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Passive Bessel Filter

Discussion in 'Electronic Basics' started by Samuel Groner, Jun 9, 2004.

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  1. Hi

    I want to design a passive Bessel LP-filter, 2nd order @ 200 kHz.
    Source imp. is 50 ohm, terminated with 250k ohm.

    From Zverevs "Handbook of Filter Synthesis", p. 323, I get the
    normalized values L1=1.3617 and C1=0.4539. And now: how do I
    denormalize this stuff?

    I got the math for 1/Rsource<=10, but with 1/Rsource=INF, I don't know
    how to choose the reference impedances.

    Thanks for the help
  2. Hi,

    It is easier with odd order networks. Change the design to a
    3-pole filter (||C, series-L, ||C), cut it in half, transform the
    hi-Z end only (C becomes C/R and L becomes R x L), stitch it back
    together (adding the series components) and then do the usual
    frequency and impedance scaling.

    This works (according to Geffe) on Butterworth and
    Tchebycheff, so it should do the same for low-order Bessel.
    Alternatively, with a 50-ohm source, 250K is practically
    open-circuit and most filter tables give values for this.

    Cheers - Joe
  3. John Larkin

    John Larkin Guest

    C' = C / (W' * Z')

    L' = L * Z' / W'

    where W' is cutoff, 2*pi*200KHz

    Z' = 50

    250K is close enough to an open circuit, so just use a 50-to-infinity
    design. The prototypes are common in the filter book tables. Ends are
    freely flippable.

    Oh, I have a cute, simple program that designs bessels up to 5th
    order. Not exactly "designs", really just stashes the prototype values
    and normalizes for you. I could post to a.b.s.e. if there was popular

  4. Z' = 50

    That was it! Thanks for the help...
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