Codan NGT 2010 RF Technical & Service Manual page 154

NGT—Technical description (2011/2012)
The volume of the tones is controlled from the Application Processor Handset I/F
3 V PCB (2011)/Application Processor Handset & Data I/F PCB (2012), based on the Rx
audio volume, to give a constant relative level between the two signals.
Voice mute
2011: Drawing
2012: Drawing
The audio from the RF/IF PCB is fed to the input of the mute stage squaring amplifier
IC12/A. The squared audio is then fed to a charge pump circuit comprising V9 and
associated components. This circuit performs a frequency to voltage function that causes
the resultant DC voltage to rise as the frequency increases.
IC13/A and associated components function as an LPF with a cutoff frequency of
approximately 10 Hz. The output from IC13/A is a DC voltage, varying at the syllabic
rate of the speech received.
IC12/C and IC12/D form a window comparator. The window width is adjusted by the
preset MUTE ADJ R85. The divider network R87, R88 and C64 provides the reference
voltage for the window comparator. If the output rises or falls below this reference by the
amount set by R85, the open collector outputs of IC12/C and IC12/D discharge C65, thus
applying a low to the negative input of comparator IC12/B. When the output from this
comparator goes high, it indicates to the Application Processor Handset I/F 3 V PCB
(2011)/Application Processor Handset & Data I/F PCB (2012) that speech has been
detected. This PCB then instructs the switch IC14/A to close, enabling the audio path.
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04-03425
04-03499 (sheet 1) and 04-03499 (sheet 2)
NGT Transceiver System Technical Service Manual