RADIOBEACON TRANSMITTER
ND500II (125 WATTS) DOUBLE SIDEBAND - NO VOICE
Page 2-3
01 November 2003
2.2.1.6
B+ Monitoring/Inhibit
: The B+
monitoring/inhibit circuit is comprised of transistors
A1A1Q1, A1A1Q3, A1A1Q4, power MOSFET
A1Q2 and associated components. Under normal
operating conditions, the
B+ VDC
input (A1J1-13)
will exceed the breakdown threshold of diode
A1A1CR3. Transistor A1A1Q1 will turn on and
transistor A1A1Q3 will turn off. A positive voltage
(approximately 27V) will be applied to connector
A1A1J1-8 and passed to the
gate
of power MOSFET
A1Q2, turning on A1Q2. +24V will be passed to the
input of +15 volt DC regulator A1U2-1. Zener diode
A1CR4 limits the gate drive to MOSFET A1Q2 at
15V. Transistor A1A1Q3 and resistor A1A1R11
provide hysteresis for the B+ monitoring circuitry.
The transmitter shuts down when the B+ voltage falls
below a nominal 42V and is restored when the B+
voltage increases to a nominal 44V. Transistor
A1A1Q4 and resistor A1A1R10 increase the B+
alarm level when operating from a DC source
(battery). This prevents oscillations from the B+
ripple voltage when AC inhibits the B+ through
transistor A1A1Q5.
If the
B+ VDC
input (A1J1-13) falls below the
desired level, zener diode A1A1CR3 will be reversed
biased and turn off. Transistor A1A1Q1 will be
turned off. Transistor A1A1Q3 will be turned on
through resistor A1A1R7. Resistor A1A1R11 will
be connected to ground through transistor A1A1Q3
and apply a ground to the gate of MOSFET A1Q2,
gating A1Q2 off. The +24V will be removed from
the source of A1Q2 and from the input of the +15
volt DC regulator circuit. The +24V and +15V will
be inhibited on connectors A1J1-7, A1J1-8/9 and
A1J2-1. The transmitter will shut down.
2.2.2
OSCILLATOR/RF DRIVE PWB (A2)
(see figure SD-3): The oscillator/RF drive circuits
generate an
RF drive
signal at the assigned carrier
frequency for the subject transmitter.
2.2.2.1
Balance/RF Drive
: The balance/RF drive
circuit is comprised of a class 'D' push-pull amplifier
formed by power MOSFETs Q7/Q8 and their
associated components. A +15V square wave, at the
assigned carrier frequency, is applied to the primary
of transformer T1 through resistor R18.
The secondary of transformer T1 contains two
identical windings. Each winding is connected across
the gate and source of a power MOSFET (Q7/Q8).
The voltage applied to the gate of Q7 will be 180
degrees out of phase with the voltage being applied to
Q8. When the gate of Q7 is positive, the gate of Q8
will be negative. Q7 will be gated on and Q8 will be
gated off. The
B+ VDC
input (J1-3) will be applied
through inductor L4 and passed through the
source/drain junction of Q7. During the next half
cycle, the gate of Q7 will be negative and the gate of
Q8 will be positive. Q7 will be gated off and Q8 will
be gated on. A ground will be applied through the
drain/source junction of Q8. The signal at the
junction of the drain/source of Q7/Q8 will be a
square wave switching at the carrier frequency
between ground and the B+ voltage level. The peak
detected output at the junction of diode CR3 and
capacitor C13 represents the RF drive level being
applied to the power amplifier/modulator assembly
A6. Test point TP4 provides a convenient
monitoring point of the RF drive output of the
assembly.
2.2.3
KEYER PWB (A3)
(see figure SD-4): The
keyer is a self-contained logic block that
automatically generates keyed audio tone signals that
are used to modulate the transmitter. This is
accomplished by means of a 64/80/96-bit code cycle
or frame, which is between 5 and 16 seconds long.
The beacon identification signal is programmed in the
keyer and may contain up to 4 characters (letters or
numbers) followed by either a long space or a long
filler dash to the end of the code cycle. The length of
a dot is determined by dividing the frame length by
the number of bits. For example, an 8-second frame
and 64 bits yield 125ms bits. A dash is 3 bits long
while a dot is 1 bit long. A tone oscillator circuit
capable of generating a 400Hz or 1020Hz tone is
also contained on the keyer PWB with an associated
keying gate. In addition to the actual beacon
identification letters or numbers, two separate minor
variations to the code may be selected via external
controls.
