|By the way, you might have found a separated component CHBBPF (U_5). Let's
replace the LPF with this BPF.
In order to see new result, please "Save" the schematic every time you modify that.
See the result of FFT (U_3) has changed. Because of BPF, only 3kHz remains.
See WAVEOUT is 3kHz of sine wave.