Codan NGT 2010 RF, Technical & Service Manual

Page: 779 / 840

8571—Technical description
340 NGT Transceiver System Technical Service Manual
Transmit audio path
2-wire or 4-wire interface
With the 2-wire link version, the transmit audio is received by land line and is terminated
into a modular telephone connector fitted on the rear panel of the 8571, and is connected
to the 2-wire interface assembly (drawing
08-03817 ). At the output of the interface
module, the audio is connected via an interface cable to the Backplane PCB and then to
the Dual Line Equaliser PCB.
When used with the 4-wire link, the transmit audio is connected to pins 1 and 2 of the
Tcvr Control
connector located at the rear of the 8571. It is applied via the
Backplane PCB to the 4-wire interface assembly (drawing 08-03995 ). At the output of
the interface module, the audio is connected via an interface cable to the Backplane PCB
and then to the Dual Line Equaliser PCB.
Hybrid
In Transmit mode, the transmit audio for the 2-wire version is applied from pins 28a
and 28b of connector P1 to the input of differential amplifier IC101a via resistors R103
and R104. As the signals at each input are opposite in phase, IC101a amplifies the signal
by 6 dB (×2).
Gain and equalisation stage
From the output of IC101a, the transmit audio is passed to a DC gain-controlled
amplifier IC103a, the gain from which is set by the microprocessor to compensate for
losses in the land line or radio link.
The gain-controlled element in the amplifier IC103a is a variable resistive cell (delta G).
Its resistance decreases as the DC current is increased at the control input pin 1. In this
circuit, maximum gain occurs when the cell is at its highest resistance (pin 1 is at zero
current). AC gain is set by the input resistor R117 and the total feedback resistance of
R123 in series with the internal 20 k
Ω
resistor, resulting in a gain of approximately
14.5 dB (×5). As the resistive cell is connected in parallel to the feedback resistance, an
increase in DC current to pin 1 results in a decrease of amplifier gain. Minimum gain is
limited by the internally fitted 20 k
Ω
resistor in series with the cell, being about 3.5 dB
(×1.5).
From the gain-controlled amplifier, the audio is passed to the first of two DC-controlled
equalisation amplifiers IC103b and IC104b. Both amplifiers operate in a similar manner
to amplifier IC103a described above, but are designed to give a DC-controlled treble
boost to compensate for high-frequency loss on the land line. As both equalisers are
almost identical, only a description of the first unit is necessary, except where detailed.
Drawings 04-02487 and 04-02541
Drawing 04-02541
Drawing 04-02541