I am trying to Filter a +-4V odd inverted square wave into a sine wave. How should I go about doing so?
So far I have computed the Fourier series of my particular input signal to be:
Which I know to be correct. I am trying to isolate the first harmonic frequency to get my sinusoidal output, which is 1Hz.
I am trying to use an active feedback filter using a TL072 op-amp. So far I have only simulated on LT-Spice and I am getting this output and I cannot figure out why.
I used the frequency band .3 Hz to 1.7 Hz with a corner frequency (.3^2 + 1.7^2)^(1/2), and an Av Gain of 40. I computed R1 and R2 and then computed a common C that would center the frequency range at the corner frequency. They are shown in the circuit diagram.
This is where I got the filter design and model equations from:
(Scroll to Infinite Gain Multiple Feedback Active Filter at
I have tried bumping up the input square wave voltage offset so that the op amp minimum output doesn't limit the output, but I seem to still be getting that effect. I have to use an Active filter design in order to do this.
The top starts to look like a sine wave but then the bottom half appears to not pass through the filter. I am thinking that I have the op amp configured incorrectly. When I remove the ground from the - terminal, it crashes the simulation.