函數(shù)信號(hào)發(fā)生器設(shè)計(jì)外文資料及翻譯-展示頁(yè)
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【正文】 he magnitude of and must be identical. This condition is then embodied in the follwing principle:Oscillations will not be sustained if, at the oscillator frequency, the magnitude of the product of the transfer gain of the amplifier and the magnitude of the feedback factor of the feedback network (the magnitude of the loop gain) are less than unity.The condition of unity loop gainis called the Barkhausen criterion. This condition implies, of course, both that and that the phase of –A F is zero. The above principles are consistent with the feedback formula . For if , then, which may be interpreted to mean that there exists an output voltage even in the absence of an externally applied signal voltage.Practical Considerations Referring to , it appears that if at the oscillator frequency is precisely unity, then, with the feedback signal connected to the input terminals, the removal of the external generator will make no difference. If is less than unity, the removal of the external generator will result in a cessation of oscillations. But now suppose that is greater than unity. Then, for example, a 1V signal appearing initially at the input terminals will, after a trip around the loop and back to the input terminals, appear there with an amplitude larger than 1V. This larger voltage will then reappear as a still larger voltage, and so on. It seems, then, that if is larger than unity, the amplitude of the oscillations will continue to increase without limit. But of course, such an increase in the amplitude can continue only as long as it is not limited by the onset of nonlinearity of operation in the active devices associated with the amplifier. Such a nonlinearity bees more marked as the amplitude of oscillation increases. This onset of nonlinearity to limit the amplitude of oscillation is an essential feature of the operation of all practical oscillators, as the following considerations will show: The condition does not give a range of acceptable values of , but rather a single and precise value. Now suppose that initially it were even possible to sati
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