How do you tune a multi-section helical bandpass filter?

Discussion in 'Electronic Design' started by mike, Apr 13, 2013.

  1. mike

    mike Guest

    A friend has asked me for help re-tuning a 4-section helical bandpass
    filter for the 2-meter ham band. It has no adjustments, so there
    won't be too many chances to recover from botched cutting.

    As I recall, there's considerable interaction between the sections.
    And since he's been bending on the resonators, the bandpass is
    quite out of whack. He wants to move it up 2 MHz. and restore
    the shape.

    A couple of decades ago, I had a procedure that involved measuring
    return loss.
    You disable (short out) all but the first resonator and set the dip
    of the curve here.
    Then enable the second section and put that dip there.
    So on till you get to the end.

    AS I recall, it got you very close in one pass.
    The second dip moved the first one, but the end result
    bandpass came out close to what you want.

    Problem is that I've misplaced the details of where "here"
    and "there" are. I've also misplaced much of my memory.

    Anybody remember the details of that technique?
    Or any technique that might be practical on a filter
    with
    no tuning adjustments and little chance to recover from
    mistakes?

    Thanks, mike
    mike, Apr 13, 2013
    #1
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  2. mike

    Jamie Guest

    mike wrote:

    > A friend has asked me for help re-tuning a 4-section helical bandpass
    > filter for the 2-meter ham band. It has no adjustments, so there
    > won't be too many chances to recover from botched cutting.
    >
    > As I recall, there's considerable interaction between the sections.
    > And since he's been bending on the resonators, the bandpass is
    > quite out of whack. He wants to move it up 2 MHz. and restore
    > the shape.
    >
    > A couple of decades ago, I had a procedure that involved measuring
    > return loss.
    > You disable (short out) all but the first resonator and set the dip
    > of the curve here.
    > Then enable the second section and put that dip there.
    > So on till you get to the end.
    >
    > AS I recall, it got you very close in one pass.
    > The second dip moved the first one, but the end result
    > bandpass came out close to what you want.
    >
    > Problem is that I've misplaced the details of where "here"
    > and "there" are. I've also misplaced much of my memory.
    >
    > Anybody remember the details of that technique?
    > Or any technique that might be practical on a filter
    > with
    > no tuning adjustments and little chance to recover from
    > mistakes?
    >
    > Thanks, mike


    Service monitor? an all in one tool!

    Jamie
    Jamie, Apr 13, 2013
    #2
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  3. mike

    Joerg Guest

    mike wrote:
    > A friend has asked me for help re-tuning a 4-section helical bandpass
    > filter for the 2-meter ham band. It has no adjustments, so there
    > won't be too many chances to recover from botched cutting.
    >
    > As I recall, there's considerable interaction between the sections.
    > And since he's been bending on the resonators, the bandpass is
    > quite out of whack. He wants to move it up 2 MHz. and restore
    > the shape.
    >
    > A couple of decades ago, I had a procedure that involved measuring
    > return loss.
    > You disable (short out) all but the first resonator and set the dip
    > of the curve here.
    > Then enable the second section and put that dip there.
    > So on till you get to the end.
    >
    > AS I recall, it got you very close in one pass.
    > The second dip moved the first one, but the end result
    > bandpass came out close to what you want.
    >
    > Problem is that I've misplaced the details of where "here"
    > and "there" are. I've also misplaced much of my memory.
    >
    > Anybody remember the details of that technique?
    > Or any technique that might be practical on a filter
    > with
    > no tuning adjustments and little chance to recover from
    > mistakes?
    >


    http://www.macomtech.com/static/PDFs/TechnicaArticles/Bandpass_Filter_tutorial2.pdf

    Design instructions but no tuning info per se. You could contact them
    and see if they have more. Then there is a book by Zverev. The best
    "McGyver style" alignment instruction I ever saw was in an ARRL handbook
    but I can't recall what year. You might want to ask in a ham radio group
    if someone still has it.

    http://www.rfcafe.com/references/electrical/helical-resonator.htm

    Not sure how far the Zverev book goes into tuning, I don't have it here.

    --
    Regards, Joerg

    http://www.analogconsultants.com/
    Joerg, Apr 13, 2013
    #3
  4. mike

    Joerg Guest

    Jeff Liebermann wrote:
    > On Sat, 13 Apr 2013 08:08:48 -0700, Joerg <>
    > wrote:
    >
    >> Not sure how far the Zverev book goes into tuning, I don't have it here.

    >
    > It does and thanks for jogging my failing memory. I it here.
    > "Handbook of Filter Synthesis" by Anatol I. Zverev. (1967)
    > 9.4 (P513 to 521) is on "Alignment of Helical Resonators" with
    > references to Dishal's method. It's too much to type here, but I can
    > scan the pages and post them if needed.
    >
    > Google finds quite a few hits for Dishal's Method.
    >


    In case Mike has IEEE library access:

    http://www.rfcurrent.com/wp-content/uploads/2010/12/Milton-Dishal-Publications-on-Filters.pdf

    Unfortunately I only have access to ultrasound and to aerospace
    publications.

    --
    Regards, Joerg

    http://www.analogconsultants.com/
    Joerg, Apr 13, 2013
    #4
  5. mike

    Joerg Guest

    Phil Hobbs wrote:
    > On 4/13/2013 11:08 AM, Joerg wrote:
    >> mike wrote:
    >>> A friend has asked me for help re-tuning a 4-section helical bandpass
    >>> filter for the 2-meter ham band. It has no adjustments, so there
    >>> won't be too many chances to recover from botched cutting.
    >>>
    >>> As I recall, there's considerable interaction between the sections.
    >>> And since he's been bending on the resonators, the bandpass is
    >>> quite out of whack. He wants to move it up 2 MHz. and restore
    >>> the shape.
    >>>
    >>> A couple of decades ago, I had a procedure that involved measuring
    >>> return loss.
    >>> You disable (short out) all but the first resonator and set the dip
    >>> of the curve here.
    >>> Then enable the second section and put that dip there.
    >>> So on till you get to the end.
    >>>
    >>> AS I recall, it got you very close in one pass.
    >>> The second dip moved the first one, but the end result
    >>> bandpass came out close to what you want.
    >>>
    >>> Problem is that I've misplaced the details of where "here"
    >>> and "there" are. I've also misplaced much of my memory.
    >>>
    >>> Anybody remember the details of that technique?
    >>> Or any technique that might be practical on a filter
    >>> with
    >>> no tuning adjustments and little chance to recover from
    >>> mistakes?
    >>>

    >>
    >> http://www.macomtech.com/static/PDFs/TechnicaArticles/Bandpass_Filter_tutorial2.pdf
    >>
    >>
    >> Design instructions but no tuning info per se. You could contact them
    >> and see if they have more. Then there is a book by Zverev. The best
    >> "McGyver style" alignment instruction I ever saw was in an ARRL handbook
    >> but I can't recall what year. You might want to ask in a ham radio group
    >> if someone still has it.
    >>
    >> http://www.rfcafe.com/references/electrical/helical-resonator.htm
    >>
    >> Not sure how far the Zverev book goes into tuning, I don't have it here.
    >>

    >
    > Dishal's method. For high-Q filters, mistuned series sections look like
    > opens, and mistuned parallel sections look like shorts.
    >
    > If you know what the resonant frequencies of the sections should be, you
    > can do that with a return loss bridge from each end.
    >


    I tuned my last one using the "wet finger and a beer" method. Worked :)

    --
    Regards, Joerg

    http://www.analogconsultants.com/
    Joerg, Apr 13, 2013
    #5
  6. mike

    Guest

    On Apr 13, 10:07 am, Jeff Liebermann <> wrote:
    > On Sat, 13 Apr 2013 03:14:10 -0700, mike <> wrote:
    > >A friend has asked me for help re-tuning a 4-section helical bandpass
    > >filter for the 2-meter ham band.  It has no adjustments, so there
    > >won't be too many chances to recover from botched cutting.

    >
    > Is this out of a commercial radio?  If so, the maker and model please.
    > Perhaps a photo.  I can't imagine anyone shipping something like that
    > without tuning adjustments.
    >
    > >As I recall, there's considerable interaction between the sections.

    >
    > That's an understatement.  It's nothing but interactions.  Change one
    > thing and all the adjacent resonator tuning changes.
    >
    > >And since he's been bending on the resonators, the bandpass is
    > >quite out of whack.  He wants to move it up 2 MHz. and restore
    > >the shape.

    >
    > 2MHz / 146MHz = 1.4%
    > That should be easy without unwinding turns.
    > Any clue on the expected bandwidth?
    >
    > >A couple of decades ago, I had a procedure that involved measuring
    > >return loss.
    > >You disable (short out) all but the first resonator and set the dip
    > >of the curve here.
    > >Then enable the second section and put that dip there.
    > >So on till you get to the end.

    >
    > Well, I'm not sure this is the correct procedure, but when I was
    > making 902-928Mhz solid cavity duplexers, I put a reflection
    > coefficient bridge on the input (no network analyzer available) and
    > swept for lowest return loss and best looking sweep.  As I vaguely
    > recall, it was 5 sections, with a total of 9 adjustments for tuning
    > and coupling:
    > <http://www.qsl.net/n9zia/wireless/pics/rtrn_loss_bridge.png>
    > <http://www.qsl.net/n9zia/wireless/rtrn_loss-pics.html>
    > <http://www.eagle-1st.com/eagle1st.nsf/17100001!OpenFrameSet>
    > <http://www.eagle-1st.com/eagle1st.nsf/10201000!OpenFrameSet>
    >
    > I might have shorted out downstream sections to reduce the display
    > clutter, but there were no provisions for that inside the cavity.  So,
    > you start at the input, tune until the curve looks best.  Move on to
    > the next stage, and tune again.  Because the insertion loss of the
    > first stage is presumably very low when it's properly tuned, the 2nd
    > stage will have a big effect on the pattern.  However, if the
    > insertion loss through the first stage is high for some reason, tuning
    > the 2nd stage will have minimal effect on the displayed pattern. After
    > you're done making the return loss look pretty, check the overall
    > response.  If the return loss looks good, the bandpass response will
    > also look good.  However, don't bother trying to tune it for best
    > bandpass response.  It just doesn't work, which is probably what your
    > friend is struggling with.
    >
    > If you have access to the helical resonators, you can see which stage
    > is responsible for which dip by waving a ferrite or brass rod near the
    > coil.
    >
    > >AS I recall, it got you very close in one pass.
    > >The second dip moved the first one, but the end result
    > >bandpass came out close to what you want.

    >
    > Yep.  That's it.  Everything affects everything else, but when you're
    > done tuning for best looking return loss, the bandpass response will
    > magically be perfect.
    >
    > >Problem is that I've misplaced the details of where "here"
    > >and "there" are.  I've also misplaced much of my memory.

    >
    > Try googling for repeater cavity duplexer tuning procedures.  It's
    > very similar, except that you're tuning a notch instead of a peak,
    > with the added bonus of overly critical, white knuckle, adjustments.
    > <http://www.eagle-1st.com/eagle1st.nsf/17100001!OpenFrameSet>
    > <http://www.amtronix.com/duplexertuning.htm>
    >
    > This Agilent app note looks interesting, but I don't have time to read
    > the whole thing:
    > <http://literature.agilent.com/litweb/pdf/5968-5328E.pdf>
    >
    > >Anybody remember the details of that technique?
    > >Or any technique that might be practical on a filter
    > >with
    > >no tuning adjustments and little chance to recover from
    > >mistakes?

    >
    > A digital camera is your friend.  Take lots of photos so you at least
    > have a chance to put Humpty Dumpty back together again.


    I dunno if it counts as an official method, but I've used a spectrum
    analyzer+tracking generator. Each resonator makes a "bump" in the
    passband; you can locate any given bump by grossly mistuning it (with
    a hand-held cap, for example), and watching the bump jump back and
    forth. Tune, repeat.

    --
    Cheers,
    James Arthur
    , Apr 14, 2013
    #6
  7. mike

    mike Guest

    On 4/13/2013 9:21 AM, Phil Hobbs wrote:
    > On 4/13/2013 11:08 AM, Joerg wrote:
    >> mike wrote:
    >>> A friend has asked me for help re-tuning a 4-section helical bandpass
    >>> filter for the 2-meter ham band. It has no adjustments, so there
    >>> won't be too many chances to recover from botched cutting.
    >>>
    >>> As I recall, there's considerable interaction between the sections.
    >>> And since he's been bending on the resonators, the bandpass is
    >>> quite out of whack. He wants to move it up 2 MHz. and restore
    >>> the shape.
    >>>
    >>> A couple of decades ago, I had a procedure that involved measuring
    >>> return loss.
    >>> You disable (short out) all but the first resonator and set the dip
    >>> of the curve here.
    >>> Then enable the second section and put that dip there.
    >>> So on till you get to the end.
    >>>
    >>> AS I recall, it got you very close in one pass.
    >>> The second dip moved the first one, but the end result
    >>> bandpass came out close to what you want.
    >>>
    >>> Problem is that I've misplaced the details of where "here"
    >>> and "there" are. I've also misplaced much of my memory.
    >>>
    >>> Anybody remember the details of that technique?
    >>> Or any technique that might be practical on a filter
    >>> with
    >>> no tuning adjustments and little chance to recover from
    >>> mistakes?
    >>>

    >>
    >> http://www.macomtech.com/static/PDFs/TechnicaArticles/Bandpass_Filter_tutorial2.pdf
    >>
    >>
    >> Design instructions but no tuning info per se. You could contact them
    >> and see if they have more. Then there is a book by Zverev. The best
    >> "McGyver style" alignment instruction I ever saw was in an ARRL handbook
    >> but I can't recall what year. You might want to ask in a ham radio group
    >> if someone still has it.
    >>
    >> http://www.rfcafe.com/references/electrical/helical-resonator.htm
    >>
    >> Not sure how far the Zverev book goes into tuning, I don't have it here.
    >>

    >
    > Dishal's method. For high-Q filters, mistuned series sections look like
    > opens, and mistuned parallel sections look like shorts.
    >
    > If you know what the resonant frequencies of the sections should be, you
    > can do that with a return loss bridge from each end.
    >
    > Cheers
    >
    > Phil Hobbs
    >

    Thanks, guys for the inputs.
    Dishal's method was what I was looking for.

    I'd found the HP article very interesting. I'll try the time-domain
    stuff next time I get an opportunity.

    I'm trying to do this remotely with someone with limited equipment.
    We'll see how it goes... ;-)


    The filter is
    http://www.dci.ca/pdf/DCI-145-2H.pdf

    Thanks, again, mike
    mike, Apr 14, 2013
    #7
  8. On Apr 13, 6:14 pm, mike <> wrote:
    > On 4/13/2013 9:21 AM, Phil Hobbs wrote:
    >
    >
    >
    >
    >
    >
    >
    > > On 4/13/2013 11:08 AM, Joerg wrote:
    > >> mike wrote:
    > >>> A friend has asked me for help re-tuning a 4-section helical bandpass
    > >>> filter for the 2-meter ham band. It has no adjustments, so there
    > >>> won't be too many chances to recover from botched cutting.

    >
    > >>> As I recall, there's considerable interaction between the sections.
    > >>> And since he's been bending on the resonators, the bandpass is
    > >>> quite out of whack. He wants to move it up 2 MHz. and restore
    > >>> the shape.

    >
    > >>> A couple of decades ago, I had a procedure that involved measuring
    > >>> return loss.
    > >>> You disable (short out) all but the first resonator and set the dip
    > >>> of the curve here.
    > >>> Then enable the second section and put that dip there.
    > >>> So on till you get to the end.

    >
    > >>> AS I recall, it got you very close in one pass.
    > >>> The second dip moved the first one, but the end result
    > >>> bandpass came out close to what you want.

    >
    > >>> Problem is that I've misplaced the details of where "here"
    > >>> and "there" are. I've also misplaced much of my memory.

    >
    > >>> Anybody remember the details of that technique?
    > >>> Or any technique that might be practical on a filter
    > >>> with
    > >>> no tuning adjustments and little chance to recover from
    > >>> mistakes?

    >
    > >>http://www.macomtech.com/static/PDFs/TechnicaArticles/Bandpass_Filter....

    >
    > >> Design instructions but no tuning info per se. You could contact them
    > >> and see if they have more. Then there is a book by Zverev. The best
    > >> "McGyver style" alignment instruction I ever saw was in an ARRL handbook
    > >> but I can't recall what year. You might want to ask in a ham radio group
    > >> if someone still has it.

    >
    > >>http://www.rfcafe.com/references/electrical/helical-resonator.htm

    >
    > >> Not sure how far the Zverev book goes into tuning, I don't have it here.

    >
    > > Dishal's method. For high-Q filters, mistuned series sections look like
    > > opens, and mistuned parallel sections look like shorts.

    >
    > > If you know what the resonant frequencies of the sections should be, you
    > > can do that with a return loss bridge from each end.

    >
    > > Cheers

    >
    > > Phil Hobbs

    >
    > Thanks, guys for the inputs.
    > Dishal's method was what I was looking for.
    >
    > I'd found the HP article very interesting.  I'll try the time-domain
    > stuff next time I get an opportunity.
    >
    > I'm trying to do this remotely with someone with limited equipment.
    > We'll see how it goes... ;-)
    >
    > The filter ishttp://www.dci.ca/pdf/DCI-145-2H.pdf
    >
    > Thanks, again, mike


    Yeah, the (Dishal) method is the classic one for coupled resonator
    filters.

    Vizmuller has a book on helical filters, if you want something in
    addition to the Zverev material.
    Simon S Aysdie, May 7, 2013
    #8
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