HOW TO DESIGN A HI-PASS FILTER (RF)

Discussion in 'Wireless & RF Design' started by DanW8KMX, Oct 1, 2015.

  1. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
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    I wish to design a HI-PASS filter so as to feed a 6 meter as well as a 70 CM antenna from a single coax.
    The 7 meter antenna and the 70 CM antenna works wonderful when fed with separate coax but I want to connect these antennas together and feed from a single coax.
    I have downloaded AADE Filter Design software but I don't fully understand the terminology so I am not sure what to insert (values) so the software can do it's thing.
    So, hopefully what I wish to end up with is a filter with a couple capacitors and a couple inductors so that I can construct in my shop etc.
    Any suggestions?
    Dan
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Hi Dan, I have the AADE software installed on the computer in my ham shack (200' from here, and I am still in my PJs). I will help you after I go over there after breakfast.

    73,

    wa7ark
     
  3. MikeML

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    Oct 2, 2009
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    I used AADE to create a 3-pole low-pass Chebyshev 2db-ripple filter with a 60Mhz cutoff to feed the 52MHz antenna. I also used AADE to create a 3-pole high-pass Chebyshev 2db-ripple filter with a 400Mhz cutoff to feed the 440 MHz antenna. I combined the two and simulated using LTSpice.

    I saw some interaction, so I tweaked the corner frequencies of the two respective filters to improve the behavior near the two bands of interest. Here is the result.

    121.gif
     
    Shaikh Wasim likes this.
  4. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
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    Thanks for the offer to help wa7ark. First I am not an engineer and I don't fully understand the terminology used in the program. Simply put, I don't know what to "input" on the first window. I am not sure of the definition of "cutoff" frequency, "Astop" etc. I am not even sure if putting the "M" after the entry means Mhz. ????? I have gone thru the tutorial and that was not much help.
    So, I just want it to attenuate everything below 400 Mhz or 300 or 200 (whatever is best to construct) and then pass everything higher then 400 Mhz, or 300 Mhz or ????? again, whatever can be constructed. Once that is done, is it possible to fix the capacitors at say 5pf or 10pf (because I have them) and optimize the filter? As you can see, I am really starting from scratch here.
    Dan w8kmx
     
  5. MikeML

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    Oct 2, 2009
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    You are trying to build a duplexer. It actually has to have two filters in it, one to pass the UHF signal, while isolating the 6m port; and a different filter to pass the 6m signal while isolating the UHF port.
     
  6. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
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    MikeML, I see you "results." I see that Mouser has the 2.7pf caps. I like the circuit. Tell me how you made the inductors and then tested.
    Dan w8kmx
     
  7. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
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    Mike tell me if I am reading the graph correct. Looks like each filter has a -7db insertion loss.
    Dan
     
  8. MikeML

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    There is an intrinsic loss of -6db where a 50Ω source drives a 50Ω load, so there is about an additional -0.5db attributable to the filters...
     
  9. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
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    Mike, can you tell me exactly what you entered in the AADE window? I want to duplicate your results. I was unaware of LTSPICE. That looks like something I can use.
    Dan, W8KMX
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    I'm not sure that the components were actually built and tested. In a simulator it is easy to get whatever components you need without worrying about physical constraints. In my experience a 2.7 pF capacitor may or may not be a 2.7 pF capacitor especially as a function of frequency. Making an inductor for HF is normally a process of finding a core material and winding some turns, then trimming to suit. At VHF and UHF the air coil inductor is more common but also much more difficult to fabricate and work with. Actual duplexers are often made with tuned cavities and tuned stubs. It is a fascinating branch of amateur radio. Having the test equipment to measure your attempts goes a long way to achieving good results. Good luck BTW.

    As for references on the terminology and theory, I recommend
    http://www.amazon.com/Analog-Filter-Design-M-E-Valkenburg/dp/0030592461/ref=sr_1_fkmr0_2?ie=UTF8&qid=1443829554&sr=8-2-fkmr0&keywords=Van+Valkenberg,+M.E.,+Analog+Filter+Design
    http://www.amazon.com/ARRL-2015-Han...29597&sr=8-2&keywords=Radio+amateurs+Handbook
     
  11. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
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    Can anyone give a defination of terms used in AADE? I.E. what is F1, F2, ASTOP etc.
    Dan
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    Generally, in the context of filter design, F1 and F2 would be corner frequencies at the lower and upper edge of the passband. The passband is the range of frequencies for which the attenuation of the filter is less than or equal to 3 dB. ASTOP would be the attenuation of the filter in the stopband. Attenuation in the stopband is usually expressed as being greater than or equal to some value expressed in dB (decibels). In between the passband and the stop band are transition bands and what we care about there is the "slope" or "rolloff" of the response. This is usually expressed in dB/decade or dB/octave. A decade is a range of frequencies where the upper end is ten times the lower end. An octave is a range of frequencies where the upper end is two times the lower end.

    Examples:
    1. 256 Hz. to 512 Hz. is an octave
    2. 3 MHz. to 30 MHz. is a decade
     
  13. MikeML

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    Oct 2, 2009
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    Start AADE
    Click DESIGN
    Select and Click on Chebyshev (works better than Butterworth, try it)
    Select Low Pass, click OK
    Click on picture to change it to input-inductor T-configuration, click OK (n.b.)
    Fill in the boxes:
    F1 =62M (cutoff above 6m band, by trial and error)
    F2= 420M (stopband starts below 70cm band)
    Apass=1. (default pass band ripple in db)
    Order=3 (I tried 2; not enough isolation), click ENTER
    Click OK to accept default title for the design.
    Look at the computed values, you will need these later.

    Check the response: Click on ANALYZE
    Fill in the boxes:
    Freq 1= 50M (bottom of 6m band)
    Freq 2=500M (above 70cm band)
    db2=-60 (you should be so lucky)
    Click Set as Defaults, then Click ENTER
    Click on plot at various frequencies to check insertion loss and ultimate rejection at ~440Mhz

    You can also plot input impedance, etc.


    n.b. You want the low-pass filter to be T-configuration so that it has a high-input impedance at 420MHz (when you combine it with a high-pass to create the duplexer.
     
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  14. DanW8KMX

    Thread Starter Member

    Oct 21, 2009
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    Hello MikeML:
    Sorry it has taken me so long to respond but I have been "playing."
    I used your data and have now used AADE to design several filters. These experiments were a great learning experience. Thanks for that last post as it "set me free." Not all bad for a guy with a Business degree with a minor in Finance (BTW which I don't remember hardly anything about the Finance because I never used it).
    My calculations came out very close to the ones you designed etc. The trick is now to build one and see how the RF responds. Hah! There are some areas in AADE that I need to explore further and learn how to do them; example: after the filter is designed, I want to plug in capacitor values (what I have or what I can get) and let the program calculate the inductor values.
    Again thanks for the "perfect" info. BTW, I am a private pilot SEG but don't fly anymore (within last 10 years) as it is just too expensive for a retired guy.
    I consider this problem solved and Thread closed.
    73, Dan, W8KMX
     
  15. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Still active as a pilot and airplane owner. Both wife and I are pilots and live an fly out of a residential airpark.
     
  16. Shaikh Wasim

    New Member

    Jan 6, 2016
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    Will U give me the idea to make bandpass filter 300Mhz to 600Mhz, I am working with 48 Mhz to 860 Mhz frequency in coaxial wire,actually it is TV Signals
     
  17. SLK001

    Well-Known Member

    Nov 29, 2011
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    Why do you need a bandpass filter for receiving TV signals? Your TV has enough selectivitiy to deal with most interferers. (Also, why did you hijack an old thread?)
     
  18. MikeML

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    The answer is the same. Obtain the AADE program, and go from there.
     
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