2-7, BASIC RT SIGNAL TYPES
There are five basic signal types used in the rt.
Analog signals include the voice audio. Analog signals can vary greatly in signal level, shape, and frequency.
Digital signals include timing clocks and digital data signals. The clocks are used to synchronize the serial data
streams between modules. Clock frequencies vary, Voltage levels are normally logic 0 or logic 1. Within the rt,
logic O is typically -0.5 to 0.5 V dc. Logic 1 is typically 6.25 to 7.25 V dc, Rt I/O digital signals use the ±5 V logic
level as required by MlL-STD188-114, 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.
Control signals include status and control lines. They will be set to logic 1 to indicate or direct a particular condi-
tion. In some cases, a -N is used to indicate that the logic is reversed. For example, a logic 1 on PTT-N
indicates the absence of a PTT; logic 0 indicates PTT.
Power signals are at constant V dc levels. Most are provided by the power supply as described in paragraph 2-5.
Rf and IF signals are present in the rt. Coaxial cables are used to pass these signals between modules. Frequen-
cies range from 12.5 MHz to 100.5 MHz.
2-8. RECEIVE RF SIGNAL PATH
The receive rf path processes the rf signal to recover audio, data, or homing signal. The signal passes through
See Figure FO-1 for the block diagram of this signal path.
Rf modulated with audio or data signals enter the rt at communications connector J3. Homing signals enter the rt
at homing connectors J4 and J5. The communications rf passes through a bandpass filter, low-pass filter, T/R
switch, and homing switch. The homing rf passes through a sampling network and homing switch. The sampling
network switches between the right and left homing antenna at a 150 Hz rate,
The rf signal from the homing switch is routed to the tuner/mixer, The tuner/mixer filters and amplifies the rf
signal and then mixes it with the local oscillator (LO) signal from the synthesizer, The LO is 12.5 MHz higher than
the operating frequency, The tuner/mixer and synthesizer are digitally tuned using the SERIAL DATA line. The
12.5 MHz IF signal is routed to the lF/demodulator. The lF/demodulator demodulates the IF signal to recover the
baseband audio or data (FM DEMOD).
During FH operation a SYNC CODE signal is recovered from the received signal and used to synchronize the
receiver with the transmitter. DATA SW-N, HOP TIME, and WB SEL are control lines from the ECCM module used
during FH operation. DATA SW-N is held at logic 1 during FH operation. HOP TIME goes to logic 1 while the
frequency is being changed. WB SEL (wideband select) goes to logic 0 when the rt looks for a cue signal. If a cue
signal is detected, the IF/demodulator sets CUE PRESENT to logic 1.
Two homing signals are taken from the lF/demodulator. They are signal strength and AGC. SIG STR RCV is
proportional to signal strength received from the transmitter on which you are homing. AGC in homing mode is a
150 Hz square wave with amplitude proportional to the difference between left and right signals, Homing is
disabled in FH mode and when the rt is keyed,
2-9. TRANSMIT RF SIGNAL PATH
The transmit mode is initiated by a PTT input from the intercom. The RADIO PTT-N line is set to logic 0. See Figure
FO-2. If the request is valid (frequency loaded and control switches set correctly), the control module responds
by setting the T/R line to logic 1.
The amplifier-oscillator and synthesizer are digitally tuned by the SERIAL DATA signal from the control module.
TUNE GATE-N and TUNE CLK are used to decode SERIAL DATA.