MODEL ST-25A
Mobilecall® Voice Encryption Module
Manual # M501-9665
September 12, 1996
Revision (960912-1)
GENERAL
The ST-25A is a voice encryption device used to secure two
way radio communication systems. The cipher process uses a
proprietary microprocessor controlled analog scrambling
algorithm. Each unit can be programmed with four User Code
Keys, with over 4 billion code keys to choose from. Special
factory set master code key groups are reserved to provide
extra security for special services. Each master code key group
has over 268 million possible code keys. To maintain security,
code keys are never transmitted. Audio processing filters
provide high quality low distortion recovered audio. The small
size and low power requirements make it ideal for portable,
mobile, and base station use.
Field programming is accomplished with either the ST-905
Keypad Programmer or the ST-907 PC based programmer.
The ST-905 Keypad Programmer must be Version 1.3 or
greater. The version can be found on the bottom of the unit
printed on a small white tag. Version 1.3 will read "V1.3". The
ST-907 must be Version 3.1 or greater. Version 3.1 will be
noted on the distribution disk for ST-CONFG, and displayed as
Ver 3.1 in the upper right hand corner of the display screen
when running the ST-CONFG program. Older versions of
either the ST-905 or ST-907 will not properly program the
ST-25A
SPECIFICATIONS
Total Code keys:
Operating Voltage:
Operating Current:
User Code keys:
Ciphered Algorithm:
Synchronization:
Delay Before Initial
Synchronization:
Audio Input Level:
Audio Output Level:
Input to Output Gain:
Frequency Response:
ST-25A Programming:
Memory:
Indicators:
Digital Inputs:
Temp. Range:
Interface:
Size:
Over 4 billion
5.2 to 18Vdc
< 8mA
Over 268 million
Real time frequency domain
Initial and maintenance bursts
Programmable 50mS to 1.2S
0.01 to 2.5 Vpp ac
0.01 to 2.5 Vpp ac
LESS THAN ± 0.5 dB
300 Hz to 2600 Hz.
External Keypad (ST-905 V1.3)
PC Programmer (ST-907 VER 3.1)
Non-volatile EEPROM
Visual (LED) or audable (Spkr. Beep)
Logic Low, less than 1 Vdc
Logic High, Greater than 4 Vdc
30
°
C to +70
°
C
18" flying leads terminated at a
low profile connector
1.00"W X 1.50"L X 0.25"H
(25.4mm X 38.20mm X 6.35mm)
OPERATION
F
Note:
Operation of radio equipment with
encrypted speech capability may be government
regulated. You are responsible for compliance with
applicable radio regulations regarding operation of this
equipment.
Operation is almost transparent to the user. The user may
select any one of 4 previously programmed code keys. The user
then enables or disables the transmit cipher mode. Once
enabled all subsequent transmissions will be ciphered using the
selected code key. Ciphered reception is automatic; other units
transmitting with the selected code key will be automatically
deciphered. Clear transmissions will also be received
automatically.
Each radio model provides different operational challenges.
This is generally due to availability of unassigned switches or
blank panel space capable of supporting accessory switches.
To accommodate these varying situations the ST-25A provides
two modes of operation for enabling and disabling transmit
cipher operation, and making User Code Key selections.
SWITCHED MODE
This is the default mode of operation, as shipped from the
factory. Clear/Ciphered switching is achieved by a dedicated
switch providing a single closure to (-) Supply. User Code Key
selection is also selected by a dedicated switch providing a
binary input to the two Code Select lines on the ST-25A.
DOUBLE CLICK MODE
This mode of operation is enabled during programming.
Operation with this mode minimizes accessory switch
requirements on the radio. The Clear/Ciphered line is connected
to any switch (except PTT) capable of providing momentary
closures to or away from ground. The switch retains its normal
functions; however the user can toggle between Clear/Ciphered
by operating the switch two times in rapid succession (Double
Clicking). The ST-25A will then provide a tone output to the
radio speaker. A high frequency beep or series of beeps
indicate subsequent transmissions will be ciphered. A low
frequency tone for .5 Sec. indicates subsequent transmissions
will be in the Clear (NOT Ciphered).
User Code Keys are selected by operating the same switch four
times in rapid succession (Quad Clicking). Quad Clicking
permits switching between User Code Keys when in the
transmit Cipher Mode (Double Click selection). Each Quad Click
transaction advances the selected User Code Key one step
around a loop of possible selections (Primary, Alt #1, Alt #2, Alt
#3, Primary ...). Following a Quad Click sequence the ST-25A
responds with speaker beeps to indicate the selection position
(Primary: 1 Beep, First Alt: 2 Beeps, Second Alt: 3 Beeps, Third
Alt: 4 beeps). When returning to cipher mode from clear mode,
