Posted on Feb 5, 2014

Clocks have been mentioned in the preceding section with regard to their action with FFs. You will recall that this circuit is a timing signal generated by the equipment to control operations. This control feature is demonstrated in both the D and J-K FFs. Remember that a clocks output had to be in a certain condition for the FFs to perform their

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functions. The simplest form is the astable or free-running multivibrator. A schematic diagram of a typical free-running multivibrator is shown in the figure below along with its output waveforms. This multivibrator circuit is called free running because it alternates between two different output voltages during the time it is active. Outputs 1 and 2 will be equal and opposite since Q1 and Q2 conduct alternately. The frequency of the outputs may be altered within certain limits by varying the values of R2C1 and R3C2. You may want to review the operation of the astable multivibrator in NEETS, Module 9, Introduction to Wave-Generation and Wave-Shaping Circuits. Although the astable multivibrator circuit seems to produce a good, balanced square wave, it lacks the frequency stability necessary for some types of equipment. The frequency stability of the astable multivibrator can be increased by applying a trigger pulse to the circuit. The frequency of the trigger must be higher than the free-running frequency of the multivibrator. The output frequency will match the trigger frequency and produce a more stable output. Another method of producing a stable timing pulse is to use a triggered monostable or one-shot multivibrator. You can recall from studying the NEETS, Module 9, that a one-shot multivibrator has one stable state and will only change states when acted on by an outside source (the trigger). A block diagram of a...

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