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楼主 |
发表于 2020-2-21 21:59:58
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The output signal from the sigma-delta modulation must be filtered to remove the high
frequency content (the quantization noise) and retain only the useful frequency band. In
order to properly design such filter, some understanding of the sigma-delta modulated signal
spectrum is required. The sigma-delta modulated signal spectrum is different from the PWM
modulation spectrum.
The PWM modulation signal is characterized by a fixed period and a variable duty cycle.
Due to the fixed modulation period (or frequency), the PWM spectrum exhibits typical peaks
of energy corresponding to the modulation base frequency and its harmonics. The removal
of these harmonic peaks is more difficult by analog filtering (RC or LC filter).
The sigma-delta modulation uses both variable duty cycle and variable frequency. As a
result, the energy on the sigma-delta spectrum is spread more evenly and is not
concentrated on regularly spaced peaks like for the PWM (there is no fixed modulation
frequency). Furthermore, there is usually more energy at higher frequencies for sigma-delta
modulations than for PWM (due to higher modulation frequencies). The noise content of
sigma-delta modulation can be easily removed by analog filtering (RC or LC filter). |
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