# Filters for DC pulses

Discussion in 'General Electronics Chat' started by Setanta, Aug 19, 2013.

1. ### Setanta Thread Starter New Member

Mar 12, 2012
17
4
So I started to play with simple RC filters for the first time and generally things have been working as expected when using sine waves.

I've been wondering about DC pulses though. In the screenshot attached the input is at 300hz and the cut off point is 2khz. If the input is a since wave it's works fine, but not if it a square wave.

Is a different technique needed for DC pulses or it it just not as simple as that? I've been checking around but haven't been able to find a concrete answer.

https://www.dropbox.com/s/01jrzgey9dcaayj/Selection_023.png?m

2. ### w2aew Member

Jan 3, 2012
219
64
What you've got there is a high-pass filter. It will only pass the high-frequency energy from your waveform, and block the DC and low frequency content. In a square-wave as you've shown, the rising and falling edges contain the high frequency content, so that's why you see a response lined up with the edges.

Another way to think about it is this: The current in a capacitor is directly proportional to the *rate-of-change* of the voltage across the capacitor. This makes sense, because in order to change the voltage on a capacitor, you have to add or remove charge - moving charge means current. That current also flows in the resistor, which is why you see the result.

A third way to think about this is - mathematically. A simple high-pass RC filter like this is a differentiator: i(t) = dV/dt. Thus, you see a response that is proportional to the rate of change of voltage across the cap.

odinhg likes this.
3. ### MrChips Moderator

Oct 2, 2009
12,623
3,451
When analyzing the effects of filters on a step function you need to know that a low-pass filter performs integration (summing) and a high-pass filter performs differentiation (derivative or rate of change).

4. ### Setanta Thread Starter New Member

Mar 12, 2012
17
4
Ok, I think I get it. Good explanation, thanks.

So a multi stage filter then. Ok, I'll see if I can dig up info on that or just play around with it.

Last edited: Aug 19, 2013
5. ### crutschow Expert

Mar 14, 2008
13,475
3,361
A square-wave has various frequency components that are affected by passing through a filter. The high frequency edge will be rolled-off by a low-pass filter and the low frequency flat portion will be rolled-off (develop a slope) by a high-pass filter. The degree of this effect depends upon the LP and HP filter corner frequencies as compared to the square-wave fundamental frequency. If you change the RC time-constant for the filter you will see how the effect changes.

6. ### Setanta Thread Starter New Member

Mar 12, 2012
17
4
Yup, that's what I didn't realize. Thanks for the help guys.