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Is this even possible? Filter/impedance question

Discussion in 'Electronic Basics' started by billcalley, Aug 9, 2005.

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  1. billcalley

    billcalley Guest

    Hi All,

    A company I'm contracting for told me to design a circuit I did not
    even think was possible. But is it? They have an amplifier that needs
    to see 1300 ohms at its output, even though the amplifier itself only
    has an output impedance of 75 ohms. So far, so good. But they want a
    *passive* highpass filter to be at the output of this 75 ohm amplifier!
    In other words, they want the input to the highpass filter to present
    the 75 ohm amplifier with 1300 ohms! (And then the HPF must also have
    an output impedance of 75 ohms after all this). I would have thought I
    needed an active buffer to accomplish this. How could I even begin to
    design such a structure, much less maintain the HP filter shape? Has
    anyone ever heard of this before?

    Thank You,

  2. John Larkin

    John Larkin Guest

    You'd need a transformer to do this, and it would result in a
    best-case voltage gain through the filter (ignoring losses) of 0.24.

    Your customer sounds a little confused.

  3. Ban

    Ban Guest

    Did you really understand rightly? It is not that difficult to make a
    highpass with 1300R input impedance, in fact the needed transformer itself
    will act as a passive highpass if you design it right. The turns relation
    must be 4.17:1and the attenuation will be 12.5dB depending on the losses in
    the transformer. But since the amp is loaded with only 1.3k, its level is
    almost +6dB higher. You will probably need a Zobel network on the output of
    the amp to prevent gain peaking. I wonder why they want the high impedance
    on the amp output, if a simple 1:1 transformer does the same thing without
    any level loss.
  4. Hi,
    Passive filters with asymmetrical in/out impedances are quite
    common. Design an odd order Butterworth (or Tchebycheff) filter
    using one of the impedances, figuratively cut it in half through
    the middle reactance, scale one half to the second impedance and
    then glue back together.

    Look up p.18 of -

    "Simplified Modern Filter Design" by Philip R. Geffe.

    Cheers - Joe
  5. jgreimer

    jgreimer Guest

    You left out most of the requirements of the filter. What is - the corner
    frequency? - the slope (db/decade)? - the insertion loss?

    My first thought is a pi or T filter but I need the above details first.
  6. billcalley

    billcalley Guest

    Hi All,

    Thanks for the replies! I know about asymmetrical filters, but
    they are used when the filter's load and source impedances are not
    equal. But in this special case both the source and load that the
    filter sees *are* 75 ohms -- it's just that the filter's picky 75 ohm
    source (the amplifier) wants to see 1300 ohms. How can I get a HPF
    response out of that, considering that there is now a huge (purposeful)
    mismatch created at the filter's input? How can any passive filter act
    as a high impedance "buffer" like that?
    The filter is a 600kHz (to 12MHz) HPF, with an insertion loss of
    <2dB, 3 poles, >15dB RL, and <0.5dB ripple.


  7. qrk

    qrk Guest

    You can build passive LC filters with asymmetric input/output
    impedance. Silly example:

    Vin 1 0 AC 18.333V
    Rin 1 2 1.3000k
    C1 2 3 247.04p
    L1 3 0 263.43u
    C2 3 4 149.28p
    Rout 4 0 75.000
    ..AC DEC 60 59.528kHz 6.0000MegHz

    However, the actual input and output impedances will change
    significantly with frequency. Probably not what your looking for. You
    could pad the input which would calm the input impedance variations
    that the source sees. Of course, the pad will attenuate the signal
    further. I think you need to chat with these guys and see what they
    are really after.
  8. Jim Thompson

    Jim Thompson Guest

    They're probably clueless... 1300 ohms load comes out to 0.5dB
    insertion loss.

    Maybe they're French ?:)

    ...Jim Thompson
  9. billcalley

    billcalley Guest

    That is what I thought guys -- thanks!

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