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bandpass filter oscillations

Discussion in 'Electronic Design' started by [email protected], Jul 17, 2006.

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  1. Guest

    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
     
  2. Mark

    Mark Guest

    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
     
  3. 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
     
  4. Tim Wescott

    Tim Wescott Guest

    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

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  5. Boris Mohar

    Boris Mohar Guest

    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) http://www.viatrack.ca

    void _-void-_ in the obvious place
     
  6. Ban

    Ban Guest

    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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