N.3.2.1
Opto-isolator
The opto-isolator input is a light-emitting diode, with both sides
above ground. When current flows, an internal LED illuminates
and causes a photo transistor to conduct. This provides a current
“sink” between the output terminals; each opto-isolator’s photo-
transistor is effectively part of a logic circuit on the Controller,
including pull-up resistors. Refer to SECTION P, Controller, for
additional information.
N.3.2.2
Control Input Requirements
To initiate or activate the control input, a momentary voltage (100
milliseconds or longer) must be applied to illuminate the opto-
isolator’s internal LED.
To prevent the transmitter control action from activating, the
voltage input to the “Control Input” terminals must be less than
+1 Volt). It is possible that voltages greater than +1 Volt could
activate the control action, because of component tolerances.
Voltages less than -1 Volt may cause component damage. Ex-
ceeding voltage or current limitations can over-dissipate the
series resistors or damage the opto-isolator.
Refer to Sheet 3 of the Schematic diagram, Characteristic Key
B, for additional requirements and limitations on control input
current and voltage.
N.3.2.3
Protection
Series resistors limit current to the opto-isolator. When input
voltage is +15 VDC, opto-isolator current is 40 mA. The resistor
network, a transzorb, and bypass capacitors protect the opto-iso-
lator input from transient voltages.
Figure N-2 shows three possible control input configurations.
N.3.3
Monitor Voltage Outputs
Monitor outputs include three types of outputs, including voltage
divider outputs and monitor voltage outputs. Each type is de-
scribed in following paragraphs.
N.3.3.1
Voltage-divider Outputs (Type C or Type D)
Voltage divider outputs are used to monitor the +22 VDC, -22
VDC, +8 VDC, and -8 VDC Low Voltage Supply outputs. Each
output circuit consists of a resistive voltage divider, with a
transzorb for over-voltage protection, and a bypass capacitor at
the input. These monitor voltage outputs appear at TB2, termi-
nals 35 through 38, and are all referenced to ground.
When there is no load, or a high impedance load, on the voltage
divider, the output will be +3.4 VDC when the input voltage is
at +22 VDC or +8 VDC, and -3.4 VDC when the input voltage
is at -22 VDC or -8 VDC.
Any loading will reduce this sample voltage. Normally, remote
control unit calibration will compensate for loading on the volt-
age divider outputs. However, if you know the load resistance
and want to calculate the reduced nominal voltage, the Thevenin
equivalent voltage and source resistance for each voltage divider
output are given on sheet 3 of the schematic diagram; if you are
not familiar with Thevenin equivalent circuits, see Figure N-3.
N.3.3.2
Operational Amplifier Buffered Outputs (Type E)
Refer to “Characteristic Key: E” on sheet 3 of the schematic
diagram for a simplified schematic diagram, and to sheet two for
complete schematic diagrams. There are six different parameters
using this type of monitor voltage output:
a. Forward Power
b. Reflected Power
c. Supply Current
d. Supply Volts
e. RF Drive (Estimate)
f. Antenna VSWR
g. Bandpass Filter VSWR
The Forward Power, Supply Current, and Supply Volts outputs
will be nom1.7 VDC when the transmitter is operating at
25 kW output power. These output levels are determined by
sample circuits in other parts of the transmitter.
N.3.3.3
Circuit Description
Each analog signal monitor output is buffered by a section of U4,
U5, U6 or U7 (half of U7 is not used). A 15-Volt transzorb and
a bypass capacitor provide output protection.
N.3.4
External Interlock
A normally closed connection between TB1-1 and TB1-2 oper-
ates 24 VAC “External Interlock” relay K3, which has a 2
Volt-Ampere coil. Contacts and interconnecting wire or cables
in the external interlock circuit must be handle this AC current,
and total external interlock circuit resistance should be kept low
to ensure reliable closure of K3. Refer to the Overall Schematic,
839-7855-151, and the DC Regulator schematic, 839-7855-163,
for the entire interlock string.
NOTE
THE TRANSMITTER WILL NOT OPERATE IF THERE IS AN
OPEN CIRCUIT BETWEEN THE EXTERNAL INTERLOCK
TERMINALS, TB1-1 AND TB1-2.
N.3.5
Audio Input
The audio input terminal board, A28TB3, is located on the
External Interface printed circuit board.
A balanced audio input is connected to TB3-2 and TB3-3.
Terminal TB3-1 is at the transmitter cabinet ground. This will be
the normal connection point for the audio input cable’s shield.
An additional AC coupled audio input circuit ground is provided
at TB3-4.
A bipolar zener diode (CR30 and CR31) from each side of the
balanced audio input to ground provides overvoltage protection.
N.3.6
Combiner Interconnect
Two BNC coaxial connectors, J7 and J8, are provided for inter-
connection to the combiner control unit when the transmitter is
used in a combined transmitter installation. The Technical Man-
ual for the Combiner Control unit will include information on
using these connectors.
N.3.7
“PA TURN OFF” and “OFF CONTROL”
Terminals 21 and 23 on TB1 are for a “PA TURN OFF” connec-
tion, and TB1 terminals 33 and 35 are for “OFF CONTROL.”
Both are optically isolated control inputs, as described earlier in
this section.
Section N - External Interface (A28)
12-12-94
888-2297-002
N-3
WARNING: Disconnect primary power prior to servicing.
