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Are active filters realistic at 30MHz?

Discussion in 'Electronic Design' started by Joel Kolstad, Feb 10, 2005.

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  1. Joel Kolstad

    Joel Kolstad Guest

    Just curious... has anyone used an active filter at 30MHz or higher? I'd be
    looking for a second order bandpass filter up there, and using a state
    variable topology I'm reading that the GBW of the amplifier needs to be at
    least 3*Q*f0... and I can live with a Q of about 10-12.5, so that would
    indicate a GBW in the ballpark of a GHz. Analog Devices will such you such
    an op-amp, but is it going to very difficult to get the filter to perform
    properly when the GBW is the same as the '3*Q*f' formula? Or does that
    formula assume you'll spec the op-amp's GBW directly from it?

    Other than LC filters, are there other common methods for HF range filters?

  2. Jim Thompson

    Jim Thompson Guest

    May I recommend you take a look at the gyrator-based filter structures
    on my website? I would think a GHz GBW OpAmp will do just ducky at

    ...Jim Thompson
  3. Joel Kolstad

    Joel Kolstad Guest

    Thanks Jim, will do. In fact I already came across your gyrator web page
    while Googling -- you had quite the discussion with Win and Peter Brackett!
  4. Ken Smith

    Ken Smith Guest

    Beware that the 3*Q*f0 thing means the GBP at the operating frequency.
    Many op-amps fall a bit faster than a single pole. If you want the filter
    to be very accurate, you may have to (gasp) trim it.

    The f0 point tends to shift downwards in frequency as the op-amps add
    excess phase to the system. You may be able to just compensate for the
    typical values and let the variation in op-amps be a variation in the
    performance. A little figuring and calculating is needed.
  5. Tim Wescott

    Tim Wescott Guest

    Crystal filters, if you need 0.01 to 0.1% relative bandwidths.

    I suspect that an LC filter will be more frequency stable than anything
    with an op-amp (sorry Jim), it'll be passive so you won't have to worry
    about oscillations, and at 30MHz it won't be much bigger, if at all.

    Whatcha trying to do?
  6. I read in that Joel Kolstad <[email protected]
    30 MHz is the border between HF and VHF.


    SAW (Surface Acoustic Wave).
  7. Joerg

    Joerg Guest

    Hello Joel,

    I would crack out Williams filter handbook and do it LC. Much cheaper,
    and easier.

    Regards, Joerg
  8. Joel Kolstad

    Joel Kolstad Guest

    Build an electronically tunabe notch filter for the entire HF band. :)

    'Electronically tunable' can mean horrible things like motor driven L/C's,
    but I'd really like to avoid that if possible.

  9. I read in that Joel Kolstad <[email protected]
    Heterodyne up to the 100 MHz band with a fixed oscillator, make a
    tunable notch filter for 100 MHz to 127 MHz and heterodyne down again.
    Or 500 MHz, if 27% bandwidth is too much.

    Or heterodyne up with a tunable oscillator to a fixed 100 MHz notch
    filter and heterodyne back down.
  10. That has to be the key question.

    Whether or not gyrators are suitable up there, gyrators tend to be
    used where the coil it replaces is way too bulky, and even where
    the coil may cause stray pickup. These are audio issues, since a big
    inductor (electrically and physically) can so easily pick up stray 60Hz,
    and the large inductance can take up space.

    At 30MHz, the coils become small, and they have too little inductance
    to pick up that 60Hz. The active circuitry is likely to use up more
    space than the inductors they replace. Expecially since a far more
    complicated filter will be needed at 30MHz to get desired bandwidth.

    One also wonders how the gyrators act under strong RF signals. Since
    one reason for filters at RF is to ensure the active elements don't
    see too much signal, does using active elements as inductors lower
    the signal handling capability of the receiver?

    When people ask about high frequency gyrators, they seem to generally
    lack a grounding in RF, and are trying to extend audio concepts that
    they know to a field they don't know.

    If one needs a lot of selectivy, you use a crystal filter at the signal
    frequency (if you can afford it), or go to a superheterodyne receiver
    so the selectivity is at a frequency where it can be done easily, with
    front end selectivity only there to limit what the active components

    One could always add some regeneration to front end filtering to
    increase Q, but of course that doesn't help skirt selectivity.

  11. Joel Kolstad

    Joel Kolstad Guest

    Hi John,

    Yeah, I've thought of that, and it is quite attractive. The idea of an
    active filter instead was to introduce less distortion in the whole process,
    but I have no hard evidence for whether or not that's really the case
    instead of just a gut feeling.

    BTW, how were you planning to build your tunable 100-127MHz notch filter?
    Varactor diodes in an LC filter?

  12. I read in that Joel Kolstad <[email protected]
    I left that as an exercise for the student. If you have room, a trough
    line or a cavity might be interesting. VERY sharp notch.
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