500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 2-15
15 January 2005
base of transistor Q12. Transistor Q12 will be reverse
biased and turn off. A shutdown control signal will
be applied to relay K2 and to a shutdown-alarm lamp
on the control/monitor's front panel. The lamp will
turn on. Both sides of the subject transmitter will be
shutdown.
2.9.10 FORWARD
POWER
CAL:
The fwd
pwr input on connector 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
applied to the non-inverting input of buffer amplifier
U1B. Capacitor C6 filters the modulation component
while charging to a dc level that is representative of
the carrier level. The output of buffer amplifier U1B
is applied through resistor R78 and the
FWD PWR CAL
potentiometer and passed through connector J5-3 to
TEST
meter M1 on control/ monitor A1's front panel.
The
FWD PWR
CAL
potentiometer provides a
calibration adjustment for the forward power
indication on the
TEST
meter.
The dc voltage representing the transmitters RF
carrier level is also passed through operational
amplifier U1C. Operational amplifier U1C plus its
associated components provide a gain of five at the
output of U1C-8 which is applied across diodes
CR5/6 to fwd pwr (remote) connector J6-3 for
external use.
2.9.11 AUDIO
MONITOR
CIRCUIT:
The fwd
pwr input on connector 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
passed to the non-inverting input of audio amplifier
U2A. The output on U2A-1 is coupled through
capacitor C23 and passed to audio monitor connector
J10.
2.9.11.1 Speaker
Control:
Normally the
SPEAKER
switch will be set to
OFF
. When it is set to
ON
,
an
audible signal will be passed through the
SPEAKER
VOLUME
potentiometer and passed to
SPEAKER
LS1.
2.9.12
RF CURRENT CIRCUIT:
The purpose
of the RF current circuit is to provide an indication
(
RF CURRENT ALARM
) that the modulation level is too
high. It actually detects that a threshold has been
reached where the RF current (on the troughs of the
modulated envelope) reduces to near zero. This in
turn protects the transmitter from excessive peak
current which would occur on the peaks of the
modulation envelope.
A sample of the RF current is applied through RF
current sample connector J8-12 and passed to a
detector circuit consisting of diode CR1 and capacitor
C3. The detected signal is passed across resistor R6
and a limiting circuit consisting of diode CR3 and
resistor R12. The limiting circuit maintains the input
on operational amplifier U8A inverting input to +15
volts dc. Resistor R20 and
OVERMOD CAL
potentiometer establish the reference threshold on the
non-inverting input of operational amplifier U8A-3.
Under normal operation the output of U8A-1 will be
low, transistor Q4 will be reverse biased and turned
off.
If the voltage being applied to U8A-2 becomes less
positive than the reference voltage at U8A-3, the
output on U8A-1 will go high. Transistor Q4 will be
forward biased and turn on. Capacitor C13 will
discharge causing the input on U8B-6 to become more
positive than the reference voltage on U8B-5. The
output on U8B-7 will go high. The
RF CURRENT-
ALARM
lamp shall turn on.
2.9.13 SWR CUTBACK THRESHOLD
CIRCUIT:
Under normal operating conditions (no
reflected power), comparator U10A's output will be
low (ground). The circuit will have no influence. If
the voltage level (DC voltage representing the
reflected power) on U10A's non-inverting input
becomes more positive than U10A's inverting input
(established by resistors R94/96 and the
SWR
CUTBACK THRESHOLD
potentiometer), U10A-1
output will go high. The voltage level, on U10A-1
output, will depend on the level of the DC voltage
representing the reflected power sample on J8-4.
Capacitor C43 shapes the transient response time
providing a lower gain by connecting resistor R100 in
parallel with resistor R101 to protect against
unwanted fluctuations.
The output on U10A-1 is passed to SWR cutback
connector J8-10 and applied to a linear attenuator
circuit within the mod driver PWB. When U10A's
output is high, comparator U10B's non-inverting input
will be more positive than U10B's inverting input
(approximately 0.6 V dc), established by resistor
R103 and diode CR31. The output on U10B-7 will
