I designed a 6th order low pass Butterworth filter using multiple feedback topology (Fc = 500 Hz). But when I run it in LTSpice, the cutoff frequency is seen shifted far below to around 10Hz. Why is this happening and what can I do to fix it? Any comments or references to reading material are greatly appreciated. I apologize if I did not ask the question in the best format, first time on here.

I am looking to accomplish a 6th order low pass filter using multiple feedback Here the topology I am using

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    \$\begingroup\$ You are aware, I hope, that the polynomial table you have above is based upon \$\omega_{_0}=1\$, yes? \$\endgroup\$
    – jonk
    Apr 21, 2020 at 5:07
  • \$\begingroup\$ You need to scale the prototype figures for both impedance and frequency (radians/s, not Hz). Show us the figures/calculations you have for one section, from prototype numbers through to component values. \$\endgroup\$
    – Neil_UK
    Apr 21, 2020 at 5:33
  • \$\begingroup\$ Here is another tipp for the design: There are two simplifications - either you can choose (a) R2=R3=R or you can (from the beginning) choose (b) two standard capacitors with C2/C1>4*Qp²(1+R2/R1). \$\endgroup\$
    – LvW
    Apr 21, 2020 at 8:12
  • \$\begingroup\$ @LvW I'd go for (b) as capacitor values have fewer options. It's easier to identify the capacitors you have and are appropriate and then let the resistors fall out from that decision. \$\endgroup\$
    – jonk
    Apr 21, 2020 at 15:54
  • \$\begingroup\$ Jonk - yes, that is the reason I have mentioned this design alternative. \$\endgroup\$
    – LvW
    Apr 21, 2020 at 17:18


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