Low Pass Filter in Falstad’s CircuitJS

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Sallen-Key Second-Order Low Pass Filter test on Falstag Circuit Simulator.

The oscillator is a logic inverter (NOT gate, here shown as a NOR gate with both inputs tied) with Schmitt trigger hysteresis, charging and discharging a capacitor.

The attached audio is the output from passing a square wave through the aforementioned filter at a series of frequencies.

The output waveform at frequencies below the filter cutoff appear something like this where we can see the overshooting feedback of the filter:

With the oscillator set to the same frequency as the cutoff frequency of the filter we can see the resonance in the waveform:

We can see the attenuation of the low pass filter as we push the oscillator frequency above the filter cut off frequency:

Lastly, with a sufficient level of gain in the filter, we can see it self-oscillate at the cut off frequency. The original input oscillator was completely removed from this example:

http://www.falstad.com/circuit/circuitjs.html?cct=$+1+0.000005+10.20027730826997+50+5+50%0Aa+592+128+672+128+1+6+-6+1000000+1.0852976508539962+1.0853247832952677%0Aw+592+112+528+112+0%0Ac+528+112+528+240+0+0.00001+1.0853247832952677%0Ar+464+112+400+112+0+100%0Aw+592+144+592+176+0%0Aw+672+128+672+176+0%0Ar+592+176+672+176+0+150%0Ar+592+176+592+240+0+100%0AO+736+96+768+96+0%0Ag+592+240+592+256+0%0Ag+528+240+528+256+0%0Ar+528+112+464+112+0+100%0Aw+672+128+672+48+0%0Aw+464+112+464+48+0%0Ac+464+48+672+48+0+0.00001+-1.605521373918652%0Aa+320+112+400+112+1+6+-6+1000000+0.999990000099999+1%0Ar+192+96+256+96+0+400%0Ar+256+96+256+160+0+100%0Aw+256+96+320+96+0%0Ag+256+160+256+176+0%0Aw+320+128+320+160+0%0Aw+320+160+400+160+0%0Aw+400+112+400+160+0%0A211+736+160+800+160+0+4+44100+1%0Aw+672+128+736+128+0%0Aw+736+128+736+96+0%0Aw+736+128+736+160+0%0Aw+128+96+128+48+0%0Aw+48+80+48+48+0%0Ac+48+112+48+160+0+0.00001+2.311794619154139%0Ag+48+160+48+176+0%0A153+48+96+128+96+2+2+5+5%0Aw+48+80+48+112+0%0A174+48+48+112+48+0+10000+0.5+Frequency%0Aw+112+48+128+48+0%0Aw+48+48+48+32+0%0Aw+48+32+80+32+0%0Aw+128+96+192+96+0%0Ax+15+215+146+218+0+24+square+osc%0Ax+241+217+368+220+0+24+attenuator%0Ax+459+300+629+303+0+24+low+pass+filter%0Ax+737+209+815+212+0+24+output%0Ax+473+329+614+332+0+24+(sallen-key)%0Ax+41+244+116+247+0+24+5V+P-P%0Ax+261+244+336+247+0+24+1V+P-P%0Ao+18+32+0+2090+1.25+0.00009765625+0+-1+0%0Ao+8+32+0+2338+4.676805239458889+9.765625e-55+1+-1+0%0A

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