Re: bandpass filter oscillations

On 17 Jul 2006 10:14:30 -0700, in sci.electronics.design
"tschoepflin@xxxxxxxxx" <tschoepflin@xxxxxxxxx> wrote:

Hi all

I'm using the dual-amplifier bandpass filter to implement a tunable
3-stage Chebyshev filter. In essence, the DABF is serving as variable
inductor via the generalized impedance converter configuration. See p.
5.93 of Analog Device's "Op-Amp Bible" by Jung
(http://www.analog.com/library/analogDialogue/archives/39-05/Web_Ch5_final_PtB_F.pdf)
for circuit details.

Anyone have experience with this filter? I am experiencing a small
oscillation at its resonant frequency (200 kHz) even with zero input.
The "envelope frequency" is about 10-20 kHz. This becomes quite
noticeable (100mVptp) after my gain stage of 128. Obviously, we could
redo the whole board to put the amplification first (which we should
have done in the first place). However, I wondered if there was an
obvious solution or some things we could try to make sure we understand
this issue before revising the entire board.

The previous design worked great and used a Toko variable inductor
instead of the generalized impedance converter.

My other suspicion is that I am using two 1000pF ceramic caps (NP0/C0G)
and I've read about microphonics being a problem. Anyone with
experience here? Could they generate an oscillation like what I'm
experiencing? I'm wondering if the 200 kHz is inherent to the filter
and whether the 10-20 kHz "envelope" is due to microphonics.

My workhorse op-amp for the filters is the Analog Devices OP467 and my
gain stage uses the AD829. I simulated the whole thing with LTSpice
and the ADI circuit models and wasn't able to reproduce the
oscillation, so I'm starting to doubt that the op-amp is at fault.

Thanks for the help.
Todd
Whats the layout like? Do you have adequate decoupling caps on the
opamps supplies? Does a 22pF fitted across R3 do anything? How are
you measuring it? a X10 scope probe?


martin
.