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From: Fred Bloggs
X-Mailer: Mozilla 4.7 [en]C-CCK-MCD EBM-Compaq1 (Win95; U)
Subject: Re: Problem with Spice simulation of Linkwitz-Riley filter
Date: Wed, 18 Sep 2002 17:43:16 GMT
NNTP-Posting-Date: Wed, 18 Sep 2002 10:43:16 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
> Hi all,
> I'm having a strange problem with a Spice simulation of a Lowpass 2nd
> order Linkwitz-Riley filter (that's two 1st order Butterworth in
> series). I have the evaluation version of Spice, which should work ok
> with two opamps and such a simple circuit.
> If I simulate the following (it's very simple so I'll just describe
> Vin -> R in series -> C to ground -> unity gain non-inverting opamp ->
> R in series -> C to ground -> unity gain non-inverting opamp -> Load,
> I do get the expected result and cutoff frequency (With R = 110 k and
> C = 18 nF, F6 = about 80 Hz).
This is a simple cascade of buffered RC low-pass so that the 3dB point
of the composite will be Fc*SQRT(2^(1/n)-1) where n=2. This puts the
composite 3dB point, from Vin->Load, at more like 50Hz and not 80Hz-
nearly an octave removed. Are you setting up DC bias points, running an
AC-analysis, and plotting the gain vs frequency on a log-log scale?
Double check your signal generator DC- bias parameter and make sure that
the 2nd amplifier is still operating in its common-mode input range. Do
a DC-operating point calculation on the circuit and make sure this works
out before you make any AC-analysis runs.
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