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So here's what I'm trying to do:

Given a square wave provided by my arduino, use a nth order active low pass filter to modify the square wave into a sine wave by filtering out upper harmonics. This is in the audio range of frequencies, of course.

I've spent a good deal of time trying to do this and I'm pretty close to giving up and buying a function signal generator ic like XR-2206. I don't have any dough for fancy equipment, and may I stress that it's partly a learning exercise.

Maybe part of the problem being is I don't have an oscilloscope, and can't afford one, so oscilloscope is out. It would definitely help to have one, sometimes wishing I could see the signal I'm creating, but here's my thinking: If I can create an audio sine wave that I can hear, then I can change my RC values to work for higher frequencies. Right? (Or wrong?)

But I can never seem to get what sounds to me like a sine wave. I play back a youtube 1000Hz sine wave (or pick your frequency) and compare that to the output of my active RC filter and they just don't sound alike. When I listen to the 1000Hz triangle wave, then that sounds exactly like what I'm hearing from the output of my active RC low-pass filter. But why?

I've read over and over, all over the net and back, that by filtering out the upper harmonics of a square wave produces a sine wave (because of the details of Fourier Transform having an infinite sum of continually doubling frequencies with phase shifts and yada-yada) but the idea just doesn't seem to work. If I filtered out all the other sine waves and am left with the fundamental sine wave, then why doesn't it sound like one? Triangle waves!!!

I'm using n many resistors, n many capacitors, and n+1 many LM358 op-amps. I need the last one to amplify the output signal. I have read that many higher capacity capacitors (like electrolytic) are just too inaccurate for a proper low-pass filter in the audio range, so I'm using 100pF caps with very large value resistors. For example, if I used a 100pF capacitor with 1.62M ohm resistor, then the cut-off frequency is 1/(2*pi*100pF*1.62Mohm) = ~982 Ohms. By using multiple RC filters, you can create a steeper "roll-off" on the upper frequencies, and because the resistance values are so high, the signal needs to be amplified, which is why I'm using LM358 op-amps. The output of one active filter feeds into another, and is filtered again in the next active filter. I'm powering everything with 0 to 5V.

And I know the filter works!! I've modified my filter so that I can hear a crisp 400Hz (triangle??) wave, but by just increasing to 450Hz, the frequency is completely inaudible. Therefore, it's definitely filtering, and pretty damn well! Yet I can still not get a clean sine wave sound. Why? What am I doing wrong? Maybe I'll just torment myself further by trying to make a crystal ladder. Pretty sure I won't do that.

I am not expecting perfection. But I know what I'm hearing and it just ain't sine. Not even close. Any suggestions?

Thanks stack community.

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    \$\begingroup\$ Draw a schematic and consider using audactity on your PC as an oscilloscope for audio signals. \$\endgroup\$ – Andy aka Aug 6 '17 at 8:41
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    \$\begingroup\$ TL;DR, draw a diagram, try reducing the words by 70% to leave only the relevant ones. \$\endgroup\$ – Neil_UK Aug 6 '17 at 9:19
  • \$\begingroup\$ You should be able to find an audio spectrum analyser for your phone too. This will show the relative strengths of the harmonics. For a symmetrical waveform you should only see odd harmonics. \$\endgroup\$ – Transistor Aug 6 '17 at 9:27
  • \$\begingroup\$ The free program that @Andyaka mentioned is Audacity (sorry to interfere, Andy, but the typo in your comment could really mislead the OP if he doesn't know the program). \$\endgroup\$ – Lorenzo Donati Aug 6 '17 at 18:50
  • \$\begingroup\$ Oops cheers @LorenzoDonati \$\endgroup\$ – Andy aka Aug 6 '17 at 18:58
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But I can never seem to get what sounds to me like a sine wave. I play back a youtube 1000Hz sine wave (or pick your frequency) and compare that to the output of my active RC filter and they just don't sound alike. When I listen to the 1000Hz triangle wave, then that sounds exactly like what I'm hearing from the output of my active RC low-pass filter. But why?

One thing to keep in mind is that even a small deviation from a sine wave will produce a sound very different from that of a sine wave. It very strongly depends on the harmonics that were not filtered out by your active filter circuit. You definitely need an oscilloscope.

The good news is, you can make an audio-frequency scope for peanuts: http://www.instructables.com/id/Use-Your-Laptop-as-Oscilloscope/

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