Specifications

Section IV
Overall System Theory
4.1 Introduction
This section of the maintenance manual will present the overall
principles of operation for the transmitter,including a description
of circuits not included on circuit boards described in sections A
through R of this Technical Manual.
This section is organized as follows:
• Block Diagram Description
• Digital Modulation Process
• Protection of Personnel
• Transmitter Protection
• RF Circuits in the Transmitter
RF Driver Combiner
Output Network
• AC Power Circuits in the Transmitter
Low Voltage Power Supply
RF Amp Power Supply
Servo Power Supply
• Driver Multimeter
• Control Multimeter
4.2 Block Diagram Description
If you refer to Figure 4-1, the Transmitter Block Diagram, you
can see thatthere are two signal paths to theRF Amplifiersection.
One is an RF signal path, and the other is a digital audio signal
path.
Most of the blocks on the block diagram represent printed circuit
boards in the transmitter.
4.2.1
RF Section
The RF Section includes the Synthesizer Interface through the
Output Network.
The RF Section generates an RF signal, then amplifies the signal
to a level adequate to drive the RF Amplifier stage. The RF
Amplifier outputs are combined and go through a Bandpass
Filter/Output Matching Network to the 50 ohm RF output con-
nector.
4.2.1.1 Synthesizer Interface
The RF signal path begins at the Synthesizer Interface board with
a frequency synthesizer module or it can be brought in externally
from another source. The most common uses of the external input
are AM Stereo or combined transmitter operations. A second
Synthesizer Interface board is available as an option.
4.2.1.2 RF Interface
The RF Interface selects which Frequency Synthesizer is to be
used to supply the transmitter. Its output drives the Buffer Am-
plifier.
4.2.1.3 Buffer Amplifier
The plug in Buffer Amplifier module amplifies the RF Interface
output from a TTLlevelto approximately20Vp-p.There are two
separate amplifiers on the Buffer amplifier module. Buffer A and
Buffer B are driven in parallel.
4.2.1.4 Predriver
The Predriver amplifies the Buffer signal to a level adequate to
drive the 14 RF Amplifiers used in the RF Driver. There are two
Predrivers, Predriver A and Predriver B. Only one of the Pre-
drivers is active at any one time. The Predriver module is inter-
changeable with the 14 Driver RF Amplifiers, and any of the 160
Big Step and Binary RF Amplifiers.
4.2.1.5 Predriver Tuning
The Predriver Tuning board resonates the active Predriver output
and provides RF drive to the next stage.
4.2.1.6 RF Drivers
Up to 14 RF Drivers are used in the transmitter to amplify the
RF signal from the Predriver to provide the proper RF drive level
for the 160 Big Step and Binary RF Amplifiers. These 14 RF
Drivers are interchangeable with any of the 160 RF Amplifiers
and the Predriver.
4.2.1.7 Driver Combiner
The outputs from the RF Drivers are combined in the Driver
Combiner. Each RF Driver feeds its own combiner transformer
and the outputs are added together by a copper pipe and con-
nected to the RF Splitter.
4.2.1.8 Driver Tuning
Driver Tuning resonates the RF Driver output.
4.2.1.9 RF Splitter
The purpose of the RF Splitter is to divide the combined output
of the Driver Combiner and provide each RF Power Amplifier
with the proper RF drive level.
4.2.1.10 Driver Encoder
TheDriver Encoder provides the control signals for the Predriver,
and RF Drivers D1-D14. The switch for Buffer/Predriver A or
Buffer/Predriver B is located on this board. The Driver Encoder
board also contains Buffer, Predriver, and High/Low Fault Sense
& metering circuitry.
4.2.1.11 RF Power Amplifiers
The RF Power Amplifiers consists of 160 plug-in modules.
Encoded audio signals from the Modulation Encoder turn on as
many RF Amplifiers as required at any instant by the required
carrier level and the modulating signal. Of the 160 RF Amplifi-
ers, 156 are Big Step Amplifiers and 4 are Binary Amplifiers. The
method of digital modulation employed in the transmitter uses a
combination of whole Big Steps and fractional Binary Steps.
4.2.1.12 Main Combiners and Binary Combiner
9 Main Combiners and 1 Binary Combiner make up the PA
Combiner. The output of each RF Power Amplifier Big Step or
Binary step feeds its own combiner transformer and the outputs
are added together and connected to the Output Network.
4.2.1.13 Output Network
The Output Network transforms the impedance of the PA Com-
biner output, at about 8 Ohms, to 50 Ohms. This is done in two
12/16/03 888-2339-002 4-1
WARNING: Disconnect primary power prior to servicing.