Page 6-22
Vector-LP Radio Beacon Transmitter Technical Instruction Manual
Issue 1.1
Section 6 Theory of Operation
6.6 RF POWER STAGE
See Figures 6-5 and SD-4. The RF power
stage produces the transmitter’s final RF
output. It contains the RF power modules
(A12 and A13, if used), the RF filter PWB
(A14) and the RF power probe (A15).
6.6.1 RF Power Module(s)
See Figure SD-22. Each RF power module
(A12 and, if purchased, A13) produces up to
250 W (for VR250) or 125 W (for VR125) RF
carrier power to the transmitter's RF output
and contains a wideband power amplifier, a
modulator, and a modulator filter. A switch
mode power supply PWB is provided to
convert the ac input voltage to the high level
B+ voltage used by the modulator assembly.
Each module’s RF output is applied to the
RF filter PWB (A14) via RF relay K1 (see
SD-4), which determines the ‘on-air’
module’. A power module interface PWB is
provided to interface control/monitor signals
between the RF power modules and the
exciter and control/monitor stages as
required. For VR250 transmitters, cooling air
for the RF power modules is provided by
fans B1 and B2.
6.6.1.1 SWITCH MODE POWER SUPPLY
PWB
See Figure SD-24. The switch mode power
supply PWB (A2) converts the transmitter’s
ac input voltage to the B+ voltage used by
the modulator. In the VR250, the ac input
voltage range is 170 – 270 V ac and the B+
voltages used by the modulator are 79 V,
136 V and 236 V. In the VR125, the ac input
voltage range is 90 – 270 V ac and the B+
voltages used by the modulator are 55 V,
97 V and 167 V.
6.6.1.1.1 Rectifier
The ac input voltage is rectified by diodes
U1 and U2, providing a dc voltage across
capacitors C12 through C17. Varistors RV1
through RV3 provide protection from high
voltage transients.
AC IND
lamp DS1
provides a status indication of the ac voltage
on the RF power module.
6.6.1.1.2 Switching Signal
IC U3 is a high current FET driver that
provides a switching PWM signal (0-18 V) to
the primary of transformer T2. The
secondaries of T2 control the on/off status of
FETs Q1 and Q2, which in turn switch the
rectified ac voltage to the primary of step-up
transformer T1/T4.
6.6.1.1.3 B+ Voltage
The secondaries of step-up transformer T1
are rectified by diodes CR9, CR10, CR11,
CR13, CR14, and CR15 and combine,
through chokes L4, L5 and L6, to form the
high level B+ voltage. This voltage is applied
to the
Dc Output
(J2-1). Samples of the B+
voltage and current are applied to inputs of
current mode PWM controller U8, which
provides PWM switching information to
regulate the B+ voltage. FETs Q4, Q5 and
resistors R10, R11, R24 and R25, in
conjunction with opto-couplers U4 and U6,
provide a means to adjust the attenuated
voltage applied to U8, thereby adjusting the
level of the B+ voltage.
6.6.1.1.4 Inhibit PWM
When an RF power module fault occurs, an
Inhibit
signal is provided to J1-7. Opto-coupler
U7 will turn on and apply a signal to current
mode PWM controller U8 to inhibit the
switching signal output of FET driver U3.
6.6.1.2 POWER MODULE CONTROL/
INTERFACE PWB
See Figure SD-23. With the exception of the
ac input for the switching power supply and
the RF output of the module, the power
module control/interface PWB (A1) provides
the input and output connections for the RF
power module. It performs the following
functions:
6.6.1.2.1 Module Status
Module status such as B+ and PA voltage,
dc current, and temperature are measured
using ADC inputs on microcontroller U7. The
Fan Tach 1
and
2
inputs are also captured
using interrupt 0 and interrupt 1 on U7. All
information is transmitted to the control/
display PWB using the internal serial bus.
