In article , Chris Morriss
wrote:
In message , Jim Lesurf
writes
The advantage of higher orders is they can cut down to size of the
region where we have an (unwanted) array effect. However you can do
this using my approach, and it saves money as you only need one
high-order LPF and then get the HPF that matches it 'for free'. :-)

Slainte,

Jim


But even if your HPF (say) is a 4-th order, the LPF you get by
subtraction is still only a first order.

I've not done this for a while, but that strikes me as rather odd (apology
for the pun! :-) ) as a general claim. I think you may find it depends upon
the details of the filter shape of the LPF filter, not just the order.

IIRC when I did some filtering like this a while ago for analysis of the
effects of HF on tweeters the HP anf LP sections (done this way) were of
the same sort or roll-off slopes. May be mis-remembering, though...

The phase shift of the summed output is zero, and it does sum flat of
course, so it still is a good thing.

(It took me ages to work this out, but it is correct, and a quick SPICE
simulation shows it)

I think this may depend upon some specific assumptions you may have made.
However I'll be interested to hear what you can report on this.

Consider designing a LPF that is approaching a 'brick wall' with a flat top
near 0dB and a fall-off to, say, -60dB that occurs over a narrow range.

Where the LPF is near 0dB the output from the 'HPF' must be very small.
However as with transit the turnover region it rises to near 0dB. The
narrower the transition region, the steeper the slope of the turnover of
both the HP and LP outputs.

OTOH if you choose a high order filter that has an inband 'droop' whose
size scales up significantly with the order, you will, indeed reduce the
slope of the HP output in the 'droop' region. But what matters here is the
filter shape, not just the order.

I've forgotten the latin for 'taking an absurd example' for the sake of
extreme illustration. However I won't let my lack of decent classical
education deter me from the following... :-)

Imagine building an analogue version of one of the 96th order low pass
filters used for digital. These can have an inband ripple that is very
close to 0dB, yet die the death in the space of about 2kHz. If you were to
apply the above methods to get the HP difference I doubt it would show just
a first order rolloff as you went down into the low frequency range. I
think the response would change very rapidly over the same 2kHz-ish band.

Or am I missing something?

Slainte,

Jim

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