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[KB6NU] 2016 Extra Class study guide: E7A  digital circuits
KB6NU's Ham Radio Blog /////////////////////////////////////////// 2016 Extra Class study guide: E7A  digital circuits Posted: 31 Jan 2016 04:43 PM PST http://feedproxy.google.com/~r/kb6nu...m_medium=email 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 E71 shows a truth table that describes the operation of a twoinput NAND gate. A and B are the two inputs; Q is the output. Table E71. This table shows how the output Q of a twoinput 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 E72 shows a truth table that describes the logical operation of a NOR gate. Table E72. This table shows how the output Q of a twoinput NOR gate changes as that the inputs, A and B, change. The logical operation that is performed by a twoinput exclusive NOR gate is that it produces a logic 0 at its output if any single input is a logic “1.” (E7A09) Table E73 shows a truth table that describes the logical operation of an XNOR gate. Table E73. This table shows how the output Q of a twoinput XNOR gate changes as that the inputs, A and B, change. Flipflops are circuits that are made from combinations of logic gates. By “latching” the state of an input at a particular time, a flipflop can be said to have memory. A D flipflop, 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 flipflop is connected as shown in the figure—with the inverted output connected to the D input—a flipflop can divide the frequency of a pulse train by 2. (E7A03) You can connect the Q output to a second flipflop to divide the frequency even further. Consequently, 2 flipflops are required to divide a signal frequency by 4. (E7A04) By connecting a number of flipflops 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 flipflop 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. The post 2016 Extra Class study guide: E7A digital circuits appeared first on KB6NUs Ham Radio Blog. 
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