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I have a low pass RC filter (R=12Ohm, C = 22µF) Which is showing some strange behavior in the phase shift. The cutoff frequency is around 600Hz, as calculated. But the phase shift goes to -45° only at 890Hz and than it starts ascending back to zero? Could this be due to inductance in the large electrlytic capacitor? enter image description here

enter image description here

Any ideas? Thanks in advance Tom

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    \$\begingroup\$ This question has been closed because it 'needs' clarification. Someone gave a perfect answer to my question, so it seems the question was clear. I don't understand why a perfectly answerable questions gets closed this way. \$\endgroup\$ Commented Nov 17, 2023 at 5:42
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    \$\begingroup\$ Good question. This is a problem I see regularly on this site. Some users with high standing decide that they can't answer the question, so it must be the questioner's fault. Of course, it can't be their fault that they don't know any better on the matter. This happens often enough for me to recognize a pattern. \$\endgroup\$
    – Velvet
    Commented Nov 17, 2023 at 8:32
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    \$\begingroup\$ They reopened it :-) anyway. i'm really thankful for the clarification by Cristobol. I've learned something and it solved my problem. \$\endgroup\$ Commented Nov 18, 2023 at 16:09

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Electrolytic capacitors have fairly large ESR values, and they're fairly constant across frequency. As the reactive impedance drops according to frequency, the total impedance becomes dominated by the ESR and you essentially have a resistor. The following figure from Murata shows this:

Screenshot from Murata paper

As you can see, the top curves show the results from a 10uF electrolytic. At 0.1k, the reactance is dominant, about 10X the ESR. By 1k, there's less difference, and by 10k they're essentially the same. Since the reactive component here is almost negligible, you have a voltage divider at this frequency, which doesn't introduce a phase shift.

Your cap goes through the transition earlier in the frequency range because 22uF is less than half the reactance of 10uf.

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  • \$\begingroup\$ Great explanation thanks! \$\endgroup\$ Commented Nov 16, 2023 at 21:52

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