500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 2-14
15 January 2005
2.9.5.2
The output from operational amplifier U4A
is applied across a differentiating circuit consisting of
resistor R38 and capacitor C12. The differentiating
circuit produces a positive pulse, during each
transition, which is applied to the base of transistor
Q6. If the positive pulse from U4A is not present
(loss of keying or a decrease of 4.0 dB on the intended
modulation depth) on the base of transistor Q6 before
the changeover circuit's time delay has elapsed,
transistor Q6 will remain turned off. This will cause
the transmitter to change over to the standby side,
indicated by a
STANDBY-ALARM
lamp turn on; or
shutdown condition, indicated by a
SHUTDOWN-
ALARM
lamp turning on, if the standby side was
providing the RF drive.
2.9.6
CARRIER THRESHOLD CIRCUIT:
The fwd pwr input on J6-1, a dc voltage representing
the forward power of the transmitter with a super
imposed ac voltage proportional to the modulation
depth, is applied through loading resistors R1/R2 and
buffer amplifier U1A to the
CARR THRSH
potentiometer R18. The
CARR THRSH
potentiometer
is normally set for the desired minimum carrier level
(normally to detect a -3.0 dB drop on the intended
carrier level). The detected dc voltage (level of dc
voltage will depend on setting of the
CARR THRSH
potentiometer) from the wiper of potentiometer R18 is
applied across capacitor C8.
2.9.6.1
Capacitor C8 filters out the modulation
component of the RF carrier and charges to the mean
dc level, representative of the RF carrier level. This
reference voltage is passed to the non-inverting input
of comparator U3A and represents the carrier
threshold level. It is compared to the voltage level,
established by resistors R22/25, on the inverting input
of U3A. Normally, the output of U3A will be high
impedance to ground and the circuit will have no
influence. When the output of U3A switches to low
impedance to ground, the positive pulses from
modulation comparator U4A will be inhibited.
2.9.6.2
If the positive pulse from U4A is not
present (modulation depth decrease of 4.0 dB) on the
base of Q6 before the changeover circuit's time delay
has elapsed, Q6 will remain turned off. This will
cause a change over to the standby exciter (
STANDBY-
ALARM
lamp shall turn on) or a shutdown
(
SHUTDOWN-ALARM
lamp shall turn on) if the
standby exciter was providing the RF drive.
2.9.7
CHANGEOVER DELAY CIRCUIT:
The changeover delay circuit can be adjusted for a
changeover time delay of between 20 and 80 seconds.
The time it takes for capacitors C15/C16 to exceed the
voltage level established on the inverting input of
comparator U5A will depend upon the setting of the
CHANGEOVER DELAY
potentiometer. With the
potentiometer set for a maximum time delay (80
seconds), capacitors C15/C16 will take 80 seconds
before the voltage level established on the non-
inverting input of comparator U5A is more positive
than the inverting input. When this occurs,
comparator U5A's output will switch to high
impedance, a changeover control signal will be
applied to the base of transistor Q8.
A high SWR status control signal being applied to
diode CR8 will cause it to be forward biased and turn
on. Q6 will be forward biased and ensure capacitor
C15/C16 are held at ground potential. This results in
the changeover circuits being inhibited during a high
SWR condition.
2.9.8
STANDBY CIRCUIT:
Under normal
operating conditions transistor Q8 will be reversed
biased and turned off. The changeover circuit will be
inhibited. If a fault occurs resulting in transistor Q8
being turned on, a low control signal will be applied
through inverters U6A/U6B and applied through
resistor R61 to the base of transistor Q9. Q9 will be
forward biased and turn on.
The input to U7-3 will be high and output on U7-1
will be high. The output on U7A-1 will be applied
through resistor R77 to the base of transistor Q13.
Transistor Q13 will be reversed biased and turn off.
A changeover control signal will be applied to K3 and
turn on a standby-alarm lamp on the control/monitor
panel. The transmitter will automatically switch to
the standby side.
2.9.9
SHUTDOWN CIRCUIT:
Normally, the
control signal applied to flip-flop device U7-11 will
be low and the shutdown circuit will be inhibited. If a
fault occurs when the standby side of the transmitter
has been selected, the collector of transistor Q9 will
switch to a ground level.
After a nominal delay (ten seconds), caused by
resistor R67 and capacitors C24/C27, the input on
flip-flop device U7-11 will go high. The output on
U7B-13 will be high and applied through R75 to the
