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Engineer It - How to test power supplies - Measuring Noise

Hari T O0
Genius 3820 points

Dear Team,

I was watching the video regarding the power supply noise measurement. It is very helpful.

I have some questions about cursor placement.

1)Why we are not placing the cursor on the top.

2)Is it possible to automatically measure using the AC RMS in the oscilloscope?

Please find the attached image.

Regards

Hari

  • 0
    TI__Expert 4905 points

    Hi Hari,

    The reason is that I was only measuring ripple noise in that case, not the higher frequency transient noise, which is often filtered out or significantly attenuated with decoupling caps at the load. 

    I believe I discuss ripple vs transient noise  within the following EDN articles:

    https://www.edn.com/testing-a-power-supply-part-1/

    Regarding RMS measurements using a scope, i believe most higher end scopes provide such a feature but you will need to reach out to the manufacturer directly for details, LeCroy in this case.

    Hope this helps, Regards, Bob

  • 0 in reply to Bob Hanrahan
    Genius 3820 points

    Hi Bob,

    Thank you very much.

    I need to measure the noise coming from a DC power supply (N6705 DC Power Analyzer) which is given as an input to the Comparator Present in my ASIC.

    The voltage range will be from 500mv to 700mv.

    My oscilloscope(Keysight,MSO-X 4054A) have AC RMS measurement capability.

    I will be following the below procedure.

    I will connect the DC power supply output to oscilloscope-->Set Coupling to AC--->set Amplitude level to 10mV/Division or low-->Then I will turn on AC RMS

    Measurement Button.

    I will be using the active probe N7020A from Keysight.

    May I know your thoughts about this.

    Thanks&Regards

    Hari

  • 0 in reply to Hari T O0
    TI__Expert 4905 points

    Hi Hari,

    I have not used that Keysight scope/probe but can share some thoughts.

    From what you state it sounds like the scope will provide a value representing the RMS AC voltage with DC removed so it might be handy for measuring noise over a specific frequency range.  Obviously you must take into consideration bandwidth limitations of the probe and scope.

    You may want to reach out to the manufacturer of the equipment you are using to understand any limitations when using these functions.

    Thanks, Bob

  • 0 in reply to Bob Hanrahan
    Genius 3820 points

    Hi Bob,

    Thank you very much.

    In some places(my google search) I read we need to use BNC cable for noise measurements.

    May I know your opinion about this.

    Regards

    Hari

  • 0 in reply to Hari T O0
    TI__Expert 4905 points

    Hi Hari,

    Using shielded cable such as coax is a good idea when making noise measurements.  Otherwise you might be seeing cell phone RF, soldering irons, and lighting switch mode noise to name just a few of the outside noise sources that creep into power supply measurements.

    From my power source I use banana to BNC adapters with 3' coax cables (RG-59 or better) on the input connections yet you must also be aware of the IR voltage drops, so keep them short.  For high currents you will need to use lower resistance coax.

    Bob

  • 0 in reply to Bob Hanrahan
    Genius 3820 points

    Hi Bob,

    Thank you very much.

    Regards

    Hari