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bandpass filter oscillations

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
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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

Reply to
tschoepflin
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what is the desired Q of the filter? component tolerances can cause high Q filters to become unstable.

is there a switching power supply in the system at a sum or multiple of

200 kHz?

look at the output signals on a spectrum analyzer and touch each component with your finger, this will tell you which components are determining the oscillation frequency.

Mark

Reply to
Mark

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

Reply to
martin griffith

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Everything everyone has said so far, plus:

In a linear circuit any instabilities result in oscillations of infinite magnitude, so any finite-magnitude limit cycles must involve circuit nonlinearities. _Small_ magnitude limit cycles usually involve small nonlinearities. Small nonlinearities are hard to model, and may not be in your SPICE component model.

Small signals appearing in band pass filters may be from oscillation, or they may be from noise pick up. It can be hard to tell the difference between oscillation and noise, particularly when the output signal is small, so you'll need your thinking cap.

Do what you can to verify that the signal isn't noise pick up and not really a circuit instability at all.

I would go at this by carefully eliminating the possibility that it's noise pickup first. If I got to the point where the input was grounded with just that sub-circuit alive and powered from batteries and there was _still_ a signal there then I'd believe oscillation (or evil magnetic forces -- is your bench next to an MRI machine?).

If it _is_ really oscillation I'd start asking myself how the actual op amps differ from the circuit model. Obvious places to consider are the environment (board layout) they're in, and subtle nonlinear effects that are too much of a pain for ADI's marketing department to include in their models.

It looks like this circuit sets the bandwidth with R1, which implies the need for a really low impedance source -- I bet that an open circuit at the input terminal would result in oscillation, and an op-amp with some crossover distortion driving that input _might_ result in oscillation. Long cable runs may do the same thing.

I don't know if distortion in an op-amp _in_ the circuit would cause oscillation, but I'd check.

Doing a poor job of bypassing your amplifiers might cause this problem as well. Ditto for poor power supply regulation or parasitics. Although parasitics shouldn't be a terrifically big deal at 200kHz you are right in considering things like microphonics (although this would show up in spades if you just tap on the board).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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Reply to
Tim Wescott

Are you sure the it is not just amplifying and filtering the input noise?

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

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void _-void-_ in the obvious place

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Reply to
Boris Mohar

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Boris seems to be right on the track. This filter attenuates a lot with the input resistor R1=8k for Q=10 we get 173mVpp noise at 128x gain. You should amplify at least 30dB before the filter and the rest afterwards. I also don't think you can just replace a coil with this circuit and it would require 3 resistors to be variable as well.

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ciao Ban
Apricale, Italy
Reply to
Ban

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