the last used User Code Key will be selected and indicated with
speaker beeps. Following power-up Cipher operation will select
the Primary User Code Key. Slow switching of this control
permits operation in a normal manner as assigned by the radio
manufacturer. Typically this is connected to the monitor switch,
but other switches can be used (freq. select).
In either mode a visual cipher indicator is available. An LED can
be used to indicate reception of a valid cipher signal as well as
transmit in cipher.
INITIAL SYNCHRONIZATION DELAY
All radio systems have an operating delay. This is the time
between PTT activation at a transmitter and speaker audio
being available at the receiving point. This time may vary
considerably from system to system or even from transmission
to transmission. For reliable cipher operation the ST-25A must
wait for this time period before signaling the beginning of a
ciphered transmission. System delays must be evaluated and
accommodated for with the INITIAL SYNCHRONIZATION
DELAY parameter.
For many radio operators it is difficult to reliably know how long
to wait before speaking in ciphered mode. This can cause loss
of the beginning of a message. The ST-25A can be
programmed to accommodate this problem. For cipher
transmissions the ST-25A will provide all the necessary timing
and beep the speaker as a "GO AHEAD" and speak indication.
USER CODE KEYS
Of the more than 268 million available code keys, four may be
selected and easily accessed as User Code Keys for each unit.
These may be used to provide different levels of security within
a particular radio system (officers, sergeants, lieutenants,
captains).
INSTALLATION
Installation should be done only by a qualified two-way radio
technician. Installation consists of programming, then mounting
the unit in the radio set, and making the electrical connections.
Installation of secure speech equipment is often time
consuming and costly due to the application
requirements presented by the radio equipment.
Selectone may be able to provide a substantial cost
savings for installation. Please contact the Selectone
Sales Department for details on factory installation of
secure speech equipment.
Selectone supports this product with application assistance on
our Toll Free phone line and with APPNOTES (Application
Notes). We encourage use of our application service to
determine hookup. Most radios do not provide an interface
connector for easy installation of voice encryption equipment as
is often provided for CTCSS applications. Radios that are not
in our application library will receive special consideration and
may qualify for sample installation of two units at the Selectone
factory for no cost.
Installation requires a minimum of eight external connections.
These connections are Power, Ground, Transmit Audio Input
and Output, Receive Audio Input and Output, PTT Input, and
the Clear/Ciphered Input. The most critical connections are
Transmit Audio Input and Output, and Receive Audio Input and
Output. Improper installation of these connections can result in
distorted audio, and the loss of either high or low frequency
voice components.
The following paragraphs describe each of the external
connections. Numbers shown in brackets [#] refer to the
connector pin number.
[3] POSITIVE (+) SUPPLY (RED)
This wire should be connected directly to a filtered source of
continuous positive DC voltage in the range of +5.2Vdc to
+18.0Vdc. This connection should be made "downstream" from
the power switch and the power supply filter components in the
radio set. If a regulated source of DC voltage is available, it
should be used. Low level microphone audio is passed through
the ST-25A and use of a quiet and stable source of DC voltage
inside the radio set will reduce the possibility of picking up
power supply noise that may affect audio signals.
[9] NEGATIVE (-) SUPPLY (BLACK)
This wire should be connected to a location inside the radio that
will supply a DC power ground return to the ST-25A. To
eliminate ground loops and power supply noise, the ground
return should be the same power supply ground used in the
transmit and receive audio stages.
[10] CLEAR/CIPHERED INPUT (BLACK/YELLOW)
Switched Mode: This wire is used to select the transmit cipher
mode when pulled to logic low (< 1Vdc). This may be
accomplished with an ON/OFF SPST switch. When this lead is
at logic high (> 4Vdc) the ST-25A will operate in clear mode
when transmitting.
Double Click Mode: This wire is used as an input line for the
Double Click and Quad Click signal. It is normally tied to the
monitor switch of the radio, however any switch can be used.
The connection point in the radio must switch between logic
high (>4Vdc) and logic low (< 1Vdc). The ST-25A analyzes
transition between these levels and is not concerned with the
resting state. This lead is intended to operate in parallel with the
existing radio function without effecting radio operation.
[12] TRANSMIT AUDIO INPUT (GREEN)
[13] TRANSMIT AUDIO OUTPUT (WHITE/GREEN)
These two wires should be connected in series with the transmit
microphone audio signal path inside the radio set. The
optimum location for connection is directly in series with the
microphone. If the microphone requires bias, then the audio
path MUST be broken "downstream" of the bias source. In
order to provide the best transmit audio quality, be sure that the
transmit audio signal path is broken BEFORE the pre-emphasis
and transmit modulation limiter circuit. Also, be sure that the
transmit audio path is broken next to a "DC blocking" capacitor,
or in such a location as not to upset any internal DC bias
voltages in the transmit audio stages.
[6] PTT INPUT (YELLOW)
The PTT Input detects a pull to ground on the PTT line in the
radio set to indicate a transmit condition. To install the PTT
Input line on the ST-25A, connect this wire to the PTT line in the
radio set at a convenient location.
