In article , Chris Morriss
wrote:
In message , Old Fart at Play
writes
You can keep the low-pass and high-pass outputs in phase with a
4th-order filter.


Not without an additional all-pass filter you can't. And with an added
all-pass you can make a second order crossover have no phase difference
between the LP and HP sections if you want.

However you may not actually want that. :-)

Personally, for active filtering, I'd tend to prefer using a LPF, then
creating a HPF output by subtracting the LPF output from the input. The
result if you keep the levels matched is a LP and HP pair of signals whose
vector sum always equals the input. Thus the combined result shows no phase
errors due to the filtering.

For the actual filters I tend to lift the basic designs from the Active
Filter Cookbook by Don Lancaster.

Slainte,

Jim

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