An AC waveform can be identified on the DC output of a S.P.S., which is shown in Figure 2.4.

There are two AC components, also known as Ripple and Noise (R&N), on the DC output. The first one, coming from sine wave rectification, is at a low frequency which is 2 times of the input frequency; the second one is at high frequency which is from the switching frequency. For measuring high frequency noise, configurations of an oscilloscope with a bandwidth of 20MHz, a scope probe with shortest ground wire possible, and adding 0.1uF and 47uF capacitors in parallel with test point for filtering out noise interference are required to be made.

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