RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 2-10
01 November 2003
2.2.6.7
Shutdown Control
: The shutdown control
circuit is comprised of operational amplifier U4B,
DELAY
potentiometer R41, transistor Q6 and their
associated components. Under normal operating
conditions, transistor Q6 will be switching on and off
at the keyed tone rate (positive pulses from the
differentiating circuit). When Q6 turns on, capacitor
C10 will discharge through Q6 to ground and the
voltage on the non-inverting gate of U4B-5 will be
less positive than the voltage on the inverting gate of
U4B-6. A low will appear on the output U4B-7 and
on the
shutdown alarm
output (J4-8). The
SHUTDOWN ALARM
lamp, on the front panel, will
remain turned off. The low output on U4B-7 will
also be applied to the base of transistor Q7.
Transistor Q7 will be reversed biased and turned off.
The
inhibit
control output (J4-7) will be an open
collector and have no effect.
If the positive pulse, from the low modulation
detector circuit is not present, (loss of keying, 3dB
decrease or 2dB increase in carrier level, or a
decrease of 4.0dB on the
intended
modulation depth)
on the base of transistor Q6 before the shutdown
circuit's time delay has elapsed (between 20 and 80
seconds), transistor Q6 will remain turned off.
Capacitor C10 will begin charging through resistor
R45/R44 and
DELAY
potentiometer R41. When the
voltage on the non-inverting gate of U4B-5 exceeds
the bias voltage on U4B-6, the output on U4B-7 will
go high. The high output of U4B-7 is applied
through
shutdown alarm
output (J4-8) and passed to
a
SHUTDOWN ALARM
lamp on the front panel. The
lamp will turn on. The high on the output on U4B-7
will also be applied through resistor R50 to the base
of transistor Q7. Q7 will be forward biased and turn
on. A ground potential will be passed to the
inhibit
output (J4-7) and passed to a low DC voltage
inhibiting circuit within power supply assembly A1.
The transmitter will shut down.
2.2.6.8
Overmodulation Detector
: The over
modulation detector circuit is comprised of
operational amplifier U5A, comparator U5B,
OVERMOD
potentiometer R56, transistor Q9 and
their associated components. Under normal operating
conditions, a DC voltage proportional to the carrier
level with a superimposed AC voltage proportional to
the modulating level is applied through
RF current
sample
input (J2-1) and passed to a detector circuit
consisting of diode CR8 and capacitor C12. The
detected signal is passed across loading resistor R53
and a limiting circuit consisting of diode CR9 and
resistor R54. The limiting circuit maintains the input
to U5A's inverting input to less than +15V. Resistor
R55 and
OVERMOD
potentiometer R56 establish the
reference threshold on the non-inverting input of
U5A-3. Normally, the output on U5A-1 will be low,
transistor Q9 will be reverse biased and turned off.
The front panel
OVERMOD
alarm lamp will be turned
off.
If the voltage being applied to U5A-2 becomes more
positive than the reference voltage at U5A-3, the
output on U5A-1 will go high. Transistor Q9 will be
forward biased and turn on. Capacitor C13 will
discharge causing the input on U5B-6 to become less
positive than the reference voltage on U5B-5. The
output on U5B-7 will go high and be applied to the
overmod alarm
output (J2-3). The
OVERMOD-
ALARM
lamp on the front panel will turn on.
2.2.6.9
Reflected Power/Refl Pwr Cal
: The
reflected power monitoring and calibration circuit is
comprised of buffer amplifiers U8A/U8B,
REFL PWR
CAL
potentiometer R80 and their associated
components. Under normal operating conditions a
DC voltage representing the reflected power of the
transmitter is passed through
refl pwr
connector J1-1
and applied across capacitor C25, through load
resistors R63/R66 and passed to the non-inverting
gate of U8B-5. Capacitor C14 will charge through
resistor R63 to an average value representing the
reflected power. The output on U8B-7 is applied
through resistor R76 and
REFL PWR CAL
potentiometer R80 and passed to
refl pwr
connector
J2-7 for internal use.
REFL PWR CAL
potentiometer
R80 provides adjustment during calibration for the
reflected power indication being provided by
TEST
meter M1 on front panel. The output on U8B-7 is
also applied through buffer U8A and passed to
refl
pwr (remote)
connector J2-3 for external use.
2.2.6.10
SWR Protection
: The SWR protection
circuit is comprised of operational amplifier U9B,
inverter U7A, transistor Q11, comparator U9A and
their associated components. Under normal operating
conditions (no reflected power) the output on U9B-7
will be low (ground) and the circuit will have no
influence. If the voltage level representing the
