8571—Technical description
338 NGT Transceiver System Technical Service Manual
Receive audio bus and audio gate
From the Rx audio bus line, the signal is fed via the Backplane PCB to the Dual Line
Equaliser PCB (drawing 04-02541) via connector P1 pins 30a and 30b. From here it
continues via an isolating transformer T2, through the audio gate IC1 pins 1 and 2, to the
input of the line driver IC102 pin 4.
The audio gate disables the Rx audio to the line driver during the Transmit mode.
Line driver and hybrid
The line driver IC102 on the Dual Line Equaliser PCB has unity gain and is a power
amplifier with a low output impedance. The output is connected to the Line Isolation
Unit via P1 pins 28a and 28b. In the 2-wire system, the transmit and receive audio share
the same land line between the 8571s. By the use of a hybrid circuit, it is possible to
combine the two signals into one path and at the same time to isolate the transmit and
receive paths from each other.
The hybrid, located on the Dual Line Equaliser PCB, consists of a bridge network.
Resistors R106 and R107 form the top two parts of the bridge and the terminated land
line plus the resistor/capacitor combination of R108/C101 form the lower third and
fourth sections. The two opposite phased outputs of the receiver line driver IC102 are
connected to the top and bottom points of the bridge (pin 5 to the junction of R106 and
R107, and pin 8 to the junction of R108/C101 and one side of the line (pin 28b)). The
two remaining bridge points (R106 to line pin 28a, and R107 to R108/C101) are
connected to the differential inputs of amplifier IC101a via resistors R103 and R104.
When the land line impedance equals the R108/C101 combination, the two receive
signals applied to IC101a are of equal amplitude and phase. Because they are connected
to the input of a differential amplifier, the signals cancel and produce no output at IC101a
pin 1. However, the line impedance rarely equals the R108/C101 combination, but still
results in sufficient cancellation of the receiver signal to prevent overloading of the
transmit path, and enable detection of the 300 Hz control tone.
2-wire interface
Four lines may be connected on the 2-wire interface. Line 1 is always connected, lines 2,
3 and 4 are optional. The receive audio for line 1 is applied to connector P2 pins 1 and 2
located on the Backplane PCB, and then via an interface cable to the 2-wire interface
assembly. Lines 2, 3 and 4 use connectors P5, P101, and P104 respectively.
The 2-wire Austel approved interface consists of an isolating transformer, a fuse, and an
overvoltage protection device assembled in a fully insulated box. The output of the
interface is connected to a modular telephone connector, and when terminated into a
600 Ω line, produces an audio level of 0 VU (–10 dBm for a single tone).
Drawing 04-02541
Drawing 04-02541
Drawing 04-02691
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