  -5.0  to  -7.25  V  dc Control  signals  include  the  status  and  control  lines.    They will be set to logic 1 to indicate or direct a particular condition.  In  some  cases,  a  signal  name  includes  a  “ –N”  to  indicate  that  the  logic  is  reversed.  For  example,  a logic  1  on  the  PTT-N  line  indicates  the  absence  of  a  PTT;  logic  0  indicates  a  PTT. Power signals are at constant V dc levels. Most are provided by the power supply as described in paragraph 2-5. The  audio  power  supply  provides  the  voltages  used  by  the  audio/data  section. RF and IF signals are also present in the RT. Coaxial cables are used to pass these signals between modules. Frequencies  range  from  12.5  (IF)  to  100.5  MHz. 2-8. RECEIVE  RF  SIGNAL  PATH. The RF signal received by the RT is processed by four modules to provide the demodulated audio or data signal. These  modules  are: Impedance  Matching  Network  (1A1) Exciter/Power   Amplifier   (1A11) Tuner/Mixer  (1A9) lF/Demodulator   (1A8) See  figure  FO-1  for  the  block  diagram  of  this  signal  path. The RF signal enters the RT at the ANT connector (J1) and passes through the impedance matching network. The impedance matching network has four bands. One is a 50-ohm network that is used when a BNC is connected to the RT. When a manpack antenna is connected to the RT, one of three impedance matching networks is used depending  on  the  frequency  selected. A  logic  1  level  is  placed  on  the  RF  input  path  by  the  impedance  matching  network.  If  a  manpack  antenna  is connected to the ANT connector, the switch shown in figure 2-9 stays closed. The logic 1 level is pulled to ground by the inductor. The impedance matching network detects the change in logic level and selects the appropriate matching network. If a BNC is connected to the ANT connector, the center part of the connector is pushed in and opens  the  switch.  The  RF  signal  is  fed  through  the  capacitor.  The  dc  level  stays  at  logic  1.  The  impedance matching  network  selects  the  50-ohm  matching  network. 2-12


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