User Manual

SECTION 1: THEORY OF OPERATION
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Modem Switching
This section displays the connector wiring and modem switching circuitry. Connector J2 is routed to the
front-panel TX, CD, and RX1-RX3 LED indicators. The radio will also accept modulation from an external
source (modem or amplified microphone audio).
Modem
This base station uses separate modems for receive and transmit functions so that full-duplex operation
may be obtained. The A0-A1 address bus in addition to the individual read (RD*), write (WR*), and chip
select (MODEMTXCS*) lines control all three (3) modems. Modem operations are timed by Y2, a 4.9152
MHZ clock module.
Modem chip U14 is dedicated to the transmit operation. Data from the D0-D7 bus is read by the chip,
and then converted to a 4-level FSK analog signal, which appears on the TXOUT pin. Op amp U21B
buffers the signal, which becomes the MODEM_TXMOD output. From this point, the signal is routed to
the modulation circuitry on the Exciter Board.
Chip U14 has the ability to demodulate receiver audio, although this capability is not used in most
systems. Incoming data-bearing audio from the Diversity Reception Controller Board (and selected
receiver) appears at DISC_AUDIO. The signal passes through resistor R54 and into the modem chip.
Resistor R52 and capacitor C41 serve as feedback elements, limiting both the gain and bandwidth of an
amplifier within U14. The modem chip demodulates the audio into 8 bits of data, which exit U14 on the
D0-D7 bus.
Chip U14 also provides a bias voltage for the analog circuitry on the Exciter Board. This voltage is about
2.5 volts DC, and it appears on the VBIAS line. The purpose of VBIAS is to bias the Exciter Board analog
circuitry for proper operation. Please note that if this voltage is low or missing, the Exciter Board circuitry
may not work.
Modem chip U15 is dedicated to the receive operation. Incoming data-bearing audio from the Diversity
Reception Controller Board (and selected receiver) appears at DISC_ AUDIO. The signal passes through
resistor R56 and into the modem chip. Resistor R55 and capacitor C46 serve as feedback elements,
limiting both the gain and bandwidth of an amplifier within U15. The modem chip breaks down the audio
into 8 bits of data, which exit U15 on the D0-D7 bus.
Modem chip U16 is also dedicated to the receive operation, although it may not be used in this
application. The operation of U16 is exactly the same as U15.
Diversity Reception Controller Board
This section displays the power supply circuitry and input/output connector wiring. The power supply
consists of a diode (CR1), metal oxide varistor (MOV) (CR2), a fuse (F1), and voltage regulators (VR2
and VR3). 13.8 volts DC from the radio’s power connector appears on the anode of CR1. CR1 provides
reverse polarity protection, while CR2 absorbs any damaging transient spikes, which appear on the
unregulated supply line. F1 provides over-current protection. The SWB+ output powers the remaining
boards in the radio, with the exception of the power amplifier module. VR2 provides a 5-volt DC source
for all of the circuitry on the Diversity Reception Controller Board. VR3 provides 5 volts DC for the three
receiver boards via a connector (TB1).
Two (2) 16-pin DIP headers connect this board to the System Controller Board. One header (J2)
provides power and control signals for the System Controller Board. The ALARMA and ALARMC lines
are of particular interest because the base station has optional external fault monitoring capabilities. For