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A ring oscillator is a device composed of an odd number of NOT gates whose output oscillates between two voltage levels, representing ''true'' and ''false''. The NOT gates, or inverters, are attached in a chain and the output of the last inverter is fed back into the first. == Details == Because a single inverter computes the logical NOT of its input, it can be shown that the last output of a chain of an odd number of inverters is the logical NOT of the first input. The final output is asserted a finite amount of time after the first input is asserted and the feedback of the last output to the input causes oscillation. A circular chain composed of an even number of inverters cannot be used as a ring oscillator. The last output in this case is the same as the input. However, this configuration of inverter feedback can be used as a storage element and it is the basic building block of static random access memory or SRAM. The stages of the ring oscillator are often differential stages, that are more immune to external disturbances. This renders available also non-inverting stages. A ring oscillator can be made with a mix of inverting and non-inverting stages, provided the total number of inverting stages is odd. The oscillator period is in all cases equal to twice the sum of the individual delays of all stages. A real ring oscillator only requires power to operate. Above a certain threshold voltage, oscillations begin spontaneously. To increase the frequency of oscillation, two methods are commonly used. Firstly, the applied voltage may be increased. This increases both the frequency of the oscillation and the current consumed. The maximum permissible voltage applied to the circuits limits the speed of a given oscillator. Secondly, making the ring from a smaller number of inverters results in a higher frequency of oscillation given a certain power consumption. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「ring oscillator」の詳細全文を読む スポンサード リンク
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