I'm trying to smooth out the steps from a digitally-generated LFO signal which changes step-wise. The digital steps are small, but still audible when the LFO is hooked up to the circuit that uses it (zipper noise).

I'm considering using a low-pass RC filter. I'd like to predict the effect it would have so I can decide if it will be a useful approach. Are there any free or affordable resources I could use? I'm pretty much a layperson when it comes to electronics, so the simpler the better.

Given that the effect of a low pass RC filter on a square wave is asymptotic, something like this (borrowed from here)...

effect of low pass filter on square wave

... my assumption about how an RC filter works is that the gradient of the filtered signal is proportional to the voltage differential between the filtered signal and the source signal; and that the proportionality is determined by the R and C values.

Edit I've TL;DR;ed the question - the accepted answer says everything I was trying to elicit.

  • \$\begingroup\$ Are you familiar with the Laplace or Fourier transform? It's much easier to talk about the response of filters in terms of frequency components. \$\endgroup\$
    – Hearth
    Commented Oct 16, 2021 at 13:12
  • \$\begingroup\$ @Hearth, unfortunately, no. They're the reason I flunked out of university 30 years ago : ) \$\endgroup\$ Commented Oct 16, 2021 at 13:14
  • \$\begingroup\$ I can sort-of get what the Fourier illustration in this post is about though electronics.stackexchange.com/a/590774/218819 - add sine waves of various frequencies and amplitudes to get a square-ish wave \$\endgroup\$ Commented Oct 16, 2021 at 13:25
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    \$\begingroup\$ The step size does not matter. For both large and small steps the curve is proportional to the step, which is why an RC filter has a time constant tau = RC which says that in time tau the capacitor is charged to 63% and in time 5 times tau it is charged to 99%. \$\endgroup\$
    – Justme
    Commented Oct 16, 2021 at 13:27
  • \$\begingroup\$ @Justme, does that mean that the curves I've guessed should all be the same basic shape, but stretched out depending on the height of the step? And that they should all get as close (proportionally) to the source signal? I'll add a graph to illustrate. \$\endgroup\$ Commented Oct 16, 2021 at 13:30

1 Answer 1


Besides analysis, another option is simulation. Here is a start. The schematic editor and simulator are free, but you can't save.

For a simple RC filter, your digital frequency must be much, much higher than the output frequency.


simulate this circuit – Schematic created using CircuitLab

enter image description here

  • \$\begingroup\$ My digital frequency is currently 500Hz, the curve I'm trying to generate from it would be around 5Hz. Is that "much much higher" enough? BTW, this answer is exactly what I needed. \$\endgroup\$ Commented Oct 20, 2021 at 9:38

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