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Thread: ripple frequency...

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    Default ripple frequency...

    hey everyone...
    lets get straight to the point....
    when we are measuring ripple (ac noise on dc output) on a smps..
    should we look in any specific frequency? or anything ac on dc output can be considered ripple (low frequencies ripple and high frequencies transient)?
    if it doesnt make sense look at these two photos and tell if there is anything wrong with em (freq on one is 500 on another its 100)...
    http://imgur.com/h59Fdbd
    http://imgur.com/CPVdeBU

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    You can't "look in a frequency" with a scope unless you are using FFT. You are measuring all frequencies over time (with DC, when DC coupled; without DC with AC coupling) together.

    And you have to differentiate between ripple and noise. Ripple is considered as the operating switching frequency of your converters in the PSU. Noise is present at higher frequencies (MHz range) and can be caused by oscillations in the PSU.

    Anyway you should set up the timebase correctly. In the ripple of a SMPS the lowest frequency component is the rectified line voltage. In a 50 Hz grid I would at least measure over 20 ms.

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    Quote Originally Posted by Microflop View Post
    You can't "look in a frequency" with a scope unless you are using FFT. You are measuring all frequencies over time (with DC, when DC coupled; without DC with AC coupling) together.

    And you have to differentiate between ripple and noise. Ripple is considered as the operating switching frequency of your converters in the PSU. Noise is present at higher frequencies (MHz range) and can be caused by oscillations in the PSU.

    Anyway you should set up the timebase correctly. In the ripple of a SMPS the lowest frequency component is the rectified line voltage. In a 50 Hz grid I would at least measure over 20 ms.
    tnx for the reply...
    yeah i did notice that time bases are not the same but does that effect the peak to peak value that i got off those observations?
    (im having an argument with someone with regard to to how valid those two "psu ripple measurements "are...)
    for sake of simplicity can we call ripple the "AC on the dc out of a SMPS"? and i guess we can use that measurement to compare two psu`s if their outputting the same amount of load ...(what i mean by that is if we are comparing two psu`s ripple on output they both should be outputting 20% to be comparable)

    -if what im saying makes 0 sense or all over the place im sorry i dont have any specific educations in these type of stuff but im really starting to like it...
    but this specific subject i couldn't find much on the internet (there is always the possibility that im not googling the right thing though)
    tnx for your time

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    Default

    From ATX design guide:

    Ripple and noise are defined as periodic or random signals over a frequency band of 10 Hz to 20 MHz. Measurements shall be made with an oscilloscope with 20 MHz bandwidth. Outputs should be bypassed at the connector with a 0.1 µF ceramic disk capacitor and a 10 µF electrolytic capacitor to simulate system loading.

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    ok sry guys for another post...
    are the measurements on the pic with 5us time div wrong?(the reported pk-pk)
    are we missing anything?
    can it even be wrong? if yes how?

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