and 1/64 step. In this series, each step is divided by two
to get the next step. A Binary series could also be 1, 2, 4,
8, 16, 32 etc.
e. BIT: A Binary digit, 0 or 1.
f.
DIGITAL WORD:
A digital word is a group of bits repre-
senting a complete piece of digital information. The term
“DIGITAL WORD,” when used here, will always refer to
a binary number, which is a series of ones and zeros. The
number of bits in a digital word refers to the total number
of digits (ones and zeros).
g.
MSB
: Abbreviation for
MOST SIGNIFICANT BIT
. In a
digital word, as in a decimal number, the first digit repre-
sents the largest change, and is the MSB.
h.
LSB
: Abbreviation for
LEAST SIGNIFICANT BIT
. In a
digital word, as in a decimal number, the last digit repre-
sents the smallest change, and is the LSB.
i.
BIT 1, BIT 2
, etc: In a 12-bit digital word, the bits are
numbered from 1 through 12, where Bit 1 is the MSB, and
Bit 12 is the LSB.
j.
A/D
: Also written “A to D.” Abbreviation for “Analog to
Digital.”
k.
D/A:
Also written “D to A.” Abbreviation for “Digital to
Analog.”
Some Basic Digital Circuit Concepts used in the following
discussion, and in circuit descriptions, are also included for
review or reference.
In logic circuits, representing a digit by either zero or one is
useful because it can be represented by a switch or a circuit that
is either “OFF” or “ON.” The digits “zero” and “one” may also
be represented by a voltage that is LOW for “zero” and HIGH
for “one.”
In circuit descriptions and on schematic diagrams, the terms
“logic LOW” and “logic HIGH” are used. These terms are also
represented by the letters “L” and “H” on schematic diagrams.
In most logic circuits, normal TTL (transistor-transistor logic)
levels are used. In these circuits, a “logic LOW” is represented
by a voltage between approximately zero and one Volt, and a
“logic HIGH” is represented by a voltage between approxi-
3.5 and +5 Volts.
On block diagrams and on schematic diagrams, when a signal
description is followed by “-L” or “-H,” the letter indicates the
logic state when the signal is ACTIVE. Examples:
a. “RESET-L” indicates that when the signal is logic LOW,
a RESET will occur, or a RESET command is being given.
b. “VSWR-H” indicates that when the signal is logic HIGH,
a VSWR fault has occurred.
A DIGITAL WORD can represent only a finite number of
quantities, or steps, depending on the number of bits in the
digital word.
a. If n = the number of bits in the digital word, then: 2
n
= the
number of quantities that may be represented by that
word, including zero. For example, if a digital word has 6
bits, it may represent 2
6
=64 quantities. If a digital word
has 12 bits, it may represent 2
12
=4096 quantities.
a. “
VALUE” OF EACH BIT
: The least significant bit (LSB)
represents one unit. The next least significant bit repre-
sents two units. The most significant bit represents one-
half of the total quantity that the word can represent. For
example, in a 6 bit “digital word”, the number of quantities
that may be represented is 2
6
=64:
1. Bit 1 (MSB) represents 32 units
2. Bit 2 represents 16 units
3. Bit 3 represents 8 units
4. Bit 4 represents 4 units
5. Bit 5 represents 2 units
6. Bit 6 (LSB) represents 1 unit.
4.9.2
Analog to Digital Conversion
An (Audio + DC) signal from the Analog Input Board is con-
verted into a series of 12 bit digital words by the Analog to
Digital Converter. The digital signal is then processed by the
Modulation Encoder to provide signals to turn individual RF
amplifier modules ON and OFF. The Power Amplifier stage acts
as the Digital to Analog converter to create a high power,
amplitude modulated, RF output signal.
The DC component of the (Audio + DC) signal controls the
number of amplifiers on for carrier power and is adjusted using
the RAISE and LOWER buttons on the front panel. The audio
signal is then added to the DC component. The (Audio + DC)
signal is then sent to the Analog to Digital Converter.
The Analog to Digital (A/D) conversion process takes place in
three steps:
a. Divide the time scale into equal intervals by a high speed
sampling circuit.
b. At each time interval, sample and record the amplitude of
the analog signal.
c. For each recorded sample, construct a 12-bit digital word
that represents the analog sample amplitude.
4.9.3
RF Amplifier Control
Refer to SECTION L, Modulation Encoder, for additional in-
formation.
The 12-bit digital word is “encoded” on the Modulation Encoder
to control the 58 “BIG STEP” and six “BINARY STEP” RF
amplifiers. The 12-bit digital word is divided into two groups of
information: The first six bits, B1 through B6, form a six bit
digital word and are used to control the 58 “BIG STEP” RF
amplifiers; the last six bits, B7 through B12, each control a
“BINARY STEP” RF amplifier.
On the Modulation Encoder, bits B1 through B6 are used to
address ROM, Read Only Memory, address locations. In turn,
the data at each address location controls the 58 “BIG STEP”
RF amplifiers. The six most significant bits of the 12-bit digital
Section IV - Overall System Theory
Rev. AA: 8-9-00
888-2297-002
4-9
WARNING: Disconnect primary power prior to servicing.
