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I have been designing a passive 4th-order RC low-pass filter in LTspice and was surprised to see that the slope is off substantially from what I would expect for a 4th-order passive filter. Instead of an 80 dB/decade slope, I am seeing a 40 dB/decade slope.

Any thoughts as to why this happens would be greatly appreciated. I have tried grounding at different points but it did nothing, which I expected.

enter image description here

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It's probably just a mistake in your reading/interpreting what you are doing/seeing. Perhaps, you were looking at the wrong node? Or perhaps you didn't allow the frequency axis enough span? (There is a transition region, after all, before the full effect takes place.)

Here's what I get:

enter image description here

Both curves (theoretical and simulated) are the same. Also, note that the slope is indeed 80 dB per decade, as expected. (Expand the image and note the blue line I drew parallel to the green slope.)

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    \$\begingroup\$ Ok. If that is what you got then it's clearly an issue with how I'm using the software. Just wanted to make sure I wasn't missing something conceptual. Thanks for the help! \$\endgroup\$
    – Jake Slavin
    Commented Feb 9, 2023 at 2:58

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