I'm simulating a sallenkey low pass filter.In LT Spice it won't work on cutoff frequency & its passing every frequency I feed.
The circuit has 107Hz cut off frequency.But when I apply frequency over 107Hz its passing.
There was a cap on the far right that has one end not connected. Removed
Had to change the value of V1 to "1".
Middle of graph, gain 9db because of U2.
Left side roll off because of C4.
Right side roll off because of U1, R1,2, C1,2.
Doing ac analyses.
You do understand that a filter will still pass frequencies above the cutoff, just at a reduced amplitude as determined by the filter rolloff order and type?
For a 2-pole filter, as the one you have, the rolloff is -3dB at the cutoff frequency, and 12dB/octave or 40dB/decade of frequency increase above that.
This can be seen in Rons plot.
You can eliminate C4 and ground R4 if you set R6 and R7 to provide a bias of 1/6 V2.
The will give a U2 output bias of 1/2 V2 as desired, due to the U2 DC gain of 3.