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How to Measure Ripple and Noise in Scope

Electronic circuits depend on direct current (DC) sources of power, well-regulated and free of electrical noise. The cleanest DC comes from a battery; however, many devices need more power than batteries provide. A power supply turns household alternating current (AC) into DC. While powerful and reliable, the electricity produced by power supplies has a small amount of 60 Hz hum, also called ripple, and noise. You can measure this with a sensitive device called an oscilloscope. The oscilloscope displays electronic signals graphically on a screen.

Things You'll Need

Oscilloscope probe
Oscilloscope

Instructions

- 1
  Turn off fluorescent lamps in the room and use only incandescent or natural lighting, if possible.
- 2
  Connect the oscilloscope probe to the BNC male connector post for the oscilloscope's Channel 1 input. Slide the switch on the probe to the "1x" setting.
- 3
  Connect the oscilloscope's probe tip to the DC power supply's positive output terminal. Connect the probe's grounding clip to the power supply's ground or negative terminal.
- 4
  Turn the oscilloscope and power supply on.
- 5
  Adjust the power supply's voltage output to a value between 5 volts and 15 volts.
- 6
  Set the input coupling selector switch on the oscilloscope's channel one to "AC." Set the horizontal sweep rate to 10 ms per division. Set the trigger mode selector switch to Channel 1.
- 7
  Adjust the Channel 1 input sensitivity control to 1 mv per division. You should see a signal on the oscilloscope display representing the power supply's noise and hum. If the signal is too strong to fit on the display, change the setting on the input channel to 10 mv per division or more.
- 8
  Count the number of screen grid divisions between the zero line on the oscilloscope display and the highest point on the signal. Multiply by the volts per division to find the signal's amplitude, or signal strength. For example, if you count 3.5 divisions and the setting is 1 mv per division, your power supply has 3.5 x 1 mv or 3.5 mv of hum and noise.

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