Specifications

Appendix A
DX Digital Modulation Technology and Concepts
a.1 DX Digital Modulation Technology
and Concepts
a.1.1 Digital Terms and Concepts
The discussion of Analog to Digital and Digital to Analog
Conversion will include some terms, abbreviations, and concepts
used in this Technical Manual which may not be familiar to some
Broadcast Station engineers and technicians. A summary is
included here for review or reference.
a. ANALOG refers to something that has a continuous range
of values, rather than changing in steps. Examples of
analog signals are the audio signals from a microphone, a
turntable cartridge, or a normal tape playback head.
b. DIGITAL is related to digits, or discrete quantities. An
analog signal changes continuously, but a digital signal
changes in steps. An analog signal has an infinite number
of possible values, and a digital signal has a finite, or
limited, number of possible values.
c. BINARY: Has only two possible values. A BINARY
NUMBER is a number represented using only the digits 0
and 1. This is useful in electronic circuitry because a circuit
can be ON or OFF (two states). A logic signal may be one
of two different Voltages, referred to as HIGH (binary 1)
or LOW (binary 0) in this Technical Manual.
d. BINARY can also refer to a series where each step is either
multiplied or divided by two to get the next step. An
example, in the transmitter, is the Binary RF combiner
steps, which are 1/2 step, 1/4 step, 1/8 step, and 1/16 step.
In this series, each step is divided by two to get the next
step.
e. BIT: A Binary digit, 0 or 1.
f. DIGITAL WORD: A DIGITAL WORD is a series of
numbers, or a group of bits, representing 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 is the total number of digits (ones
and zeros). Examples of a six bit digital word are “010010“
and “110101.” A 12 bit digital word is “0100 1000 1101.”
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 “AtoD.” Abbreviation for “Analog to
Digital.”
k. D/A: Also written “DtoA.” Abbreviation for “Digital to
Analog.”
l. ADC: Abbreviation for “Analog to Digital Converter.
m. DAC: Abbreviation for “Digital to Analog Converter.
Some Basic Digital Circuit Concepts, which will be used in the
followingdiscussion 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 may also
be represented by the letters “L” and “H,” particularly on sche-
matic diagrams. In most of the digital logic circuits, normal TTL
(transistor-transistor logic) levels are used, and a “logic LOW”
is represented by a Voltage near zero (between approximately
zero and one Volt), and a “logic HIGH” isrepresentedbya
Voltage near +5 Volts (between approximately +3.5 and +5
Volts). On block diagrams and on schematic diagrams in this
Technical Manual, when a signal description is followed by “-L”
or “-H,” the letter indicates the logic state when the signal is
ACTIVE. Examples: “RESET-L” indicates that when the signal
is logic LOW, a RESET will occur, or a RESET command is
being given. “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.
If n = the number of bits in the digital word, then:
2
n
= the number of quantities that may be represented by that
word.
If a digital word has 12 bits, it may represent 2
12
=4096 quantities.
“VAL UE ” OF EACH BIT: The least significant bit (LSB) repre-
sents one unit. The next least significant bit represents two units.
The most significant bit represents 2
n-
1 units. Example: In a 5 bit
digital word":
Bit 1 (MSB) represents 16 units
Bit 2 represents 8 units
Bit 3 represents 4 units
Bit 4 represents 2 units
Bit 5 (LSB) represents 1 unit.
The basic principles of this new modulation technique are not
difficult to understand, especially if we first review some basic
principles of amplitude modulation and digital electronics tech-
nology. A basic discussion is included in the following para-
graphs, as an introduction or review for technical personnel who
888-2339-002 a-1
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