It is the small unwanted residual periodic variation of the direct current (DC) output of a power supply which has been derived from an alternating current (AC) source. The wave form is shown as figure below.
![](https://meanwellpowersupplies.com/wp-content/uploads/2023/03/image-27.png)
There are two AC contents, 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, a configuration of an oscilloscope with a bandwidth of 20MHz, a scope probe with shortest ground wire possible, and add 0.1uF and 47uF capacitors in parallel with test point for filtering out noise interference are required.
![](https://meanwellpowersupplies.com/wp-content/uploads/2023/03/image-28.png)