Fun with Filters!

Posted on Feb 5, 2014

I built a simple 1KHz 6Vpp Sine Wave Generator from a handful of opamps and passives I had laying around. You can check out that project here. I thought it would be fun to quantify just how good of a Sine Wave this circuit was; after all it was just a wave-shaped triangle wave, so I wasn`t expecting too much. But if I quantify h

Fun with Filters!
Click here to download the full size of the above Circuit.

ow good of a sine wave it is I can tweak the circuit to try and make if better. Essentially, a perfect sine wave should only have a single frequency component at the fundamental frequency, but real sine waves have harmonic distortion. This harmonic distortion is the part of the sine wave that we really don`t want; a poor sine wave has lots of harmonic content and a good sine wave has little to no harmonic content. So if we measure the amplitude of the harmonics we can quantify how good our sine wave is! What we need to do is design a Notch filter at 1KHz. This will remove the fundamental frequency and leave the Harmonic content. Below is an LTSPICE schematic of Sine Wave Generator and Notch Filter. When the 1KHz signal is passed through the 1KHz centered notch filter the fundemental frequency is removed and what is left is the Harmonic content. You can see the 3rd harmonic at ~3KHz is what is left. Below is the FFT of the input Sine Wave and the Output of the Notch Filter; you can see the fundamental frequency amplitude is lower with minimal effect on the harmonic content. Now that I had a base line simulation working I grabbed my soldering iron and built up the 1KHz Notch Filter dead bug style in an Altoids box. I wish I had a Network Analyzer or at minimum a Spectrum Analyzer with a tracking output to measure my filters frequency response, but all I had was a function generator and a scope. I swept the function...

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