Table of Contents
Advertisement
Chapter
1
Description
MSF
5000
Instruction Manual
Secure
Board Functional Description
Secure
Board Functional
Description
Transmit Audio Circuitry
1-40
The Transmit
Audio
circuitry is divided into two
sections
which provide two
different modes of operation:
oa
Transparent operation
©
Transmit audio encryption
Figure
1-18
shows
a
block diagram of
the
transmit audio circuitry.
Transparent Operation
The input
to the transmit
audio circuitry is
via
the
SSCB. The input signal (RAW
TX AUDIO)
consists of audio or filtered data.
The RAW TX AUDIO signal
is routed
through
a
noise filter
which band-limits
the
line
data.
This
removes
high
frequency noise components from the transmission channel that could cause
spurious code detections.
The RAW
TX
AUDIO
is then limited
by
a
comparator to transform the signal into
discrete
logic level
data.
A
single pole low-pass filter determines
the average
DC
value of
the
input signal and provides
a
bias to set
the comparator threshold.
The RAW
TX
AUDIO
is also checked for
a
Beginning-Of-Message (BOM) or an
End-Of-Message
(EOM) bit
stream,
consisting of alternating one-zero patterns
which
are
used
to
signal
the
beginning
or end
of encrypted
messages.
Whenever
a
valid
12
kBit/Sec
transmit code, BOM, or
EOM
is detected, the
SSCB
front panel
Tx CD
LED
lights.
The RAW
TX
AUDIO
is continually reclocked, buffered, and converted
by
the
on-chip circuitry of
the secure
ASIC.
Once
valid
code is detected, the serial
transmit data at the output of the secure
ASIC
is sent to the low-pass splatter
filter.
The splatter filter band-limits
the data to
6
kHz,
eliminating
the
higher frequency
noise
which could
otherwise cause interference on adjacent channels.
In addition, a filter attenuates
high
frequency components above
50
kHz. This
circuit also limits
the
peak-to-peak voltage of
the data signal.
A
low
pass filter
element is used to filter out
switching
noise at the output and change the
DC
bias
of
the
signal.
The output level of
the splatter
filter is adjusted
via
EEPOT
6
(coded deviation
level). The Secure board
EEPOTs
are
controlled
by
the
Control Tray front panel.
The CODED MOD AUDIO output of
the Secure board is sent to the
SSCB
to be
routed to the modulator.
During
alignment, an internal
1
kHz
square-wave
generator
sets the
coded deviation level.
68P81092E02
3/14/94
ma]
Advertisement
Table of Contents
