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22.1
DESCRIPTION
The Secure Com option contains two
independent
sections:
an encode section
and
a decode section. These
sections
interface
with
the
R2001D
transmitter
and
receiver to allow encrypted
messages
to
be
transmit-
ted and
received
via
RF
channels.
In
addition,
line
drivers
and
receivers
are
provided
to
permit
signaling
to occur through
twisted
pairs and
modems. The
Secure Com option provides
programming
capability,
wherein a
radio can be programmed with
a test key or
by
a
hand-held
key
inserter.
Measurements made by
the Secure Com
option
include
the following:
•
deviation measurement
(both transmitted
and
received)
•
line level
measurement-ac
voltage
being
output to
the line drivers
and received from the line receiv-
ers
•
bit
error rate measurement-number of
bits in
error
to
0.01
percent.
Other Secure Com
features
are:
•
scaled oscilloscope
displays
of various
portions
of
the t ransmitted
and received messages
•
end of message (EOM)
detection
and generation
ability
•
self
test functions
•
dual
code
radio
testing capability.
22.2 THEORY OF OPERATION
22.2.1
TRANSMIT
The
Secure Com Communications System
Analyzer
is
designed
to test narrowband
FM
radios
which
transmit encrypted audio
signals.
The
radios to be
tested use a Continuously Variable Slope
Delta
Mod-
ulator
(CVSD) to
digitize
the microphone audio at a
12
kB/sec
rate.
After
digitization,
the
plain
text to
be
transmitted is input
to
the encryption
circuitry.
The
resulting
cipher text
is
then splatter-filtered at 6
kHz
to
remove upper
harmonics
and transmitted.
The
nominal
frequency
deviation
for these
radios
is 4.0
kHz. Carrier frequencies of these radios can
range
from
100kHz to 999.999 MHz
and still remain
compatible
with the
test
system.
22.2.2
RECEIVE
In
receive
mode,
the radios to be tested demodulate
the received
signal
and feed the resulting encrypted
data
stream
to a
bit synchronizer
circuit. The
receiver
SECTION
22.
SECURE COM OPTION
is self-synchronizing,
meaning
that
no preamble is
required to acquire the
signal.
Reclocked
data
is
input
to the encryption circuitry which
decodes
the
data. The
received plain text
is
converted to an analog
waveform
in
the CVSD. After amplification, this audio
wave-
form
is sent to
the speaker.
22.3
ENCODER SECTION
22.3.1
INPUTS
The
encoder
has
inputs
for
the
following signals:
internal 1-kHz,
microphone
audio, and external mod-
ulation
(input through the
R2001D
front
panel). These
signals are internally summed so they can
be used
individually or combined to
produce
various wave-
forms.
The adjustments provided
on the front
panel
for
the
1-kHz level
and ext.
mod. level,
control
the level of
these signals at the encoder input and have
no
effect
on the transmitted encrypted data
deviation
level.
22.3.2
ENCRYPTION
TYPE
The
Secure Com option
has
the ability to encode
and
·
decode
encrypted
messages.
There
are
provisions
for
t he R2004/5D
to
hold, as
many
as
four
encryption cir-
cuits.
The R2004/5D
can
be
loaded with
multiple
encryption circuits of
the
same type
in
order that
secure systems operating on
different
code keys can be
monitored and
tested
without
reprogramming.
Only
radios which encrypt the audio as
described
in
para-
graphs 22.2.1
and
22.2.2
will
be able to operate
with
the
Secure Com test
option.
Manufacturers
other than
Motorola produce encrypted
radios
which are
not
compatible
with
the Secure Com tester.
22.3.3
ENCODER OUTPUTS
The
encoder
section
outputs are baseband encrypted
data and modulated RF. The
baseband
encrypted
data
is available at the MOD OUT
jack
on the R2004/5D
front
panel,
and also at the LINE OUT and SINGLE
ENDED OUT ports
on
the junction box.
LINE
OUT
is a
balanced-line
output. Modulated RF is output at
the antenna
port,
the RF
1/0
port, and the
duplex
generator port. The
internal
modulation
signal gener-
ated
by the audio
code
synthesizer
is
summed
into the
encoder
outputs.
This is to allow the
injection
of
an
undesired
signal
component to introduce bit errors.
The
summing occurs
before RF modulation is
com-
plete so
the
distorted
waveform can be transmitted.
The CODE SYNTH LVL adjustment on the front
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