Codan NGT 2010 RF Technical & Service Manual page 784

From the output of the gain-controlled amplifier, the 300 Hz is applied to the detector
circuit consisting of IC107a, IC107b, IC105b and associated components. IC107a and
IC107b function as comparators. Their DC inputs are set by the resistor divider chain R2,
R3, R4 and R5 connected across the 10 V supply to form a window detector. When the
control tone exceeds 2.0 V p–p, the positive peaks applied to the non-inverting input
(IC107a pin 3) produce a positive output pulse at pin 1. The negative peaks applied to the
inverting input (IC107b pin 6) produce a positive output pulse at pin 7. The outputs are
ORed by D101 and D102 and connected to the input of control amplifier IC105b.
Capacitor C130 and resistor R137 form a filter network to average the output pulses
from the two comparators. When the average DC on pin 5 exceeds 5 V (threshold), the
output pin 7 goes high. The 300 Hz detect line from IC105b is inverted by IC5b and
connected to the 300 Hz detect bus on the Backplane PCB via connector P1 pin 31b.
From the Backplane PCB, the 300 Hz detect is fed to the Microprocessor & TCVR
Interface PCB via connector P1 pin 31b. It is inverted by IC1a and is detected by the
microprocessor on pin 18.
Signalling modem
Drawing
The signalling modem is compatible with the Bell 103 format, converting digital data
into FSK using frequencies 1070 Hz and 1270 Hz at 550 baud rate.
The modem IC6 in the 8571 is located on the Microprocessor & TCVR Interface PCB. It
uses a crystal-controlled oscillator at a frequency of 3.579 MHz to drive the FSK
modulator, digital Rx filters and the FSK demodulator.
Control signals from the microprocessor enable the FSK modulator to transmit data by
applying a low to pins 2 (ALB) and 14 (SQT), and a high to pin 13 (O/A). Serial data
from the microprocessor's internal UART exits from pin 25 and is connected to the FSK
modulator pin 11. Output from the modulator is mixed with the 300 Hz control tone, and
the combined output signal at pin 17 follows the receive path to the output of the 8571
(see
FSK data, and on completion, the modulator is disabled by the control line pin 14 (SQT)
going high (controlled by the microprocessor).
FSK data from the master 8571, received via the land line or radio link, follows the
transmit audio path to the Tx audio bus. From pin 32a of P1, the FSK data is applied to
the input amplifier of the modem IC6 pin 15 (RXA2). After amplification, it is fed via
the Rx digital filters to the FSK demodulator and the carrier detector circuit. The carrier
detector informs the microprocessor that data is being received by applying a low on
pin 13 (CD). From the output of the FSK demodulator, the digital data is applied to the
microprocessor input pin 24 (RxD) for processing.
NGT Transceiver System Technical Service Manual
04-02690
page 337, Receive audio path). The receive audio is disabled during the sending of
8571—Technical description
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