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2016 Extra Class study guide: E7A - digital circuits

Posted: 31 Jan 2016 04:43 PM PST
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E7A Digital circuits: digital circuit principles and logic circuits:
classes of logic elements; positive and negative logic; frequency dividers;
truth tables

Digital circuits are used for a variety of functions in modern amateur
radio equipment. Unlike analog circuits, the output voltage of an ideal
digital circuit can only be one of two values. One of these
voltages—normally a positive voltage—represents a digital 1. The other
value—normally near 0 V—represents a digital 0.

This type of logic is generally called positive logic. Positive Logic is
the name for logic which represents a logic 1 as a high voltage. (E7A11)
The logic may be reversed, though. That is to say that a high voltage may
represent a logic 0. Negative logic is the name for logic which represents
a logic 0 as a high voltage. (E7A12)

The microcomputers that control today’s transceivers are very complex
digital circuits. These complex digital circuits are made by combining many
smaller building blocks called logic gates. These gates perform basic
digital logic functions.

One of the most basic digital circuits in the NAND gate. The logical
operation that a NAND gate performs is that it produces a logic 0 at its
output only when all inputs are logic 1. (E7A07)

This logical operation can be described by a truth table. A truth table is
a list of inputs and corresponding outputs for a digital device. (E7A10)
Table E7-1 shows a truth table that describes the operation of a two-input
NAND gate. A and B are the two inputs; Q is the output.
Table E7-1. This table shows how the output Q of a two-input NAND gate
changes as that the inputs, A and B, change.

Other types of gates perform different logical functions. The logical
operation that a NOR gate performs is that it produces a logic 0 at its
output if any or all inputs are logic 1. (E7A08) Table E7-2 shows a truth
table that describes the logical operation of a NOR gate.
Table E7-2. This table shows how the output Q of a two-input NOR gate
changes as that the inputs, A and B, change.



The logical operation that is performed by a two-input exclusive NOR gate
is that it produces a logic 0 at its output if any single input is a logic
“1.� (E7A09) Table E7-3 shows a truth table that describes the logical
operation of an XNOR gate.
Table E7-3. This table shows how the output Q of a two-input XNOR gate
changes as that the inputs, A and B, change.

Flip-flops are circuits that are made from combinations of logic gates. By
“latching� the state of an input at a particular time, a flip-flop can be
said to have memory. A D flip-flop, and its truth table is shown in the
figure below.



As shown, the output changes only on the rising edge of the clock (CLK)
signal. That is to say, when the signal goes from 0 to 1. If D = 1, Q = 1.
If D = 0, then Q = 0. The other output, denoted by a bar over the Q, is the
inverse of Q.

When a D flip-flop is connected as shown in the figure—with the inverted
output connected to the D input—a flip-flop can divide the frequency of a
pulse train by 2. (E7A03) You can connect the Q output to a second
flip-flop to divide the frequency even further. Consequently, 2 flip-flops
are required to divide a signal frequency by 4. (E7A04)

By connecting a number of flip-flops together, and resetting the circuit
once ten pulses have been input, you can build a decade counter. The
function of a decade counter digital IC is to produce one output pulse for
every ten input pulses. (E7A02)

A flip-flop is a bistable circuit. (E7A01) That means its output is stable
in either state. A monostable circuit is one that is stable in one state,
but not the other. One characteristic of a monostable multivibrator is that
it switches momentarily to the opposite binary state and then returns,
after a set time, to its original state.(E7A06) A trigger pulse causes the
monostable vibrator to switch to the unstable state, and it stays in that
state for a set period, no matter how long the trigger pulse. An astable
multivibrator is a circuit that continuously alternates between two states
without an external clock. (E7A05) In other words, it is an oscillator.

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