The Bandpass Filter stage completes the digital to analog con-
version by filtering out the individual “step” voltages generated
by the RF amplifiers.
The Pi Network provides for impedance matching into antenna
systems that are not perfect 50 Ohm loads.
The Output Network also includes RF sample circuits for the
Output Monitor circuitry to provide power monitoring, and
protection against high VSWR conditions.
4.2.2
Audio and Modulation Section
The Modulation Section of the transmitter accepts an analog
audio input signal and converts it to a digital signal. The digital
signal is then processed, or “encoded”, to control the RF ampli-
fiers which produce the “Digital Amplitude Modulation”. Circuit
boards in the Modulation section include the Analog Input
Board, Analog to Digital Converter, Modulation Encoder, and
DC Regulator.
4.2.2.1
Audio Input
Audio is fed into the Analog Input Board where it is processed.
This processing includes attenuating the high audio frequencies
for Medium Wave Band channel spacing, and adding a DC
component to determine the carrier power. This (Audio + DC)
signal is sent to the Analog to Digital Converter. A second (Audio
+ DC) signal is sent to the DC Regulator.
4.2.2.2
Analog to Digital Converter
The (Audio + DC) signal is sampled at a 400 to 800 kilohertz
rate, depending on the transmitter operating frequency, by the
Analog to Digital Converter circuits. The Analog to Digital
Converter converts the (Audio + DC) signal into a 12 bit digital
signal.
4.2.2.3
Modulation Encoder
The Modulation Encoder converts the 12-bit digital audio infor-
mation into control signals which turn the RF amplifier modules
in the Power Amplifier stage ON and OFF, to generate the
transmitter carrier and the instantaneous modulation level.
Other inputs to the Modulation Encoder include a PA OFF signal
from the Output Monitor. Supply voltages for the Modulation
Encoder are supplied by the DC Regulator.
The PA OFF signal immediately turns the Power Amplifier
control signals OFF, if a VSWR condition is detected, to protect
the RF amplifier modules. The OFF signal only lasts as long as
the VSWR condition is present, and the transmitter will imme-
diately return to normal operation.
4.2.2.4
DC Regulator
The DC Regulator produces the B+ (+5 VDC) and B- voltages
used by the Modulation Encoder. The (Audio + DC) sample from
the Analog Input Board “modulates” the DC Regulator “B-”
output voltage. The Modulated B- is a bias voltage for the RF
amplifier modules in the Power Amplifier stage which varies the
turn on/turn off times of the modules to optimize distortion and
noise performance.
4.2.3
Controller Section
The Controller section consists of the Controller, LED Board,
External Interface, and the Switch Board/Meter Panel.
4.2.3.1
Controller
The PA Turn-On/Turn-Off command is recognized by the Con-
troller from any of the LOW, MED, HIGH, or OFF buttons on
the Switch board or from external inputs. Circuits on the Con-
troller energize the main contactors for the PA Power Supply and
provide carrier power control.
4.2.3.2
LED Board
The LED Board contains fault and overload sensing and logic.
It provides 26 LED ColorStat™ panel indications to monitor
transmitter operation. These status indications are also available
as remote status outputs from the External Interface. Many status
indications are “latched” to provide fault indications until they
are “reset,” even if the transmitter is turned OFF. A battery
backup supply holds status indications in memory if AC power
fails or is turned off. The backup supply also enables the trans-
mitter to automatically restart when AC power is restored.
4.2.3.3
External Interface (Remote Control)
The External Interface provides “remote control” inputs, status
outputs, and selected metering outputs. The External Interface
isolates transmitter circuits from remote control inputs to mini-
mize the possibility of damage if improper voltages are acciden-
tally applied to the terminal boards.
4.2.4
Power Supplies
Power Supplies are derived from two transformers: T1 provides
the PA Power Supply voltage and T2 provides the Low Voltage
supply.
4.2.4.1
Low Voltage Supplies
The Low Voltage supply in30 VDC, +22 VDC, -22 VDC,
+8 VDC and -8 VDC unregulated supplies. These supplies are
regulated on individual modules for circuit supply voltages.
4.2.4.2
PA Power Supply
The PA Power Supply in230 VDC, +115 VDC and +60
VDC supplies for the 58 “Big Step” and 6 “Binary Step” ampli-
fiers.
4.3
Low Voltage Power Supply: Circuit
Description
Refer to Sheet 1 of the DX-25U Overall Schematic, 839-7855-
151, in the Drawing Package.
A 208 to 270 VAC input is protected by Metal Oxide Varistors
RV1 and RV2 in the Step-Start panel, fed to Low Voltage circuit
breakers CB1 and CB2 and then to transformer T2 in the trans-
mitter cabinet.
Low Voltage Supply transformer T2 has two secondary wind-
ings. One winding provides 24 Volts AC to bridge rectifier CR15.
The bridge rectifier positive output is +30 VDC and filtered by
C37. The +30 VDC output is used for contactor status and
DX-25U
4-2
888-2297-002
Rev. AA: 8-9-00
WARNING: Disconnect primary power prior to servicing.
