500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 2-1
15 January 2005
SECTION 2
THEORY OF OPERATION
GENERAL
2.1
The theory of operation for the subject
transmitter is presented in this section. The
information is presented in detail using the electrical
schematics as a reference.
TRANSMITTER DESCRIPTION
(see figures SD-1
and SD-2)
2.2
The transmitter operates in the LF/MF
band (190 kHz to 535 kHz) at 500 watts maximum
(carrier) power. It automatically transmits specific
beacon identification signals at a repetition rate of 8.0
seconds. Standby codes may also be transmitted
when commanded from an external source. Provision
is made for local or remote operation of the
transmitter. Emission is continuous carrier with
double sideband amplitude modulation modes, A2A
beacon-keyed identification tone and A2A and A3E,
simultaneous beacon and voice. Provision is also
made for automatic changeover from the selected
main side of the transmitter to the standby side when
the selected main side's critical parameters are not
met.
Outputs, from tapped current shunt resistors R1/R4,
provide dc current indication on
TEST-Volts/Current
meter that are representative of the current being
consumed by side
A
or
B
of the transmitter. Current
A
Cal potentiometer R2 and Current
B
Cal
potentiometer R3 provide adjustment for a precise dc
current indications during special calibration
procedures (side
A
or side
B
). Links between
terminals 1/2 and 3/4 on TB3 are either connected or
removed, dependent on the status (removed or
installed) of optional battery panel A11.
CONTROL/MONITOR PANEL
(A1) (see figure
SD-3)
2.3
Control/monitor panel A1 contains switched
metering circuits that monitor forward power,
reflected power, modulation percentage, dc operating
voltages and dc current of both sides of the
transmitter. The monitor also contains alarm lamps,
which turn on, when the following conditions exists;
overmodulation
, high SWR, standby status and
shutdown status. A battery alarm, when turned on,
indicates the transmitter's B- operating voltage is
being produced by an external dc power source
(battery etc).
The RF monitor connector provides a BNC coaxial
connection for external test equipment to monitor a
sample of the RF output of the transmitter. Other
switching controls included are; local or remote
selection, monitor (changeover inhibit), RF on/off and
a select
SELECT MAIN Tx
switch (side-
A
or side-
B
selection). Individual circuit descriptions are as
follows:
2.3.1
LOCAL/REMOTE CONTROL:
The
local/remote switching circuit controls the electrical
power for the transmitter when the
RF
switch is set to
ON
. During normal operation, the
CONTROL
switch
is set to
REMOTE
and
REMOTE
lamp is turned on.
Power trim A/B function and on/off switching facility
can be controlled from a remote location. An external
A/B main select (ground) on A1P4-11, when present,
changes the selected main-side. When changeover
occurs, remote and local standby alarm signals are
generated. The
STANDBY-ALARM
lamp shall turn on.
2.3.2
NORMAL/BYPASS
CONTROL:
During normal operation the
MONITOR
switch will be
set to
NORMAL
. When the
MONITOR
switch is set to
BYPASS
, the
BYPASS
lamp will turn on. A control
signal (ground) will be passed through connector
A1P4-10 and applied to the monitor PWB. The
automatic changeover circuits (main to standby)
within the monitor PWB will be inhibited. When the
MONITOR
switch is set to
NORMAL
, the
BYPASS
lamp
will turn off and the changeover circuits will no
longer be inhibited.
2.3.3
RF CONTROL SWITCH:
The RF
switch controls the
ON/OFF
function of the transmitter
and resets the transmitter to the selected main-side
when set to
OFF
then
ON
(if the transmitter is
operating in the standby-side, transmitter will
automatically be reset to the selected main-side). The
RF ON
status lamp will turn on when the RF switch is
set to
ON
.
