500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 2-11
15 January 2005
2.7.2
B- VDC To +24 VOLT DC
CONVERTER PWB (A7A2):
The B- voltage is
applied through fuse A7F1 and inductor A2L1 to the
emitter of A2A1Q1. When B- VDC reaches
approximately -40 V dc, A2A1Q1 will be forward
biased and turn on. The resultant output from the
collector of A2A1Q1 will cause A2A1Q2 to be
forward biased and turn on. The voltage at the
junction of resistor A2A1R6 and the collector of
A2A1Q2 will go to a nominal 13.0 V dc more
positive than the B- voltage and be applied to timer
A2A1U1. Resistors A2A1R7 through A2A1R10,
diode A2A1CR2 and capacitor A2A1C2 determine
the switching frequency and duty cycle being
generated by timer A2A1U1. The output of A2A1U1,
an adjustable pulse width control signal, is passed to
the gate of power MOSFET A2Q1.
2.7.2.1
Power MOSFET A2Q1, inductor A2L3,
diode A2CR1 and capacitor A2C2 form a basic step-
down flyback converter which inverts the polarity of
the source voltage. When power MOSFET A2Q1
switches on, current from capacitor A2C2 will be
applied to the load while current from power
MOSFET A2Q1 will store energy in inductor A2L3.
When power MOSFET A2Q1 switches off, the stored
energy in inductor A2L3 will be applied through
diode A2CR1 and capacitor A2C2 and applied to the
load. This ensures that a constant current will be
applied to the load when power MOSFET A2Q1 is
switching on and off. Zener diode A2CR2 protects
against excessive voltage on the output when the load
is removed and resistor A2R3 limits the peak current
through power MOSFET A2Q1. The output from the
converter circuit, a +24.0 V dc control signal, is
passed through resistor A2R4 to +24 V status lamp
DS1. The lamp will turn on. The +24.0 V dc control
signal is also passed to A2TB1-5 for external use.
2.7.2.2
The -15 V dc generated by components
A2L3, A2CR3 and A2C3 is not used in ND radio
beacons.
2.7.3
AC SUPPLY MONITOR (A7A3)
ASSEMBLY:
The AC supply monitor circuit
contains transistors A3Q1/A3Q2/A3Q4, power
MOSFET transistor A3Q3 and their associated
components. Under normal operating conditions (ac
power source), the breakdown threshold of zener
diode A3CR3 will be exceeded, transistors
A3Q1/A3Q4 and MOSFET A3Q3 will be forward
biased (turned on). Transistor A3Q2 will be reverse
biased (turned off). +15 V dc will be passed to ac
sense connectors J1-8 and J1-12. Diodes A3CR1/
CR2 and capacitor A3C1 form a full-wave filtered
rectifier for the input voltage to the ac supply monitor
PWB. Resistors A3R1/R2 provide attenuation. If the
breakdown threshold of zener diode A3CR3 is not
exceeded, transistor A1Q1 will be reverse biased
(turned off). When transistor A3Q1 turns off,
transistor A3Q2 will be forward biased (turned on).
MOSFET A3Q3 will be reverse biased (turned off)
and transistor A3Q4 will be reverse biased (turned
off). The AC sense outputs on connectors J1-8/J1-12
will be inhibited. The transmitter will either switch to
the standby side, providing the fault is not generated
from the AC power source of if already operating on
the standby side, will switch to the dc power source
(battery, if installed).
HARMONIC FILTER ASSEMBLY (A9)
(see
figure SD-11)
2.8
The harmonic filter assembly is a band
pass filter that attenuates the harmonics of the square
wave output being applied from the modulator/power
amplifier circuits. The filter has a flat response
characteristic over the operating bandwidth. The
circuit also filters out the frequency on the lower side
of the carrier frequency. Relay K1 or relay K2 will be
energized depending on which side
A
or
B
of the
subject transmitter has been selected. The harmonic
filter assembly contains a forward/reflected power
probe (A9A2) and an RF current probe (A9A1).
2.8.1
RF CURRENT PROBE (A9A1):
The
RF current probe contains transformer A1T1 and its
associated components. The probe monitors the RF
output
current
. The RF input from the contacts of
relay K1 or K2 is passed across the primary winding
of transformer A1T1 and applied to terminal '1' of
inductor L1. A sample of the RF current is applied
across transformer A1T1's secondary winding and
passed through resistors A1R1 through A1R4 to
connector A1J2. The current sample is passed to logic
circuits within the monitor/interface panel.
