Transmit Path; Handset Microphone Amplifier Transmit Path; Junction Box Transmit Path - Codan NGT 2010 RF Technical & Service Manual

Transmit path

Handset microphone amplifier transmit path

Drawing
The handset has an electret microphone. When PTT is not pressed, the microphone audio
is clamped by V5.
When PTT is pressed, V5 is turned off, allowing audio from the microphone to be passed
to the amplifier IC9/B. The output of IC9/B is split into two paths, one going to the
handset audio bus, the other passing through the inverter IC9/A before going to the
handset audio bus. In this way, push-pull balanced audio is provided to the junction box
via the handset bus.

Junction box transmit path

Drawing 04-03201, or
ALTERNATIVE TEXT
Drawing
The junction box has four transmit audio inputs. These four inputs are mixed together at
the input of IC7/A as described below:
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•
•
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The output of IC7/A goes to IC11/B and then to IC10/A. These two ICs form a 3-pole
low-pass anti-aliasing filter with a nominal 3 kHz cutoff frequency.
The output of IC10/A passes to IC10/B. IC10/B acts as a peak limiter due to H2 and H3
in the feedback path. The normal level for the audio is 1 V p–p. Where the audio level is
much higher than this (such as when someone shouts into the microphone), IC10/B
limits the output of IC10/A to approximately 4 V p–p. This protects the synchronisation
pulse generated in the RF unit from becoming corrupted by large audio levels.
The output of IC10/B is split into two paths, one going to the TDM gate IC9/B, the other
passing through the inverter IC8/B before going to the TDM gate IC9/A. This produces
push-pull balanced audio.
NGT Transceiver System Technical Service Manual
04-03125
04-03423
Tx Audio Input 1: When PTT is pressed on the handset, the handset audio on P7,
pins 6 and 7, is passed to IC11/A. IC11/A converts the balanced audio to single-
ended audio that is then passed to IC7/A.
Tx Audio Input 2: When PTT is pressed on the desk console, the desk console
audio is mixed via R57 at the input of IC7/A.
Tx Audio Input 3: The audio input of the GPIO port is on pins 5 and 15 of J108.
When the GPIO PTT is active, the balanced audio is passed via T1 before passing to
the volume control IC2/C. IC2/C is activated to a preset level, controlled by
firmware. The audio is then mixed via R58 at the input of IC7/A.
Tx Audio Input 4: A tone used for Morse is generated by IC9 on the
Microprocessor PCB in the junction box (drawing 04-03133). The tone is passed to
the tone volume control IC2/B before mixing via R56 at the input of IC7/A.
NGT—Technical description (2010)
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