# resistor values of a 4th order bessel high pass filter

I have a question about cascading the 2nd order bessel high pass filter to form a 4th order bessel high pass filter, I have an equation to solve for R1 and R2, will the same equation be used to solve for R3 and R4 of the cascaded 2nd order bessel high pass filter?

## 1 Answer

From wiki, the transfer function polynomials for several Bessel filters are: -    If you took two 2nd order filters and multiplied the polynomials you do not end up with the polynomial of a 4th order Bessel filter. You end up with this: -

$s^4 + 6s^3 + 15s^2 + 18s + 9$

This informs you that cascading two independent 2nd order Bessel filters does not produce a 4th order Bessel filter or, in other words you have to calculate the values of the components in both 2nd order filters taking into account that you are trying to implement a 4th order system. You might get lucky of course but if you just cascade 2 2nd order Bessel filters it won't be a 4th order Bessel filter.

It's not just Bessel filters that this rule applies to.

Here's a website that might help. The calculator appears to keep a common value for the capacitors used but there are 6 resistors all having different values: - 