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Designing LC Ladders

Discussion in 'Electronic Design' started by Rob Gaddi, Jan 5, 2005.

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  1. Rob Gaddi

    Rob Gaddi Guest

    So I need to do some wideband low RF filtering (flat to 0.5 dB from
    40-100 MHz, fairly sharp cut off on the high end, low end can cut off or
    not as convenient), and have found that my lossless LC ladders are in
    fact anything but. Who knew that several ohms of resistance in my 0603
    inductors would add up, other of course than anyone who had stopped to
    think about it.

    I've managed to massage the filters back into shape through trial and
    error in RFSim99, but in the process have beaten my nice little
    Butterworth and Chebyshev filters into unrecognizable scattered heaps of
    very sore poles, making irrelevant decades of filter theory. It strikes
    me that there must have been an easier way of doing it.

    So, for future reference, how should one go about doing wideband RF
    filtering? I've got books here with plenty of suggestions if I felt
    like blowing ten thousand dollars designing an analog ASIC, but I'd like
    a solution with a few less zeros, not to mention not having to learn
    analog ASIC design.

    -- Rob
  2. I read in that Rob Gaddi
    Are you using separate low-pass and high-pass filters with a buffer
    between them, rather than a band-pass? There are formulae for filters
    with lossy inductors in the classic books, Zverev and Williams et al.

    I suggest you stick to Butterworth; Chebs are much more sensitive to
    component values.
  3. Max

    Max Guest

    I think you were on the right track. Karl Willy Wagner dealt with some
    of the same issues in his very classic paper "Kettenleiter und
    Wellensiebe" in 1919 (_Arkiv für Elektroteknik_ was the journal that I
    recall, though I should verify). There are also some good solid-state
    monolithic filters today with some flexibility.
  4. Nico Coesel

    Nico Coesel Guest

    Besides that, component variations will kill your filter as wel. I
    recently designed a low pass filter in about the same frequency region
    using an active sallen-key second order filter. To make it roll off
    more sharply I added an LC series snubber.

    The topic is more or less covered on page 3 of this document:
  5. Max

    Max Guest

    My previous reply on this will teach me not to rely on memory.
    Wagner's 1919 paper is "Spulen und Kondensatorleitungen." _Arkiv für
    Elektrotechnik,_ Band 8, Heft 2 u. 3 (July 1919). His paper
    "Kettenleiter und Wellensiebe" appeared in _Elektrische
    Nachrichten-Technik,_ Band 5, Heft 1 (January 1928).
  6. Joerg

    Joerg Guest

    Hi Rob,

    It depends on what the words "fairly sharp" mean. If you have to be down
    to -30dB or more at 110MHz it'll be quite a challenge. Then those tiny
    0603 inductors won't cut it. I'd start with 'real' inductors.

    Also, depending on band rejection outside your range you may have to
    make compartments or at least spread it out a bit. Another question
    would be whether this is a one-off or something for mass production and
    what the cost and other constraints are.

    One trick that may seem a bit obscure but has worked for me in the past
    is to up-convert the whole chebang to above a GHz where you then can
    design a regular filter, abandoning the lowpass-highpass combo and using
    resonant filtering instead. Then just convert back down with the same
    LO. Make sure to suppress the unwanted sideband. Who knows, maybe you
    could even convert to some range where you can buy commercial filters
    that happen to fit a 60MHz bandwidth. If you want to scope out this
    route check Murata's 'Gigafil' series of dielectric filters. I believe
    there was one that has 60MHz bandwidth. In case you go for it be
    prepared for some really suspicious looks from the other engineers in
    the design review.

    With respect to mixer chips for such a task that has become much easier
    these days, thanks to the bargain pricing going on in the cell phone
    industry. Same for filters.

    Regards, Joerg
  7. Mac

    Mac Guest

    In this particular application, you could just cascade simple op-amp
    filters. You could use a spreadsheet as a design aid to let you try
    different R and C values (including tolerance) and different numbers
    of stages. The more stages you use, the farther up you have to move
    the 3 dB point to satisfy your 0.5 dB at 100 MHz spec.

    If I remember my schooling correctly, with 4 stages, you get 80 dB per
    decade of rolloff. Of course, the op-amp is probably running out of steam
    at that frequency, anyway, so you may get even more rolloff.

  8. Leon Heller

    Leon Heller Guest

    I use ELSIE for filter synthesis. The limited free version is OK for my
    purposes, but the full version isn't expensive.

  9. Rob Gaddi

    Rob Gaddi Guest

    Depending on which set of components I have soldered down at any given
    time, I either have just strict low-pass 6th order Butterworth or a
    low-pass 5th order Chebyshev with a couple big DC blocking caps that put
    a zero at about 5 MHz. What goes on on the low end is fairly
    unimportant, I'm just trying to knock out the D/A harmonics from the
    direct digital synthesis chips generating the RF.

    Went to the local university library, blew the dust off of Zverev. Lots
    of good information on Q pre-distortion. Useful stuff to know about;
    makes me wish my undergraduate years hadn't been spent after the
    prematurely announced death of analog.
  10. Rob Gaddi

    Rob Gaddi Guest

    It depends on what the words "fairly sharp" mean. If you have to be
    Did some poking around on Coilcraft's site. Yep, lo and behold a little
    less stinginess on the footprints could have doubled or better my sorry
    Q of 40. Yeah, fairly sharp in that I'm trying to keep that whole band
    while putting at least 20dB of hurt on the 3rd order harmonics that
    start at 120 MHz. Hence I was using 6 pole filters, and the parasitics
    added up.
    Ideas like that make me wish I had budget enough to design with
    honest-to-god impedance controlled boards.
  11. Tony

    Tony Guest

    Probably no good to you, but AFAICR the early 10baseT magnetics
    included a 7th order Butterworth LPF, which might be a cheap easy way
    to filter a lesser DDS.

    Tony (remove the "_" to reply by email)
  12. Joerg

    Joerg Guest

    Hi Rob,

    As an experiment, try to start with a not so aggressive filter, more in
    the direction of Butterworth rather than Chebychev. Then place a few
    notches (resonant circuits) above your passband to steepen the slope
    there so you get to the 20dB.

    Actually, the final circuitry was remarkably cheap. Much cheaper than
    low tolerance inductors and all that labor involved with tuning. What
    really blew the others away was that it also ended up being smaller in
    real estate.

    With respect to impedance control you can get that with plain old FR4.
    Maybe not like on a hybrid or a teflon board but often it will be good

    Regards, Joerg
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