Nautel XL12, Technical Instructions

Page: 114 / 238

12 000 WATT AM BROADCAST TRANSMITTER
XL12
Page 6-1
01 January 2002
SECTION 6
THEORY OF OPERATION
GENERAL
6.1
The theory of operation for the XL12 AM
broadcast transmitter is presented in this section.
SYSTEM OVERVIEW
6.2
The transmitter circuitry is subdivided into
five functional blocks - exciter stage, RF drive stage,
RF power stage, control/monitor function and
AC/DC power stage (see figure SD-1).
The
exciter stage
accepts audio, carrier reference,
exciter selection, reset and PDM inhibit controls
from the control/monitor function. Low level DC
voltages and a B+ sample are applied to the exciter
stage from the AC/DC power supply. A sample of
the PA volts is applied to the exciter stage from the
RF power stage. PDM signals are applied to the RF
power stage from the exciter stage. Low level
voltages and control/status signals are also applied to
the control/monitor function.
The
RF drive stage
accepts carrier frequency
signals and control voltages from the exciter stage to
provide RF drive signals to the RF power stage.
The
RF power stage
accepts RF drive, control and
DC voltages from the exciter and RF drive stages
stage as well as 312 VDC B+ supplies, fan supplies,
and +24 VDC supplied from the AC/DC power
stage. Reset and fan control inputs to the RF power
stage originate in the control/monitor function. RF
power monitoring and RF power stage status
information is applied to the control/monitor function.
The final RF output to the antenna system is
provided by the RF power stage.
The
control/monitor function
accepts critical
signal samples, status and alarm signals from the
exciter stage, RF power stage and AC/DC power
supply. Based on the values of these inputs, the
control/monitor function will output the necessary
control signals to the exciter stage, RF power stage
and AC/DC power supply to ensure proper operation
and protection of the transmitter system.
The
AC/DC power stage
provides the required
unregulated power supply voltages.
EXCITER STAGE
6.3
The exciter stage contains two independent
exciter sections (A and B) which can be selected
automatically or by local or remote control. Each
exciter generates its own regulated DC supply
voltages, RF frequency source, PDM and digital
control signals for use in the RF power stage. The
exciter stage consists of an exciter interface PWB
(A2A1), RF drive or optional synthesizer PWB A
and B (A2A1A1 and A2A1A4), interphase PDM
driver PWB A and B (A2A1A5 and A2A1A6), and
DC power supplies A and B (A2A1A2 and
A2A1A3). Provision is made to accept an optional
C-QUAM stereo exciter in place of the A RF drive
source (see figure SD-2).
Two BNC connectors -
RF V
(voltage)
SAMPLE
and
RF I
(current)
SAMPLE
- are available on exciter
panel A2. They provide voltage and current samples
from the RF output filter. When the transmitter is
properly tuned, the voltage waveform should lead the
current waveform by between 50ns and 120ns.
6.3.1 EXCITER INTERFACE PWB:
The
exciter interface PWB, provides signal distribution as
well as the physical interconnection for A and B DC
power supplies, RF frequency oscillator PWBs and
interphase PDM driver PWBs (see figure SD-8).
6.3.1.1 Exciter Selection:
The selection of
unregulated and regulated voltages, modulating audio,
and FPDM signals is dependant on the position of
relays K1, K2, K3 and K5. Until a +5 VDC
Exciter
B
signal is applied to inverter U1/A, relays K1, K2,
K3 and K5 remain in their non-energized state. The
±15 VDC and ±5 VDC sources will be the A DC
power supply. The unregulated ±24 VDC and ±8
VDC source voltages will be directed to the A DC
power supply.