# Rewriting piecewise voltage function using Heaviside?

Recently I've had an idea to create an assignment where we have low-pass filter connected to the voltage source which is described by piecewise function. I've already dealt with periodic functions such as sinusoidal waveforms and square waves and I got an idea - why not to try something new? I described the voltage source like this:

In the above description of piecewise function, a stands for a time period the signal is at its peak. The signal should be 50% positive and 50% negative. We can choose a to stretch the signal or shrink it, depending on which type of function we prefer. What I did next was to rewrite this piecewise function to Heaviside so I can use Laplace transform afterwards. Using Heaviside, it looks like this

As I mentioned previously, I applied this signal on low-pass filter circuit where I tried to figure out the voltage on capacitor with respect to time. Here is the schematic of a simple RC filter

After I had everything set up, I headed for the KVL equation for this circuit which should look like this

I substituted my Heaviside voltage function to the left-hand side of the equation and I tried to use Laplace transform which looked like this

Then I rearranged the equation, used the residue theory to get to the time domain.

Calculating the first limit gives us the following result

Calculating the second limit gives us this

If we sum them up, we get the final voltage with respect to time on the capacitor

Are these steps correct? Because in the end, I got the voltage function on the capacitor that was total nonsense. Can I even use the piecewise function to describe the voltage source like this? If there is something incorrect, I would be glad if someone explained me why. I would really appriciate it.

Note: p is the Laplace parameter, other literature can use s as a parameter. Initial voltage on capacitor is zero at t=0

• You've bothered to enter all these equations, why not also show what it is you got, both result and plot that doesn't make sense (to you, we can't see or read minds). You should get a piecewise waveform made of three exponentials: rising, falling, rising. Nov 9, 2022 at 17:41
• Thank you for your answer. I'll add the result as soon as possible. Nov 9, 2022 at 18:21
• @aconcernedcitizen I've added everything you asked for. Nov 9, 2022 at 19:48

The source supplies a discontinuous waveform, since it's a PWL type, therefore the output will also be discontinuous. Two things can be stated right from the beginning: first, the type of circuit is a 1st order lowpass filter, and second, the input waveform is formed by two rectangular pulses which, taken separately, can count as step functions. Therefore what's needed is the step response of the 1st order:

\begin{align} H(s)&=\dfrac{1}{\tau s+1} \tag{1} \\ s(t)&=\mathcal{L}^{-1}\left(\dfrac{H(s)}{s}\right)=1-\exp\left(-\dfrac{t}{\tau}\right) \tag{2} \end{align}

1. In the interval $$\0...a\$$ you have a step function, so the 1st segment will be (2) times the amplitude of the source:

$$s_1(t)=U_Mh(t)=U_M\left[1-\exp\left(-\dfrac{t}{\tau}\right)\right] \tag{3}$$

1. In the interval $$\a...2a\$$ the input is a negative step having the initial condition given by the value of the end of the last interval. Its amplitude will be given by this initial condition + $$\U_M\$$:

\begin{align} s_2(0)&=s_1(a)=U_M\left[1-\exp\left(-\dfrac{a}{\tau}\right)\right] \\ U'_M&=s_2(0)+U_M \\ s_2(t)&=s_2(0)+U'_M\cdot s(t-a) \\ {}&=U_M\cdot s(a)-U'_M\cdot s(t-a) \tag{4} \\ {}&=U_M\left[2\exp\left(\dfrac{a-t}{\tau}\right)-\exp\left(-\dfrac{t}{\tau}\right)-1\right] \end{align}

1. In the interval $$\2a...\infty\$$ there is a positive step starting with the value at the end of the last interval:

\begin{align} s_3(0)&=s_2(2a)=-U_M\cdot s(a)^2=U_M\left[2\exp\left(-\dfrac{a}{\tau}\right)-\exp\left(-\dfrac{2a}{\tau}\right)-1\right] \\ U''_M&=|s_3(0)| \\ s_3(t)&=s_3(0)+U''_M\cdot s(t-2a) \\ {}&=-U_M\cdot s(a)^2+U_M\cdot s(a)^2\cdot s(t-2a) \tag{5} \\ {}&=U_M\left[2\exp\left(\dfrac{a-t}{\tau}\right)-\exp\left(\dfrac{2a-t}{\tau}\right)-\exp\left(-\dfrac{t}{\tau}\right)\right] \end{align}

The total system response is (3), (4), and (5), concatenated, and appropriately time-limited (each of them multiplied by one or two $$\\theta(t)\$$ functions). By now you should see how, the more PWL segments you add, the more this can get out of hand. This is one of the reasons why simulators have been invented, with which you can verify the above:

V(1) (green) is the response of the filter to the whole waveform, which overlaps with V(2) (blue) for the 2nd segment andV(3) (red) for the last segment. I've omitted the 1st one since that one is straightforward (1) times $$\U_M\$$.

This analysis is true for any PWL waveform you can think of: for each segment there is a separate analysis, and each segment will use the end values of the last segment. So, while I applaud your attempt at wanting to see how things evolve, or how they look like, mathematically, at the same time, I'd also encourage you to take a step back and admire the whole picture.

I got a bit lazy as I finished this but, when I said that (3), (4), and (5) are concatenated and time-limited, I meant that the full response is this:

$$y(t)=U_M \left( \left(\mathrm{u}(t)-\mathrm{u}(t-a)\right)\cdot\left(1-\exp\left(-\dfrac{t}{\tau}\right)\right) ... \\ \quad+\left(\mathrm{u}(t-a)-\mathrm{u}(t-2a)\right)\cdot\left(2\exp\left(\dfrac{a-t}{\tau}\right)-\exp\left(-\dfrac{t}{\tau}\right)-1\right) ... \\ \quad+\mathrm{u}(t-2a)\cdot\left(2\exp\left(\dfrac{a-t}{\tau}\right)-\exp\left(\dfrac{2a-t}{\tau}\right)-\exp\left(-\dfrac{t}{\tau}\right)\right) \right) \tag{6}$$

(Not sure why \begin{align} complains about "misplaced &", it looks like I can't split lines to align them.)

I've also corrected (3) ($$\\tau\$$ instead of $$\a\$$) and (5) (- instead of + for the 2nd exponential argument). And, to prove it, SPICE to the rescue, again:

The blue text below is the SPICE netlist entry for the whole expression, split for easier reading. I plotted them separately, otherwise they would have overlapped completely.

• Thank you for finding enough time to write all of this, I appriciate it so much. Plus, you plotted the graphs which is good. The thing is, I went through my calculations again and it seems like my final equation I calculated which is also posted at the end of my post, matches your equation (5), but yours is further simplified. But it's still the same. I plotted it and it is the same as mine. Look at the graph I added at the end of my post. But I don't understand why it doesn't work the way I calculated it. I just got half of the result. Why is that? Is it really necessary to divide the graph? Nov 11, 2022 at 11:03
• @tom_ger I've already said it in the beginning (and hinted at it in the comment under your question): the input is discontinuous, meaning that at each inflexion point of the PWL there will be either a Dirac or a Heaviside -- therefore from that point on the output will act as if there was eiter a δ or a θ applied. But, since before that there was already a previous segment with an input signal, that means that the δ or θ will start with the initial conditions given by the previous input. I'm not sure how clearer I can say this, it's pretty much word for word as in the answer. Nov 11, 2022 at 12:22
• Your approach was to consider the whole input as a continuous segment, so you simply added them up, resulting in something similar to f(a,b)+f(b,c)+f(c,d)=f(a,d). That's not correct. Nov 11, 2022 at 12:23
• Okay, thank you for your answer, this summed it up for me and made it clearer. Nov 11, 2022 at 12:44

Well, the transfer function of your circuit is given by:

$$\mathscr{H}\left(\text{s}\right):=\frac{\text{V}_\text{out}\left(\text{s}\right)}{\text{V}_\text{in}\left(\text{s}\right)}=\frac{\frac{1}{\text{sC}}}{\frac{1}{\text{sC}}+\text{R}}=\frac{1}{1+\text{CRs}}\tag1$$

Using the convolution thoerem of the Laplace transform, we can write:

$$$$\begin{split} \text{v}_\text{out}\left(t\right)&=\int\limits_0^t\mathscr{L}_\text{s}^{-1}\left[\frac{1}{1+\text{CRs}}\right]_{\left(t-\tau\right)}\cdot\mathscr{L}_\text{s}^{-1}\left[\text{V}_\text{in}\left(\text{s}\right)\right]_{\left(\tau\right)}\space\text{d}\tau\\ \\ &=\int\limits_0^t\frac{\exp\left(\frac{\tau-t}{\text{CR}}\right)}{\text{CR}}\cdot\text{v}_\text{in}\left(\tau\right)\space\text{d}\tau\\ \\ &=\frac{1}{\text{CR}}\int\limits_0^t\exp\left(\frac{\tau-t}{\text{CR}}\right)\cdot\text{v}_\text{in}\left(\tau\right)\space\text{d}\tau \end{split}\tag2$$$$

So, when:

$$\text{v}_\text{in}\left(t\right)= \begin{cases} 0&\text{if}\space t<0\\ \\ \text{n}&\text{if}\space0\leq t\leq\text{k}\\ \\ -\text{n}&\text{if}\space\text{k}2\text{k} \end{cases}\tag3$$

We get:

$$$$\begin{split} \text{v}_\text{out}\left(t\right)&=\frac{1}{\text{CR}}\int\limits_0^t\exp\left(\frac{\tau-t}{\text{CR}}\right)\cdot\text{v}_\text{in}\left(\tau\right)\space\text{d}\tau\\ \\ &=\frac{1}{\text{CR}}\cdot\left\{\int\limits_0^\text{k}\exp\left(\frac{\tau-t}{\text{CR}}\right)\cdot\text{n}\space\text{d}\tau+\int\limits_\text{k}^{2\text{k}}\exp\left(\frac{\tau-t}{\text{CR}}\right)\cdot\left(-\text{n}\right)\space\text{d}\tau\right\}\\ \\ &=\frac{\text{n}}{\text{CR}}\cdot\left\{\int\limits_0^\text{k}\exp\left(\frac{\tau-t}{\text{CR}}\right)\space\text{d}\tau+\int\limits_{2\text{k}}^\text{k}\exp\left(\frac{\tau-t}{\text{CR}}\right)\space\text{d}\tau\right\}\\ \\ &=\frac{\text{n}}{\text{CR}}\cdot\left\{\left[\text{CR}\exp\left(\frac{\tau-t}{\text{CR}}\right)\right]_0^\text{k}+\left[\text{CR}\exp\left(\frac{\tau-t}{\text{CR}}\right)\right]_{2\text{k}}^\text{k}\right\}\\ \\ &=\text{n}\cdot\left\{\left[\exp\left(\frac{\tau-t}{\text{CR}}\right)\right]_0^\text{k}+\left[\exp\left(\frac{\tau-t}{\text{CR}}\right)\right]_{2\text{k}}^\text{k}\right\}\\ \\ &=\text{n}\cdot\left\{\exp\left(\frac{\text{k}-t}{\text{CR}}\right)-\exp\left(\frac{0-t}{\text{CR}}\right)+\exp\left(\frac{\text{k}-t}{\text{CR}}\right)-\exp\left(\frac{2\text{k}-t}{\text{CR}}\right)\right\}\\ \\ &=\text{n}\cdot\left\{2\exp\left(\frac{\text{k}-t}{\text{CR}}\right)-\exp\left(-\frac{t}{\text{CR}}\right)-\exp\left(\frac{2\text{k}-t}{\text{CR}}\right)\right\} \end{split}\tag4$$$$

• Jan, you can't treat a discontinuous input as if it were continuous. Have you tried plotting your result? There's a reason I said that the output will be a PWL sort, like the input, and it will need little Heaviside helpers, individually. I've updated my answer, see the little monster at the end (6). Nov 10, 2022 at 22:40