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Low pass filter Design

Discussion in 'General Electronics Discussion' started by san, Jun 17, 2010.

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  1. san

    san

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    0
    Jun 17, 2010
    can any one suggest a layout pattern(compact) for low pass filter..at freq 1.5GHz..and equations to get the dimensions of the layout
     
  2. davenn

    davenn Moderator

    13,808
    1,943
    Sep 5, 2009
    hi San,
    have a look through some amateur radio handbooks eg. .....

    International Microwave Handbook or The ARRL UHF/Microwave Experimenters Manual
    to name 2 there are many others look for low pass filters for 1296MHz the measurements will be easily adjustable for 1500MHz

    cheers
    Dave
    VK2TDN
     
  3. san

    san

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    Jun 17, 2010
    Thanks buddy..ll look into tthose books
     
  4. NickS

    NickS

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    Apr 6, 2010
    Can you be a little more specific.

    What kind of filter Butterworth, Chebyshev, Bessel, Elliptic, M-derived/constant-k composite?

    We know you want Low Pass but what order(how many poles)?

    Do you have requirements on being down X-dB by Z-MHz?

    Is the primary goal size/performance or something else?

    There are many options at that frequency but it all depends on your needs. There are so many variable that make a difference. But I think filter synthesis can be rewarding design and a lot of fun(that makes me a dork I know).
     
  5. davenn

    davenn Moderator

    13,808
    1,943
    Sep 5, 2009
    awww Nick I would NEVER call you a dork ;) hahaha

    as for the filters, I aimed him towards the amateur radio publications as its the first place I always go :)
    He should be able to find a few different designs that should state the parameters that are achievable.

    Hopefully some time he calls back in and lets us all know how he got on :)

    ciao
    Dave
     
  6. san

    san

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    Jun 17, 2010
    thanks for reply...(NickS and davenn)

    to davenn--->i tried to find the book...but its not available in net...

    to NickS --> elliptical filter...bandwidth is 400 MHz and high cuttoff of abt 60 dB at BW +150 MHz.
     
  7. NickS

    NickS

    367
    0
    Apr 6, 2010
    I thought you said Low Pass Filter... Are you changing to a Band pass filter(400MHz bandwidth).

    Assuming a LPF with fc = 1500MHz I interpret your need of 60dB attenuation at 1650MHz. Is that correct?

    Also I feel the need to mention that elliptical filters are very inviting because of their steep transition bands but that comes at a price(group delay). So as we proceed I ask are you trying to pass a single tone or are you passing a band of frequencies(say 400MHz) and if so how close are they to the cut-off frequency?

    Moving on if it is a band pass filter you are after you may want to take a look at coaxial resonators. They are not used nearly enough in my opinion. There Q is much better than discrete components and you can tune them with a diamond bit and a dremmel tool(or order them pre-made here).
     
  8. NickS

    NickS

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    Apr 6, 2010
    Ok so with a Fcutoff of 1500MHz and an Fstop of 1650 Fs/Fc = 1.1 is really tight stopband. I ran some preliminary synthesis and found that even a realistic 7th order elliptic filter cannot meet those requirements(and that was assuming pretty high Q for the inductors: 100). Take close notice of the group delay plot.

    The result was for 1dB passband ripple and it yielded an attenuation of -50dB at 1650MHz
     

    Attached Files:

  9. NickS

    NickS

    367
    0
    Apr 6, 2010
    I think this might be a good place to try the composite filter. The advantages are.
    1) the group delay spike is primarily in the transition band and flattens out quickly.
    2) The passband is very flat(unlike elliptical and chebychev)
    3) You can place pole emphasis in the stop band to target your maximum attenuation on a narrow band.

    Below are plots showing first the filter insertion loss/return loss, group delay and phase.
    The second and third plots are close-ups of the trasition band that show the ideal filter vs what you can actually build with real components. The component architecture can be seen below(with part values).
    Code:
    Results With combined Inductors 
      Bisected pi       Constant k        m derived        Bisected pi      
    o-----o----(Lbp+Lk)-----o-----(Lk+Lm)-----o-----(Lm+Lbp)-----o-----o    
          |                 |                 |                  |          
         Lbps               |                Lms                Lbps        
          |                Ck                 |                  |          
         Cbps               |                Cms                Cbps        
          |                 |                 |                  |          
         GND               GND               GND                GND         
      
    Lbps:   5.66e-009  Best Match: 5.60e-009 error 1.04% 
    Cbps:   1.27e-012  Best Match: 1.20e-012 error 5.75% 
    Lbp+Lk: 8.49e-009  Best Match: 8.20e-009 error 3.40% 
    Ck:     4.24e-012  Best Match: 3.90e-012 error 8.11% 
    Lk+Lm:  7.52e-009  Best Match: 8.20e-009 error 9.11% 
    Lms:    5.26e-009  Best Match: 5.60e-009 error 6.42% 
    Cms:    1.77e-012  Best Match: 1.80e-012 error 1.80% 
    Lm+Lbp: 5.39e-009  Best Match: 5.60e-009 error 3.83% 
    
     

    Attached Files:

  10. davenn

    davenn Moderator

    13,808
    1,943
    Sep 5, 2009
    the net is not the only place to find books :)

    there are these buildings people used to go to called libraries, I hear they are full of all sorts of books and usually have good technology sections ;)

    HINT ... try your local library, if its any good it will have a good selection of decent publications that will help you out. most also allow you to photocopy a few pages

    Dave
     
  11. san

    san

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    Jun 17, 2010
    hey thanks for the reply........

    @NickS....i need BPF....by mistake i wrote LPF...sorry for that.......i ll try with composite filter..but i doubt whether i ll be able to get -60 dB response in stopband.( i.e less than 60 dB..for wide band)..and less ripple in pass band (less than 1dB)

    @davenn ....we have library...but i doubt whether i can get those books there..
     
    Last edited: Jun 24, 2010
  12. NickS

    NickS

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    0
    Apr 6, 2010

    You are in a tough spot for making this filter yourself. The fact that you are going to BPF now further complicates the problem and will cost you in the passband. Regardless I have had far better results implementing BP filters as a cascade of LP and HP rather than setting up the combined BP resonant structures.

    You will be hard pressed to have the inductor Q you need for that much attenuation so close to your cutoff. I would recommend winding your own inductors if you have the ability to do so(it may buy you a little better Q and they are tunable by separating/compressing the windings). Ceramic resonators will not work well with that wide a bandwidth(~26%BW).

    You may be at a point of asking yourself how critical some of your specs are. If they are firm then you may need to resort to contracting the filter design to a filter design house. It is expensive but some filters can't be made with DIY techniques and COTS parts.

    Good luck.
     
  13. san

    san

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    Jun 17, 2010
    oooo...ll see what can be done..anyways thanks for the reply...

    can u just pass some good filter design papers...which have layouts....i am trying to search them but i am not able to find...
     
  14. NickS

    NickS

    367
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    Apr 6, 2010
    All my design notes have come from my textbooks and practice. I can send you Matlab code I have written that I use for most of my lumped element filter calcs. But if you don't have Matlab you may find this free calc tool helpful.

    As for layout are you familiar with good RF layout practices? I guess if you are going to be laying out a board you may even consider microstrip filters, the book I would recommend for that is "HF Filter Design" by RHEA

    Other books I use are "Microwave Engineering" by Pozar
    "RF Circuit design" by Bowick
    "RF Design Guide" by Vizmuller
    But they all reference the kings of filter design documentation which are

    "Microwave Filters, Impedance-Matching Networks, and Coupling Structures (Artech Microwave Library)" by Mattaei, Young, Jones

    "Handbook of Filter Synthesis" By Zverev
     
  15. san

    san

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    Jun 17, 2010
    thank you very very much dude....ll get back to you once i finalize the deign for this filter
     
  16. NickS

    NickS

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    Apr 6, 2010
    Found this diagram to help show the importance of Q when building filters.

    I do not know the publication it came from or I would add the ref info.
     

    Attached Files:

    • Q.JPG
      Q.JPG
      File size:
      70.9 KB
      Views:
      452
  17. san

    san

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    Jun 17, 2010
    does anyone has elliptical filter design table...for order till 15..
     
    Last edited: Jun 30, 2010
  18. NickS

    NickS

    367
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    Apr 6, 2010
    Good luck on that one. Elliptical filter synthesis does not involve a closed loop solution and tables for filters of that order are rare because of the problems involved with high order filters.

    I have never seen a 15th order elliptic filter table perhaps someone else has.
     
  19. san

    san

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    Jun 17, 2010
    hey how does the Q of capacitor affect the band pass filter
     
  20. NickS

    NickS

    367
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    Apr 6, 2010
    Absolutely, but the Q of ceramic capacitors are typically orders of magnitude higher than that of inductors.

    Some filter pros will actually add resistive loss to their capacitors to bring the Q down to a closer match of the inductor Q. But I wouldn't worry about that if I were you.
     
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