500 WATT RADIOBEACON TRANSMITTER
ND2000A-02x-xx0
Page 2-3
15 January 2005
A modulator/power amplifier can provide up to 2000
watts of peak envelope power or 500 watts of
continuous carrier power, over a frequency range of
190 kHz to 535 kHz. Each modulator/power
amplifier contains four identical modulator assemblies
(A1, A3, A5 and A7), four identical power amplifier
assemblies (A2, A4, A6 and A8) and a display
interface PWB (A9).
2.4.1
MODULATOR (A2A1)
(see figure
SD-4)
:
The description for the four modulators are
identical. Only modulator A1 will be explained.
Modulator PWB assembly A1A1, power MOSFET
A1Q1 and their associated components form a
switched regulator circuit that changes the logic level
of the variable width modulator drive pulses from a
ground reference to a B- voltage reference (logic 0).
2.4.1.1
The mod drive (0 to +15 V dc pulses) on
J1-1 is applied to switching transistor A1A1Q1.
A1A1Q1 will switch on/off as the 0 to +15 VDC
pulses are applied to its emitter. The output on its
collector will be passed to transistors A1A1Q2/Q3.
The bases of A1A1Q2/Q3 will be switching between
the B- level and a voltage that is 13 V less negative
than the B-level.
Transistors A1A1Q2/Q3 and their associated
components form a balanced drive for power
MOSFET transistor A1Q1. Zener diode A1A1CR1
and resistor A1A1R2 establish a reference voltage that
is 13 VDC less negative than the B- voltage, this
reference is applied to the collector of transistor
A1A1Q2. When the bases of transistors A1A1Q2/Q3
are at B- level, A1A1Q2 will be turned off and
A1A1Q3 will be turned on. The voltage at the emitter
junction of transistors A1A1Q2/A1Q3 will be at the
B-level, MOSFET A1Q1 will be switched off. When
the bases of transistors A1A1Q2/Q3 are 13 volts
less
negative than the B- level, A1A1Q2 will be turned on,
A1A1Q3 will be turned off, MOSFET A1Q1 will be
switched on. The output at the emitter of
A1A1Q2/Q3 will be a rectangular waveform at the
pulse width modulator switching frequency and shall
be switching between the B-voltage and a level that is
13 VDC less negative.
2.4.1.2
Zener diode A1A1CR1 maintains the
voltage on the collector of A1A1Q2 at 13.0 V dc
(logic 1) more positive than the B- voltage (logic 0),
regardless of the state of transistor A1A1Q2, this
ensures the voltage across the gate/source junction of
MOSFET A1Q1 does not exceed 13.0 V. Transistors
A1A1Q2/Q3 act as switches between a logic 1 and a
logic 0 to charge and discharge the capacitive load
presented by the gate of MOSFET A1Q1. MOSFET
A1Q1 acts as a switch that connects its load to the B-
line when turned on and to ground when turned off.
2.4.1.2.1
When a logic 1 is applied to the base of
transistors A1A1Q2/Q3, A1A1Q2 will be forward
biased (turned on) and A1A1Q3 will be reverse biased
(turned off). A logic 1 will be applied to the gate of
MOSFET A1Q1 and it will turn on. When MOSFET
A1Q1 is turned on, current will flow through inductor
A1L1.
2.4.1.2.2
When a logic 0 is applied to the bases of
transistors A1A1Q2/Q3, A1A1Q2 will be reverse
biased (turned off) and A1A1Q3 will be forward
biased (turned on). A logic 0 will be applied to the
gate of MOSFET A1Q1 and it will turn off. Current
in inductor A1L1 will collapse through free wheeling
diodes A1CR1/CR2.
2.4.1.3
Inductors A1L1 and L1 and capacitors
C2/C3 form a low pass filter which removes the
switching frequency (70 kHz) but allows audio
information (300 to 3000 Hz) to pass without
attenuation. The resultant output of the low pass filter
is a negative dc voltage that varies at the frequency
and amplitude of the modulating audio being applied.
The level of the negative voltage is the average of the
fixed width pulses (from the modulator driver)
required to produce the desired RF carrier output from
the transmitter when there is no modulation present.
The superimposed audio's frequency is determined by
the rate the pulse width changes. The amplitude is
determined by the amount the pulse width differs
from the fixed width of the reference pulses.
2.4.1.4
The
PA VOLTS 1
lamp monitors the output
of modulator A2A1. It will be full brilliance when the
output power level is at maximum (125 watts). The
average voltage of the four modulators may be
monitored at test point TP1.
2.4.2
POWER AMPLIFIER (A2A2):
The
description for the four power amplifiers are identical.
Only power amplifier A2A2 will be explained. The
RF drive input on A2J1 is applied to the primary
windings of 8:4 step-down transformer A2T1. The
output of each secondary winding is applied across
the gate and source terminals of a power MOSFET
transistor. When the gate of MOSFET A2Q1 is
