Demodulator; Rf Unit-Audio Interface Pcb Receive Path - Codan NGT 2010 RF Technical & Service Manual

Demodulator

Drawing
ALTERNATIVE TEXT
Drawing
NOTE
The output of the 455 kHz IF amplifier is passed to the input of a double balanced mixer
IC208. The IF signal is mixed with the local oscillator that is applied to pins 8 and 10 to
produce an audio output at pin 6. The local oscillator operates at 456.5 kHz for USB
operation and 453.5 kHz for LSB.
The audio is then filtered by the LPF comprising IC209 and associated components,
before being passed to the Audio Interface PCB.
RF unit—Audio Interface PCB receive path
Voice mute
Drawing
The audio from the RF/IF PCB is fed to the input of the mute stage squaring amplifier
IC15/A. The squared audio is then fed to a charge pump circuit comprising V10 and
associated components. This circuit performs a frequency to voltage function that causes
the resultant DC voltage to rise as the frequency increases.
IC16/A and associated components function as an LPF with a cutoff frequency of
approximately 10 Hz. The output from IC16/A is a DC voltage, varying at the syllabic
rate of the speech received.
IC15/C and IC15/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 IC15/C and IC15/D discharge C65, thus
applying a low to the negative input of comparator IC15/B. When the output from this
comparator goes high, it indicates to several microprocessors that speech has been
detected. The microprocessors then signal the junction box (via the CIB) to switch the
audio path on by enabling volume control IC2/E (drawing
Time division multiplex audio generation
Drawing
The raw receive audio passes through an LPF comprising IC3/B and IC3/A. It is then
sampled by IC1/A and IC1/D at a rate of 12.5 kHz, which produces the TDM audio for
the CIB.
The raw CIB audio is also passed via IC7 and IC8 to the Application Processor PCB
where the DSP enhances the received audio. This audio enhancement is a proprietary
function called Easitalk
NGT Transceiver System Technical Service Manual
04-03106 (sheet 2), or
04-03378 (sheet 2)
If the PCB issue status is -06 or earlier, see drawing
04-03109
04-03109
®
.
NGT—Technical description (2010)
04-03378 (sheet
04-03201 or 04-03423).
2).
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