RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 2-4
01 November 2003
These coding variations are referred to as standby '1'
and standby '2' and may be used to transmit
information such as changeover from selected main to
standby transmitter or changeover of the power
source from AC supply to the (optional) battery
backup or diesel backup. The keyer also has the
capability to generate whole frames of tones or
spaces to meet particular customer coding
requirements. If the transmitter is in the MCW
operating mode, a keying override signal is applied to
override the coded keying and actuate a continuous
tone as the keyed tone output.
The keyer consists of four alphanumeric displays,
two EEPROMs, four push-button switches, a tone
generator, a watchdog circuit, and a reset circuit.
2.2.3.1
The
COM
mon input (pin 18) on the
alphanumeric displays (U11 through U14) is
connected to the programmed microcontroller (U5)
through two transistors which are used as level
converters. All other alphanumeric display pins are
connected to U5 via Darlington transistor arrays (U2
and U3). LED segments of the alphanumeric
displays are turned on by controlling U5’s output
pins. As a power-saving feature, software will shut
off the alphanumeric displays after the keyer has
output 255 bits. To turn the display back on, any of
the four push-button switches can be pressed.
2.2.3.2
Each EEPROM (U9 and U10) has three
inputs (
S
,
C
and
D
) and an output (
Q
). The three
inputs are tied to the corresponding pin on the
alternate EEPROM and connected to microcontroller
U8, while the output of each EEPROM is connected
to a unique input on U8. This writes the same
information to both EEPROMs. When a read is
conducted, software checks to see if the EEPROMs
respond with the same information. If not, an error is
indicated.
2.2.3.3
The four push-button switches (S1 through
S4) are connected to microcontroller U8 via pull up
resistors. A low indicates that the button is pushed.
2.2.3.4
A supervisory circuit (U15) is used to hold
the reset (
RST
) input (pin 10) high for 350ms
(minimum) at initial turn-on. The circuit holds the
reset high whenever the +5V DC supply falls below
4.6V. The reset pin is held high until the voltage
increases above 4.6V, or for a minimum of 350ms,
whichever period of time is longer.
2.2.3.5
The watchdog circuit consists of capacitors
C19/C20, diodes CR7/CR8 and resistor R48.
During normal operation, the P0.6 output (pin 37) on
microcontroller U8 switches on and off at a high
frequency. A low is provided, by the watchdog
circuit, to Q10’s base. The
keyer fault
output (J2-2)
will be open circuit. When a fault occurs, the
keyer
fault
output switches to ground potential (Q10 turned
on). When shorting jumper E4 is installed in the
ENBL
position (shorting pins 1 and 2), the fault acts
to inhibit the
keyed tone
output (J1-1). When
shorting jumper E4 is installed in the
DSBL
position
(shorting pins 2 and 3), the fault, although indicated
by
KEYER FAULT
lamp DS1, has no effect on the
keyed tone
output.
2.2.3.6
Operational amplifiers U1A/U1B and their
associated components form a stable, free running
RF oscillator. Positioning of shorting jumpers E1
and E2 in the feedback network allows frequencies of
400Hz or 1020Hz to be used for beacon tone (E1 and
E2 shorting pins 1 and 2 provides a 1020Hz tone; E1
and E2 shorting pins 2 and 3 provides a 400Hz tone).
To enable an
external tone
input (J2-1), shorting
jumper E3 must be installed in the
EXT
position
(shorting pins 2 and 3) to disable the free running
oscillator.
2.2.4
MODULATOR DRIVER PWB (A4)
(see figure SD-5): The modulator driver circuits
process the audio signal and convert the processed
audio to a pulse-width modulation signal that
contains the carrier level and audio information.
Logic circuits monitor the level of the RF drive and
the modulation pulse-width output and produce
mod
drive
and
RF drive alarm
control signals that turn on
associated alarm lamps located on the front panel
when these thresholds are exceeded.
2.2.4.1
Mod Enable Switch
: The mod enable
switch circuit is comprised of analog switches
U1B/U1C, operational amplifier U6A,
MOD%
potentiometer R2 and associated components. Under
normal operating conditions the keyed tone (audio) on
J1-6 is applied through capacitor C26,
MOD%
potentiometer R2, resistor R3 and passed to analog
switch U1B-4.
MOD%
potentiometer R2, adjusts the
level of the audio input to the modulator driver PWB.
With
MOD
switch S2 (see figure SD-1), set to on, a
+15 volt DC
mod enable
control signal will be
