Section L
Modulation Encoder (A37)
L.1
Introduction
This section includes a description of the Modulation Encoder
and troubleshooting information.
Refer to SECTION V, Maintenance, for maintenance and re-
placement procedures. There are no adjustments on the Modula-
tion Encoder.
The 12-bit digital audio signal from the Analog to Digital Con-
verter is fed to the board and “encoded” to provide the correct
Turn-On/Turn-Off signals for the 64 PA Modules.
The DX-25U uses Modulation Encoder A37.
a.
Modulation Encoder A37 controls “Big Steps” RF33
through RF90 and “Binary Steps” B7 through B12 (RF96
through RF91).
The Modulation Encoder includes cable interlock and blown fuse
circuitry to monitor the PA Modules. The Driver Encoder/Temp
Sense Board cable interlock circuit is also interfaced on A37 to
monitor the Driver Module interlocks.
L.2
Principles Of Operation
L.2.1
Modulation Encoding: Explanation and Example
The digital audio signal consists of a stream of 12-bit digital
“words”. The 12 bits are referred to as B1 through B12, where
B1 is the MSB (Most Significant Bit) and B12 is the LSB (Least
Significant Bit). The 12 bits are divided into two groups of binary
information to be decoded: B1 through B6 control “Big Step”
RF amplifiers RF33 through RF90; B7 through B12 control
“Binary Step” RF amplifiers RF96 through RF91.
T h e f o l l ow i n g ex a m p l e d e c o d e s t h e b i n ar y wo rd
“010111/001011”.
L.2.1.1
“Big Steps”(RF33-RF90).
If the status of B1 through B6 is:
B1
B2
B3B4B5B6
0
10 111
The total number of “Big Step” RF amplifiers turned ON can be
calculated by changing the binary number to its decimal equiva-
lent. This example would indicate that 23 amplifiers are turned
ON. Typically, 23 “Big Step” RF amplifiers are required for a
carrier power of 25 kW.
L.2.1.2
“Binary Steps” B7-B12 (RF96-RF91).
The “Binary Steps” are controlled directly by the individual bit.
If the bit is a logic “HIGH”, then the associated RF amplifier is
turned ON. If the bit is a logic “LOW”, then the associated RF
amp is turned OFF.
a. B7 = 1/2 Step
b. B8 = 1/4 Step
c. B9 = 1/8 Step
d. B10 = 1/16 Step
e. B11 = 1/32 Step
f. B12 = 1/64 Step
If the status of B7 through B12 is:
B7
0
In the 12-bit example, the status of the “Binary Step” amplifiers
would be:
a. B7 = OFF
b. B8 = OFF
c. B9 = ON
d. B10 = OFF
e. B11 = ON
f. B12 = ON
L.2.1.3
Modulation Encoding: Read Only Memories
The Modulation Encoder uses eight 256x8 ROM (Read Only
Memory) IC’s. Each ROM contains 256 eight-bit addressable
memory locations that are permanently programmed at the fac-
tory with an eight-bit digital word at each memory location.
The eight MSB’s (Most Significant Bits: B1 through B8) of the
encoded audio address any of the 256 memory locations in each
ROM. Each memory location contains the information to control
the number of “Big Step” RF amplifiers necessary to correctly
represent the eight MSB’s of the encoded audio. When a memory
location is addressed, the 8-bit digital word stored at that location
appears at the outputs of the IC (pins 6-9 and 11-14). Each bit of
the digital word provides a turn-on/turn-off signal for a “Big
Step” PA Module.
L.3
Circuit Description
Refer to the Mod Encoder Schematic, 839-7855-174, in the
Drawing Package.
L.3.1
Supply Voltages
Three supply voltages from the DC Regulator are fed to both
Mod Encoders:
a. B+ Supply (+5.75 VDC)
b. Modulated B- Supply
c. +22 VDC
To prevent overdissipation of the regulator pass transistors on the
DC Regulator when the cooling fans are not operating, the B-
and B+ supplies are disabled when the transmitter is OFF.
L.3.1.1
B+ Supply
The B+ Supply is filtered by C1 through C3 and pr5.7
VDC to Inverter/Drivers U1 through U30, U39 and U40. Zener
diode CR2 regulates the B+ to +5 VDC and is fused by F1. All
remaining integrated circuits on the board operate from the +5
VDC Supply.
Rev. T2: 06-27-97
888-2297-002
L-1
WARNING: Disconnect primary power prior to servicing.
