Plantronics 1632 Manual page 65

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SECTION IV
THEORY OF OPERATION
4.1 GENERAL
This section contains the functional description of the
Model 1632A Voice Frequency Carrier Telegraph Demodulator
(VFCTD). The explanations are in sufficient detail to permit
a trained electronic technican to understand the operation of
the unit.
4.2 FUNCTIONAL DESCRIPTION
The VFCTD consists of a mainframe which is subdivided into two
compartments. The larger of these two compartments contains
the input circuits, computer control logic, timing logic and
two identical demodulator units. The smaller compartment
contains the power supply and the output circuits. This
compartment may also contain the optional high level keyers •
The simplified overall block diagram, Figure 4-1, shows that
the external signals are introduced to the input circuits, here
they are conditioned and routed via a multiplexer to either one
or both demodulator units. The multiplexer selection is
controlled through a computer control unit or locally by front
panel switch selection.
The signal routed from the input circuits to the demodulator
unit is applied to the mixer circuit. Here it is combined with
a local oscillator frequency to produce a 20.4 kHz intermediate
frequency (IF) •
The local oscillator frequency is produced by a highly stable
synthesizer which utilizes the time base circuits for reference
frequencies. The synthesizer is controlled by the computer
control unit or the front panel frequency selection switches.
The mixer output is then routed through a digital filter, which
eliminates all components except the 20. 4 kHz difference
frequency. The filter bandwidth is controlled by front panel
switch selection or the computer control unit.
The 20.4 kHz signal is then passed through a 21 kHz low pass
filter. From this filter the signal is applied to the
demodulator circuits. Here it is rectified, detected and
introduced to a slicer circuit which combines the mark and
space signals.
4-1