[11] RECEIVE AUDIO INPUT (BLUE)
[1] RECEIVE AUDIO OUTPUT (WHITE/BLUE)
These two wires should be connected in series with the receive
audio path inside the radio. The optimum location for
connection is directly off the receiver detector circuit BEFORE
any audio processing circuits (de-emphasis). This connection
location will provide high quality audio recovery. Be sure not to
break the audio path between the detector and the squelch
circuit, or between the detector and the CTCSS decoder if one
is used. Also, be sure that the receive 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 receiver audio
stages.
[7] EE CS (BROWN)
Switched Mode: This input is only used for programming and
the lead should be removed from the connector.
Double Click Mode: Speaker beep is vital to operation in this
mode. This lead is the beep output. It may usually be connected
directly to the high side of the speaker. In some applications
direct speaker connection may not be usable. In these cases
connect to the input of the receiver audio amplifier through a
series resistor. The value of the series resistor will have to be
determined experimentally.
NOTE: Many portables power down the receiver audio amplifier
when squelched. Refer to [4] Code Select 1 Input, Double
Click Mode for additional information.
[2] EE DI/DO (VIOLET)
This input is only used for programming and the lead should be
removed from the connector.
[4] CODE SELECT 1 INPUT (WHITE/ORANGE)
[5] CODE SELECT 2 INPUT (BLACK/ORANGE)
Switched Mode: These two inputs are used for selecting up to
four unique code keys on the ST-25A. These inputs may be left
unconnected if only the Primary User Code Key is to be used.
The three Alternate User Code Keys are selected by connecting
these leads to a binary coded switch. The leads are pulled to
logic high by 47K resistors on the ST-25A board. The switch
must cause switching to ground (logic low) as indicated in
Table 1 (Logic High = Open, Logic Low = Grounded).
CODE KEY
Code Select 1
Code Select 2
Primary
Open
Open
1st Alternate
Grounded
Open
2nd Alternate
Open
Grounded
3rd Alternate
Grounded
Grounded
TABLE 1
Double Click Mode: In this mode CODE SELECT 1 is
redefined and provides an output to enable the receiver audio
amplifier for speaker beeping. When beep tones are not present
this lead is high impedance. During beeping it is switched to
ground through a series 10K resistor. If the receiver audio
amplifier is turned off while the receiver is squelched, this
output may be used to enable the amplifier for speaker beeping.
[8] EE SK & CIPHERED/CLEAR LED (BLACK/BROWN)
This output is used to drive an LED. Since the output drive
current is only 8mA, a low current high efficiency LED must be
used. The CIPHERED/CLEAR LED output should be connected
directly to the anode of the LED. The cathode of the LED must
be connected directly to ground. This lead is also used for
programming and may be left unconnected if not required.
MOUNTING
Use of a double-sided adhesive pad eliminates hardware
requirements. Mount the ST-25A on a clean, dry surface
oriented to allow future adjustments should they be necessary.
Press firmly after mounting to insure good contact of adhesive.
Do not touch the adhesive or attempt to reposition the unit after
mounting.
The clear shrink wrap attached to this manual can be used to
insulate the ST-25A after all of the adjustments have been
made.
ADJUSTMENTS
GAIN SETTINGS:
As shipped from the factory, the ST-25A is fully functional for
many applications. However experience indicates the ST-25A
functions best when audio signals internal to the ST-25A are at
or near pre-determined levels. To optimize operation, receive
and transmit levels must be evaluated independently and the
proper gain setting resistors must be installed. Use the following
procedures to determine the input level for the receive and
transmit audio paths. The measured levels are for evaluation
with TABLE 2. Use TABLE 2 to select resistors values for
optimum operation. Measurements may be made with an
oscilloscope or an AC level meter. For convenience TABLE 2
presents input levels in mV peak to peak and mVrms.
Input Level
Gain Resistor
mVpp
From To
mVrms
From To
TX R47
RX R7
TX R46
RX R22
10
22
40
73
134
244
447
819
1500
21
39
72
133
243
446
818
1499
2500
3.5
7.6
15
26
51
86
161
291
531
7.5
14
25
50
85
160
290
530
885
7.5 K
15 K
27 K
51 K
100 K
180 K
430 K
1.2 M
---
8.2 K
15 K
27 K
56 K
100 K
200 K
470 K
1.5 M
---
TABLE 2
