In message , Jim Lesurf
writes
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.


It struck me as rather odd as well when it was first put to me. Do you
have SPICE available to you? It's easy enough to make an arbitary LP or
HP filter and use an "ideal subtractor" element to produce the other
output. It's easy enough to verify it.

I'll try it with a high order filter, (6-th order or so) and run the
simulation. I guess your email address is valid, so I'll email you the
gain/phase plot of the sim run.

Regards,

--
Chris Morriss