Nautel XL12, Technical Instructions

Page: 116 / 238

12 000 WATT AM BROADCAST TRANSMITTER
XL12
Page 6-3
01 January 2002
The PDM sample is also compared against a bias
voltage proportional to the
carrier ref
voltage to
determine if the PDM pulse width is in correct
proportion to the applied gain control voltage.
6.3.1.6 Low Voltage Supply Detector:
Comparators U5/A and U5/B monitor the +5V,
+15V, -5V and -15V regulated supply voltages
against a fixed reference voltage to detect a high or
low supply voltage condition. If all supplies are
within allowable tolerances the outputs of U5/A and
U5/B are at ground potential. If one or more supply
voltages increase or decrease, such that they are no
longer at an acceptable level, the associated
comparator's output will switch to a high impedance
(open circuit). A logic 1 (+5V)
LV P/S fail
signal
will be applied to J2-10 for use by the associated
system control PWB in the control/monitor function.
6.3.2 RF DRIVE SOURCE:
The RF drive can
be sourced from one of three different circuits.
6.3.2.1 RF Drive PWB:
The RF drive PWBs
(A2A1A1 and A2A1A4) provides the carrier
frequency
ƒ
c signal and a signal at twice the pulse
duration switching frequency
FPDM
for the
associated transmitter. Refer to the RF drive PWB
instruction manual for additional information (see
figure SD-2).
6.3.2.2 Frequency Synthesizer PWB:
The
optional frequency synthesizer PWBs (A2A1A1 and
A2A1A4) provides the carrier frequency
ƒ
c signal
and a signal at twice the pulse duration switching
frequency
FPDM
for the associated transmitter (see
figure SD-2).
The frequency synthesizer is a microprocessor-
controlled synthesizer which uses direct digital
synthesis (DDS) techniques to generate the required
RF carrier frequency. Rotary DIP switches allow
for the selection of any frequency in the broadcast
band.
The output of the synthesizer is applied to the RF
drive amplifier through the exciter interface PWB.
The
2
ƒ
PDM
output is applied to the associated bi-
phase PDM driver PWB through the exciter
interface PWB and is used to drive the PDM ramp
integrator circuit.
Refer to the frequency synthesizer PWB instruction
manual for detailed information.
6.3.2.3 C-QUAM
1
AM Stereo Exciter PWB:
The optional C-QUAM AM stereo exciter PWB
provides the carrier frequency
ƒ
c signal and a signal
at twice the pulse duration switching frequency
FPDM
for the associated transmitter. Provision is
made to phase modulate the carrier signal with L-R
information. The left and right channel information is
summed and applied directly to the interphase PDM
drive PWB. The stereo exciter is optional for exciter
A only.
1
C-QUAM is an acronym for Compatible
QUadrature Amplitude Modulation and it is a
registered trademark of Motorola. Nautel's C-
QUAM AM stereo exciter is manufactured under
license to Motorola Inc.
6.3.3 INTERPHASE PDM DRIVE PWB
The
interphase PDM driver PWBs (A2A1A5 and
A2A1A6) generates the required interphase, PDM
(pulse duration modulation) drive signals for the
modulator assemblies in RF power modules A thru
H. These PDM drive signals determine the
transmitter output power level as well as the output
modulation level (see figure SD-2).
The
Carrier Ref
input is applied from the system
control to determine the output power of the
transmitter. A sample of the B+ supply voltage is
applied to the carrier level control circuit which is
designed to hold the transmitter output power
constant with variations in the unregulated B+
supply. An
inhibit PDM
input is applied from the
system control PWB to inhibit the interphase PDM
drive during a system during alarm/fault conditions.
A
reset
input provides a reset pulse to reset
protection circuits internal to the interphase drive
PWB. Refer to the interphase driver PWB
instruction manual for detailed information.
6.3.4 DC POWER SUPPLY PWBS:
The DC
power supply PWBs (A2A1A2 and A2A1A3)
accepts unregulated DC voltages from the AC/DC
power supply and provides reg15V, +5V, -
15V and -5V supplies for the exciter stage and the
remaining functional blocks of the transmitter. Refer
to the DC power supply instruction manual for
additional information (see figure SD-2).