Codan NGT 2010 RF Technical & Service Manual page 776

Receive audio path
Receive audio from transceiver
Drawings
The receive audio is connected from pin 17 of the 15-way GPIO connector on the split-
site interface controller via an interface cable to the 8571 Backplane PCB (drawing
04-02691) connector P1 pin 8. From here it is connected to the input of transformer T1
on the Microprocessor & TCVR Interface PCB (drawing 04-02690) via P1 pin 16a
(ground pin 14a).
300 Hz notch filter and audio compressor
Drawing
From the secondary of transformer T1 on the Microprocessor & TCVR Interface PCB,
the receive signal passes to an analog changeover switch IC2c and connects to the input
of a 2-section digital filter, consisting of IC4, IC3a, IC3b, and associated components, to
produce a 300 Hz notch filter. It has unity gain at frequencies above 300 Hz, but provides
high attenuation (> –40 dB) at 300 Hz. IC4 is a dual active filter and is driven by a
30 kHz external clock generated from IC16. The high-pass output (pin 18) and the low-
pass output (pin 20) of the first active filter are phase mixed at the input of amplifier
IC3b, resulting in the required notch response. The output from IC3b is applied to the
second active filter and again the high- and low-pass filter outputs are phase mixed at the
input of IC3a. The two notch filters are slightly offset from each other to ensure the
complete rejection of any signal on or close to 300 Hz that may possibly corrupt the
operation of the system.
From the output of IC3a, the audio is fed via a second analog changeover switch IC2a to
the input of the compressor amplifier IC7, the gain from which is controlled to maintain
a constant output level for varying input levels.
In this circuit, the gain-controlled element (resistive cell delta G) is DC controlled by
internally rectifying the amplified audio that is applied to IC7 pin 15. Above the
threshold of AGC, set by resistor R29, an increase in the audio level at the input
proportionally decreases the resistance of the cell. As this is part of the negative
feedback, the output level remains constant. R26 and C18 form part of the DC feedback
to the amplifier. C21 averages the rectified audio that is applied to the resistive element.
The receiver audio at the output of IC7 is connected to a mixer amplifier in the Bell 103
modem IC6 pin 1, and the output (pin 17) is coupled to connector P1 pin 30a (designated
as Rx audio bus).
NGT Transceiver System Technical Service Manual
04-02691 and 04-02690
04-02690
8571—Technical description
337