Nautel XL12, Technical Instructions

Page: 117 / 238

12 000 WATT AM BROADCAST TRANSMITTER
XL12
Page 6-4
01 January 2002
RF DRIVE STAGE
6.4
The RF drive stage amplifies the carrier
frequency signal
Fc
generated by the active RF drive
PWB. The RF drive stage comprises A and B
regulated power supplies and RF drive amplifiers,
series RF drive tuning and a level detector all located
on RF drive panel A11. When exciter A is the
active exciter, regulated supply A11A4 provides the
power source for RF drive amplifier A11A1. RF
drive amplifier A11A1 amplifies the
Fc
signal, which
is then filtered and applied to the host transmitter RF
power stage (see figure SD-3).
6.4.1 REGULATED POWER SUPPLIES:
Regulated power supplies A11A4 and A11A5 are
boost type switching power supplies which provide a
regulated DC supply voltage for the transmitter's RF
drive amplifiers. A n48V supply is applied
to the drain of switching FET Q1 thru inductor L2.
The gate of FET Q1 is driven with pulses from pulse
width modulation (PWM) switching power supply
control U1. This device is a fixed-frequency PWM
control circuit, incorporating the functions required
for the control of a switching power supply. Internal
to the device is a sawtooth oscillator which is set to a
nominal frequency of 100kHz by external
components R6 and C6. The output, PWM control
pulses to the gate of FET Q1, is generated by the
comparison of this sawtooth waveform with a
feedback voltage sample from the junction of U2-
Cathode and the bias voltage created by
VOLTAGE
CONTROL potentiometer R3. The resultant variable
pulse width output (U1-8) is applied to the gate of
FET Q1. Q1 turns on and off, ringing inductor L1
with the appropriate pulses to maintain the desired
set DC output voltage. Diode pair U2 rectifies the
signal and inductor L3 and capacitor C9 provide
filtering for the
RF DR B+
output voltage. A sample
of the DC output voltage level is applied thru zener
diode CR1 to the base of transistor Q2 which
provides an alarm signal should the RF drive power
supply fail. The
RF DR B+ FAIL alarm output is
normally at ground potential and switches to an open
circuit condition should a failure occur in the RF
drive power supply.
6.4.2 RF DRIVE AMPLIFIER:
The RF drive
amplifier is a class D amplifier switching at the
carrier frequency
Fc
. The carrier frequency signal
Fc
is applied to FETs Q1 and Q2 in opposing phases
thru buffer transformer T1. The supply voltage (
RF
Dr B+
) is applied to FET Q1 with inductor L1 and
capacitors C1/C2 providing RF decoupling. The
resultant
RF Drive
output will be a square wave at
the carrier frequency with a peak amplitude equal to
supply voltage
RF Dr B+
.
6.4.3 RF DRIVE TUNING:
RF drive tuning
inductor A11L1 is used in conjunction with
capacitors C1 thru C5 on the RF drive tuning PWB
(A11A2). The network is tuned to be series
resonant at the carrier frequency, providing a high
impedance to the harmonics generated by the RF
drive amplifier.
6.4.4 RF DRIVE TUNING PWB:
The RF
drive tuning PWB (A11A2) contains series drive
tuning capacitors C1 thru C5 and an RF drive level
detector. The RF drive level detector comprises
comparator U1/A and associated components. A
sample of the RF drive voltage is rectified and
applied to U1-4. If this voltage falls below the
threshold determined by
RF DRIVE THRESHOLD
potentiometer R5, a 5 VDC
RF drive fail
signal will
be present at J1-1. The RF DRIVE THRESHOLD
potentiometer is adjusted to ensure the RF drive level
in the associated power module RF amplifiers does
not fall below 10 volts peak at the FET gates.
There are eight current transformers and detector
circuits provided to tune the RF drive to the individual
RF power modules. Positive temperature coefficient
(PTC) thermistors RT1 though RT8 provide RF
power module isolation. When a power module's RF
amplifier module fails, the current flowing through its
corresponding PTC will increase above the PTC
threshold. The PTC will switch to a high impedance
and buffer the failed module from the remaining RF
power modules.