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How can we test the working of a decoupling capacitor with an oscilloscope. I think I understood the need for a decoupling capacitor in a circuit. One of their use is to remove the noise from the input signal. But I wanted to test this practically.

Q1. Firstly, I want to clarify myself when we say "DC signal have noise" do we mean below(the highlighted red boxes)? Here I have used a battery. enter image description here

I have used 22v battery across a resistor(1Mohm) enter image description here

Q2. Assuming that is the noise, To test the behavior of a decoupling capacitor. I have connected a capacitor(tried both 100uf and .1uf) in parallel to the resistor as shown below and connected the Oscilloscope probes across resistor(where one end is at the ground). But I still notice those small fluctuations. enter image description here enter image description here

Sorry if my terminology is confusing and inaccurate as I am still learning the basics.

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  1. That's more likely to be some other noise than the battery and resistor. All systems, including oscillosscopes are not perfect and have noise too. Maybe some external noise gets coupled into the system, like many people carry around a multitude of RF transmitters with them, like laptops with WiFi, mobile phones that communicate with cell towers, and Bluetooth headsets.

  2. It's likely that putting any amount of capacitance will get not rid of those small glitches if they come from external disturbance sources or it's just the noise of the scope.

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  • \$\begingroup\$ Thanks for correcting my understanding. Now I am able to simulate a glitch by adding a switch in between and tried replacing multiple capacitors(seems like 10uf working best). So when I press the switch, the circuit is complete and the voltage is maintained across the resistor and but when I momentarily disconnect the switch I could still see the voltage is still maintained(of course not for long duration). The same in the absence of the capacitor would bring down the voltage to zero immediately. \$\endgroup\$
    – NTOS Linux
    Oct 10, 2023 at 22:48

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