RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 2-1
01 November 2003
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
(Figure SD-1)
2.2
The ND500II double sideband (no voice)
radiobeacon transmitter can operate in the LF/MF
band (190kHz to 650kHz) at 125 watts maximum
carrier power or in the MF band (651kHz to
1250kHz or 1600kHz to 1800kHz) at 100 watts
maximum carrier power. It automatically transmits
specific beacon identification signals at pre-selected
repetition rates. Special codes may also be
transmitted when commanded from an external
source. Provision is made for local or remote
operation of the transmitter as well as antenna fine-
tuning through controls on the transmitter's front
panel. Emission is continuous carrier (NON mode)
and beacon keyed identification tone (A2A mode).
Figure 2-2 illustrates the block diagram for the
ND500II transmitter.
2.2.1
POWER SUPPLY (A1)
(see figures SD-
1 and SD-2): The power supply pr50V (B+),
an unreg24V, a reg15V and can be
operated from either an AC or DC voltage source
(battery etc.). Monitoring circuits detect low levels
on the AC power source or B+ voltage line and
automatically switch to the DC supply (battery)
and/or inhibit the +24V and +15V voltages. Status/
alarm lamps, located on the transmitter's front panel,
will turn on indicating the power supply is operating
from either a DC or AC power source. An
inhibit
(ground) control signal from monitor PWB A5, when
applied (dependent on the setting of
MONITOR
switch
S5), will result in the low DC voltages being inhibited
and will shut down the transmitter.
2.2.1.1
AC Power Source Operation
: With
POWER
switch S3 set to
ON
, AC power (115 or
230 volts AC) will be applied, via AC line filter U2,
through the contacts of switch S3 and
AC POWER
fuse F2, through connector A1J1-1/2 to the primary
taps of transformer A1T1. The secondary windings
of transformer A1T1 provide tap selection to
compensate for a low or high AC power source. Tap
selection is determined during installation (see section
3). Under normal operating conditions, a nominal 57
volts RMS is applied through full-wave rectifier
A1U1, passed across choke inductor A1L1 and
applied to terminal 1 on current shunt resistor A1R2.
The output on terminals 3(+) and 4(-) of resistor
A1R2, a DC voltage representing the DC current of
the transmitter, is passed to connector A1J1-10/11
and can be monitored on
TEST
meter M1 when
TEST
switch S6 is set to
DC CURRENT
.
The B+ voltage on
terminal 6 of resistor A1R2 is applied across diode
A1CR3, MOSFET A1Q1 and resistor R4 and passed
through connector A1J1-6/14 for distribution
throughout the transmitter.
Power MOSFET transistor A1Q1 and its associated
components form a limiting circuit that prevents the
B+ voltage from exceeding 60V during no load
conditions (i.e., no output power). When the
breakdown voltage (56V) of zener diode A1CR3 is
exceeded by the B+ voltage, a positive voltage will be
developed at the junction of resistor A1R3/zener
diode A1CR3 and applied to the gate of A1Q1,
A1Q1 will be forward biased and turn on. Resistor
A1R4 will be connected across the B+ line to ground
and prevent the B+ voltage from exceeding 60V.
2.2.1.2
Unreg24V:
The unreg24
volt DC circuit consists of power MOSFET
transistor A1Q2, zener diode A1CR4 and associated
components. Under normal operating conditions, the
'B+ VDC' (fused) input on connector A1J1-13 is
applied through resistor A1R5 to the drain of A1Q2
and through resistors A1A1R1/R11/R17 to connector
A1A1J1-8. A nominal 27V on connector A1J1-8 is
passed to the gate of MOSFET A1Q2 and A1Q2 will
be gated on. +24V will be developed on the source of
A1Q2 and passed to the input of the +15 volt
regulator circuit and to connector A1J1-7 for
distribution throughout the transmitter. Zener diode
A1CR4 ensures the gate of A1Q2 is held at 15V.
Loss of unreg24V will result in loss of the
reg15V and cause transmitter shutdown.
