Connect with us

Wanted: bandpass filter design

Discussion in 'Electronic Design' started by Paul Burridge, Mar 4, 2004.

Scroll to continue with content
  1. Hi all,

    I need a simple design for a BPF with a centre frequency of 17.2Mhz.,
    corner frequencies one Meg or therabouts either side of that and a
    reasonably good shape factor (nothing critical here). I don't mind
    having to do a bit of tweaking of values myself if anyone has
    something close to that they can post. Passive L/C combinations only,
    please. (assume the usual 50R nominal Zs.)

  2. Paul:


    Such a design can be achieved with a vert simple formula and a pocket
    calculator or slide rule. Why don't you just use Bennett's simple formula
    for the maximally flat approximation all pole element values, or Takashi's
    simple formulae for the equiripple approximation all pole element values?

    Oh... perhaps you want elliptic [Cauer or Darlington] designs or perhaps a
    completely general parameter design in that case ypu will need a computer
    and will need to do some programming or buy some programming, or
    better yet...

    Hire a consultant... :)

    Hey, I've designed literally thousands of LC filters over decades in most
    frequency ranges, for clients who have paid me well. I have access to very
    sophisticated and comprehensive general parameter LC filter design
    programs that I have written [In Fortran BTW] over lo these many years
    and they are immediately at my disposal.

    But... unfortunately for some... I never design filters for free!

    I'll do it for you at my usual consulting rates.

    My rates are reasonable. Just call or email me to discuss the terms and


    Professional Consultant - Signal Processing and Analog Electronics
    Indialantic By-the-Sea, FL
    peter dot brackett at "eye" triple "e" dot org.
    USA area code three two one, number seven two three, nine two five five.
  3. Conversely, go to the WB6BLD website and download the free student version
    of Elsie and do it yourself, will take an hour to learn the software and 5
    minutes to design the filter. Another good filter program that is free and
    available and relatively easy to use is at the AADE website.

    Would be worth your while to learn even if you seldom design a filter.

  4. John Larkin

    John Larkin Guest

    The usual technique is to get a book that has tables of normalized LC
    lowpass filters of various types, pick a response you like, and scale
    it and map to a bandpass. Williams' "Electronic Filter Design
    Handbook" has all this stuff.

    It's mildly tedious, so I won't offer to do one for you.

  5. If you have Office 2002 (XP), you can use this web site.

    A man, a plan, a cat, a canal - Panama!

    Ho, ho, ho, hee, hee, hee
    and a couple of ha, ha, has;
    That's how we pass the day away,
    in the merry old land of Oz.
  6. maxfoo

    maxfoo Guest

    Try this 5 pole Chebychev with 0.1dB ripple

    Fc=17.2MHz BW=1MHz

    CAP 1 2 C=27PF
    PLC 2 0 L=1uH C=56PF
    CAP 2 3 C=4PF
    PLC 3 0 L=.1uH C=80PF
    CAP 3 4 C=3PF
    PLC 4 0 L=1uH C=80PF
    CAP 4 5 C=3PF
    PLC 5 0 L=1uH C=80PF
    CAP 5 6 C=4PF
    PLC 6 0 L=1uH C=56PF
    CAP 6 7 C=27PF


    Remove "HeadFromButt", before replying by email.
  7. maxfoo

    maxfoo Guest

    how did that decimal point get there on plc 3 0??
    inductors should all be 1 microhenry.

    Remove "HeadFromButt", before replying by email.
  8. qrk

    qrk Guest

    Lots of books out there with normalized tables. Go to a technical
    university library and spend a few hours pouring thru some filter
    books. You can use Linear Technology's FilterCad to see what sort of
    filter type and order you need. Then hit the tables and make an LC
    filter. The best book I have found is "Simplified Modern Filter
    Design" by Philip Geffe, circa 1963. You can sometimes find this book
    in a university technical library. It has an excellent treatment of
    basic filter design and how to go from a low-pass prototype to other
    filter types.

    For your particular filter, you might consider a loose-coupled design,
    also called coupled resonators. Good for Q > 10. Your design will just
    fit into this category if you don't mind some imperfection. This type
    of design produces nice component values. "Electronic Filter Design
    Handbook", by Arthur Williams and Fred Taylor (ISBN 0-07-070434-1),
    page 5-19 has info.

  9. Download the freeware filter design software from

    Their UI is, uh, colorful, but it seems to work. Pay attention to the source
    and load impedance. Also, it doesn't like 'engineering' notation (eg:
    17.2Meg), so use scientific notation like 17.2e6 instead.

    The freeware only designs filters to 3rd order, so if you need better than
    that, either dig it out of a book or buy the software. However, it does both
    passive and active filters. Its a bargain at twice the price... ;)

    Bob Monsen
  10. Tom Bruhns

    Tom Bruhns Guest

    Download RFSim99 and let it do it for you. Download Filter Design
    from AADE's site and let it do it for you. Download NuHertz's demo
    version of their filter design software and let it do it for you.
    There may even be web aplets that will do it for you. And then you
    can play with configurations and shape factors and the inevitable
    tradeoffs between complexity and shape factor.

  11. Reg Edwards

    Reg Edwards Guest

    For a very simple bandpass design program download program BANDPAS1 from
    website below. Also obtain response versus frequency. Download in a few
    seconds and run immediately.
    Regards from Reg, G4FGQ
    For Free Radio Design Software go to
  12. Active8

    Active8 Guest

    He already has a book with a fair amount of tables and all the
    explainations on how to go about the design. I'm wondering WTF
    here. Hey paul. wtf?
  13. Fair point. I just wondered if anyone had a design similar to
    requirements *already* lying around they might post it, that's all.
    Save me a bit of scrawling and button-pushing. I didn't wish to put
    anyone out.
    Thanks to all who responded, BTW. Some very useful steers.
  14. Pete  KE9OA

    Pete KE9OA Guest

    Hi Paul,
    E-mail me directly and let me know how many poles you need. I
    have the AADE program, and I will be glad to do that for you.

  15. John Jardine

    John Jardine Guest

    The simplest filter that'll get you by, is this one. (a couple of tuned
    circuits with a bit of top-coupling :) ...

    ___ ||C1
    -|_R_¦------.----.-------||-------.----.-------, O/p
    Source |L2 |C2 || |C2 |L2 |
    I/p | | | | .-.
    C| --- --- C| | |load R
    C| --- --- C| |R|
    C| | | C| '-'
    | | | | |
    === === === === ===

    F1=16.2e6 'lower limit MHz
    F2=18.2e6 'upper limit MHz
    R=50 'Ohms source and load impedance

    C1 = (F1+F2) / (4 x Pi x F1 x F2 x R)
    C2 = F1 /(Pi x F2 x (F2-F1) x R)/2
    L2 = 2xR /(4 x Pi) x (1/F1 - 1/F2)

    The inductors end up being quite small but a couple of loops of wire cost
    nothing. At 25megs the output is down to about 3% , which is not bad.

    This particular style is the "3 element" design and from the traditional
    "Constant K" filter stable. The pleasantly simple to design "constant K's"
    and associated "M derived" filters, can be easily and freely used for most
    day-to-day electronics work.

    Exact Filter design, or 'synthesis' as they nowadays prefer to call it, can
    be a nightmare if you're coming in from scratch. Even using a filter design
    programme requires a good knowledge of filter types/performance/spec's
    before data can begin to be entered.
    If not using a prog' you move onto the more vexing method of having to
    design an initial (prototype) low-pass filter, either via precalculated
    tables or computing the required co-efficients, then mathematically
    transforming the low-pass values into a bandpass (etc) design. For higher
    order filters it can be ball-acheing drudgery and even then inductor
    resistance has a nasty habit of spoiling that pristine work of art. (not a
    problem with DSP filters).

    I can't suggest a decent book, as the general purpose books seem over
    complicated and destined to be written by maths people for maths people.
    (The old GPO line-comm's manuals have useful notes on the 'K and M '
  16. bg

    bg Guest

    Go to Alt Binaries Schematics -
    Paul Burridge wrote in message
  17. Joel Kolstad

    Joel Kolstad Guest

    I'd suggest Temes and LaPatra, "Circuit Synthesis and Design." Now out of
    print, but occasionally you can find a cheap old copy used.

    I'd also suggest the original poster does a couple of the exercises (he
    might have already), doing what you describe with building a low frequency
    prototype filter and then transforming it to a different type and frequency
    range, but just for, e.g., a second order filter. There's not much to be
    gained by doing the 'drudgery,' as you say, for a higher order filter other
    than proficiency in aithrmetic.

    ---Joel Kolstad
  18. Active8

    Active8 Guest

    Yes. I had to check my disorganized drive to make sure I'd been to
    all those links. BTDT. I like having tools like that around to
    compare what they say with what I figgered out on paper.
  19. qrk

    qrk Guest

    Since your too lazy to do this, here, have a few designs. You can run
    these on LTSpice. What is reasonably good shape factor?

    Butterworth, pi-input, 1st-order low-pass prototype
    Vin 1 0 AC 2.0000V
    Rin 1 2 50.000
    L1 2 0 26.899n
    C1 2 0 3.1831n
    Rout 2 0 50.000
    ..AC DEC 396 253.58kHz 1.1667GHz

    Butterworth, pi-input, 2nd-order low-pass prototype
    Vin 1 0 AC 2.0000V
    Rin 1 2 50.000
    L1 2 0 38.041n
    C1 2 0 2.2508n
    L2 2 3 5.6270u
    C2 3 4 15.216p
    Rout 4 0 50.000
    ..AC DEC 792 5.7378MegHz 51.560MegHz

    Butterworth, loose coupled, 2nd-order low-pass prototype
    Vin 1 0 AC 2.0000V
    Rin 1 2 50.000
    C1 2 0 2.0654nF
    L1 2 0 38.046nH
    C2 2 3 185.01pF
    C3 3 0 2.0654nF
    L2 3 0 38.046nH
    Rout 3 0 50.000
    ..AC DEC 792 5.7378MegHz 51.560MegHz

    Butterworth, pi-input, 3rd-order low-pass prototype
    Vin 1 0 AC 2.0000V
    Rin 1 2 50.000
    L1 2 0 53.798n
    C1 2 0 1.5915n
    L2 2 3 7.9577u
    C2 3 4 10.760p
    L3 4 0 53.798n
    C3 4 0 1.5915n
    Rout 4 0 50.000
    ..AC DEC 1188 10.224MegHz 28.937MegHz

    Butterworth, loose coupled, 3rd-order low-pass prototype
    Vin 1 0 AC 2.0000V
    Rin 1 2 50.000
    C1 2 0 1.4607nF
    L1 2 0 53.798nH
    C2 2 3 130.84pF
    C3 3 0 1.3299nF
    L2 3 0 53.798nH
    C4 3 4 130.84pF
    C5 4 0 1.4607nF
    L3 4 0 53.798nH
    Rout 4 0 50.000
    ..AC DEC 1188 10.224MegHz 28.937MegHz

    Butterworth, loose coupled, 8th-order low-pass prototype
    Vin 1 0 AC 2.0000V
    Rin 1 2 50.000
    C1 2 0 511.07pF
    L1 2 0 137.94nH
    C2 2 3 109.63pF
    C3 3 0 457.95pF
    L2 3 0 137.94nH
    C4 3 4 53.121pF
    C5 4 0 527.60pF
    L3 4 0 137.94nH
    C6 4 5 39.985pF
    C7 5 0 543.91pF
    L4 5 0 137.94nH
    C8 5 6 36.809pF
    C9 6 0 543.91pF
    L5 6 0 137.94nH
    C10 6 7 39.985pF
    C11 7 0 527.60pF
    L6 7 0 137.94nH
    C12 7 8 53.121pF
    C13 8 0 457.95pF
    L7 8 0 137.94nH
    C14 8 9 109.63pF
    C15 9 0 511.07pF
    L8 9 0 137.94nH
    Rout 9 0 50.000
    ..AC DEC 3170 15.208MegHz 19.453MegHz
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day