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Why Bessel?

Discussion in 'Electronic Design' started by Roger Bourne, Mar 19, 2006.

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  1. Roger Bourne

    Roger Bourne Guest

    Hello all,

    I recently was going over an old electronic design, and I noticed that
    the designer used a 4rth order Bessel low pass filter where low pass
    filtering was needed. (I know it was a Bessel filter because it so
    marked ;) ). The bessel filter was realized with 2 op-amps and many
    resistors and a few capacitors. Knowing the designer (designer is
    overseas now for a few years...) was always making his designs as
    compact as possible, - (had to twist both of to his arms to make him
    insert failsafe redundancies :) ) - why would he use such a bulky
    method for low pass filtering?
    The bessel low pass filter cuttoff frequency is ~100Hz. It's sharpness
    would have to be, hmmm, well pretty sharp. After all it was initially
    intended for a medical application. I would hazard a guess that is what
    warrants the 4rth order. But why a Bessel topology? I know bessel
    filters are usefull in audio applications because of the linear phase
    property they have, but this is not an audio application.
    Basically, I am asking why Bessel ? Does Bessel have anything special
    that I am anaware of?

    -Roger
     
  2. Bessel filters don't ring when a step is applied to their inputs.
    Would that mean anything to this design?
     
  3. A Bessel filter offers the lowest possible time delay and group delay
    distortion. Its falloff rate is an utter atrocity.

    See my Active Filter Cookbook.

    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
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  4. Err.. no it doesn't. Your statement is actually a bit vague.

    The Bessel filter is a filter invented to produce a linear phase with
    frequency (or constant delay) that actually has an *explicit*
    mathematical method available to calculate its element values. This is a
    key, fundamental point for all of the standard filters. The ability to
    actually be able to mathematically calculate component values of a
    filter of arbitrary order with some explicit procedure.

    If one uses other computer techniques, not available when "modern filter
    theory" was being invented, such as brute force least squares fitting it
    is strait forward to design filters with better performance then pretty
    much any of the standards such as chebychev, Butterworth, Bessel etc.

    For example, the Q=2/pi 2nd order is a "better" least mean squares fit
    to a linear phase response, i.e. better than the Bessel filter of 2nd
    order.
    Since it wasn't designed to be a frequency filter this is not
    surprising. The fact that it rolls off at all is incidental to its
    initial invention.



    Kevin Aylward B.Sc.

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "There are none more ignorant and useless,than they that seek answers
    on their knees, with their eyes closed"
     
  5. Could you tell us who invented it - and a bit more about the design
    process they went through if they didn't intend it to be a 'frequency
    filter'?
     
  6. Roger Bourne

    Roger Bourne Guest

    A Bessel filter offers the lowest possible time delay and group delay
    How bad is the rolloff (falloff) rate?
    We are talking about bessel low passes, rigth? Wouldn't it have to have
    a decent rolloff (falloff) rate to be used as a low-pass ?
    I assume that the rolloff rate would be ALSO among the parameters that
    make or break a filter. Having linear phase is a great property, but it
    would also need to attenuate. Rigth?

    -Roger

    -Roger
     
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