Codan NGT 2010 RF Technical & Service Manual page 783

8571—Technical description
300 Hz control tone—generation and detection
Drawings
The 300 Hz control tone is generated by the tone generator IC16 on the Microprocessor
& TCVR Interface PCB. The clock frequency of 3.579 MHz is supplied from the
oscillator in the Bell 103 modem (IC6) and connected via buffer IC1e to the three clock
inputs of the tone generator (pins 9, 15, and 18).
The internal dividers of IC16 are programmed by the microprocessor via the eight data
lines and initiated by the two address lines (A0, A1) to produce a 300 Hz and two 30 kHz
square wave outputs.
The first 30 kHz from the tone generator (pin 17) clocks the notch and digital filters
(Dual Line Equaliser PCB, drawing 04-02541) and runs continuously.
The 300 Hz square wave from the tone generator (pin 10) is applied via an RC network
to the input of a digital LPF IC17 (pin 8). The second 30 kHz square wave from pin 13 is
connected to the clock input (pin 1). The 300 Hz (pin 10) runs continuously, but the
30 kHz (pin 13) is enabled only when the 300 Hz sine wave is required at the output of
the digital LPF (pin 5). The digital filter is disabled when the 30 kHz is removed.
From the output of filter IC17, the control tone is connected to the input of the mixer
amplifier in the Bell 103 modem IC6. From the output (pin 17), the signal is passed via
connector P1 pin 30a to the Rx audio bus on the Backplane PCB, and continues on the
same path as the Rx audio through the Dual Line Equaliser PCB to the output of the
8571.
The 300 Hz received from the master 8571 is connected via the 2-wire or 4-wire
interface and Backplane PCB to the Dual Line Equaliser PCB and follows the same path
as the transmit audio to the output of the hybrid amplifier IC101a (see
Transmit audio
is applied to a 300 Hz digital BPF IC106.
The digital filter consists of two sections, the first connected as an LPF, followed by the
second as an HPF. The two filters overlap each other, resulting in a very narrow BPF
centred at 300 Hz. This removes the transmit audio but retains the 300 Hz control signal.
The 30 kHz clock is supplied from the tone generator IC16 on the Microprocessor &
TCVR Interface PCB.
From the output of the BPF, the control tone with the transmit audio removed is passed to
a DC gain-controlled amplifier IC104a. The gain of this amplifier is set by the
microprocessor for 6 dB above the threshold of the 300 Hz detector described below.
The gain-controlled element in amplifier IC104a is a variable resistive cell (delta G). Its
resistance decreases as the DC current is increased at the control pin 1 (controlled by the
microprocessor via DAC IC2 (Dual Line Equaliser PCB). Because the resistive cell is
part of the input circuit to the amplifier, a rise in DC current at IC104a pin 1 results in an
increase in the amplifier gain.
344
04-02690 and 04-02541
path). At this point, the 300 Hz control tone (when in the Transmit mode)
NGT Transceiver System Technical Service Manual
page 340,