Because of the high frequency content of this ripple, special measurements techniques must be employed so that correct measurements are obtained. First a 20MHz bandwidth oscillosocpe is normally used for the measurement so that all significant harmonics of the ripple spike are included.
This noise pickup is eliminated as shown in Figure 3 by using a scope probe with an external ground band or rind and pressing this band directly against the output common terminal of the power converter while the tip contact the voltage output terminal. This makes the shortest possible connection across the output terminals.
Figure 3.
Figure4 show a complex ripple voltage waveform that may be presentage on the output of a switching power supply. There are three components of the waveform, first is a 120Hz component that originates at input rectifier and filter, next is the component at the switching frequency of the power supply, finally there are small high frequency spikes imposed on the high frequency ripple.
Figure 4.
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