Hello all, I'm incredibly new to electronics and am taking an instrumentation course this fall to finish out a chemistry degree. Intro aside, we did our first lab today which involved making a low pass filter to turn triangular waves into sine waves - turned out to be pretty simple after I figured out how a circuit board works and where to plug in cords. The part we were supposed to figure out on our own was turning a square wave into a sine wave using a low pass filter (or some variation). Like I said, I'm really new to this so I just basically started sticking capacitors and resistors in every order I could think of on the circuit board. That didn't work, so I started varying the frequency and turning knobs on the oscilloscope while I was at it. Nothing I did worked, so I came home to research what I could on the internet. There seems to be plenty of active filter information (using opamps which we just learned about today in lecture), but the only useful information I could find was in this forum in a post that mentioned "resonance" using more than one low past filter http://forum.allaboutcircuits.com/showthread.php?t=12501 (last comment). Is this the right direction, or is there some other method that I'm failing to figure out here? I get the feeling that the solution is probably something simple, even by introductory standards, but for the life of me I'm at a loss. Thank you for your help.

 

I think you have the answer at hand and don't realize it. Both a square wave and triangle wave only consist of odd harmonics; of course, the Fourier coefficients are different. Hopefully, that's enough of a clue to get you going...

 

Thank you for the advice, I really am trying to figure this out on my own since that was my professor's intent. The only problem is that I think he is assuming that I have more background knowledge than I really do (he didn't even assign a textbook for the course, and his notes seem lacking). His style seems to be learn by doing, and he suggested just randomly trying stuff out on the circuit board, which I did for longer than I would have liked. If I could ask just one more question to be sure, are the three variables I'm concerned with the frequency, resistor used, and capacitor used? Is it just a basic low pass filter, and I'm looking for specific values on those three variables? The reason I ask was because pretty much any resistor/capacitor was working to make triangle waves into sine waves. Thanks again for your help though. I'm not as intuitive in math and otherwise new material as I would like to be, but I'm going to go over my notes again to try and find what I missed.

 

You need more poles/zeroes in your filter network to get rid of the transients of a square wave. Basically string two RC filter stages together such that the second one has higher impedances than the first (bigger R, smaller C). This way the first one can drive the second and still behave like a first-order low pass filter.

Or think of it this way: Find a way to turn a square wave into a triangular wave. Then deal with the triangular waveform.

 

That's what we were thinking, but it turns out we didn't know how to wire it in correctly on the circuit board. The professor eventually showed us how to put the second RC set in and explained the same thing that you just did. I know this was an exercise in intuition and experimenting, but it seems to make it 10x as hard when we're not even told how the hardware actually works. Oh well, hopefully things aren't as bad next time. Thank you for the help marktchaos.

Edit: Was looking around on the internets and realized I was actually working with a breadboard and not a circuit board. Pardon my ignorance . Plus side is that I found a how-to on breadboards lol.