Dishal tuning method question

Discussion in 'Electronic Design' started by Joel Kolstad, Aug 23, 2006.

  1. Joel Kolstad

    Joel Kolstad Guest

    After the not-so-great bandpass filter I built last week, I built another one
    this week using a capacitor-coupled resonator topology. Happily, it works
    pretty much as designed! However, I did discover that the "real world" method
    of Dishal tuning isn't quite so clean as the literature would have you
    believe.

    Articles such as this one -->
    http://www.johansontechnology.com/technicalnotes/dbf/ (which, ironically
    enough, is trying to sell you on laser timming rather than trimmer caps, which
    is what I used :) ) have, e.g., figures 3 and 4 where the first and second
    resonator, respectively, are tuned. Well, I can get figure 3 just fine, but
    when I go to the second resonator, the two peaks in figure 4 are asymmetrical
    in amplitude, and I have to re-adjust the first resonator to make them
    symmetrical again, kind of iterating between centering the second resonator's
    dip and making the first resonators response symmetric.

    As you could imagine, this problem tends to compound itself as you move from
    resonator to resonator. So, I'm curious:

    1) Presumably the assymetries in one section arise due to the additional
    parasitics present when you un-short the next section? Or perhaps they're due
    to a little through-the-air coupling between sections? I'm making another
    board that shields each resonator section to find out...
    2) I eventually was able to tune up the entire filter reasonably well by
    deciding that I was only going to tweak the "current" section and the
    immediately prior sections, getting things as centered and symmetrical as
    possible (a somewhat iterative process). Trying to go through multiple prior
    sections became as exercise in futility, and I rationalized that the further
    away a section was, the less impacts the parasitics of the current section had
    on it -- hence the idea of only considering the current section and the one
    immediately prior. How's that sound for a tuning procedure?

    I believe that all the plots in the Johanson paper were computer-generated.
    Anyone have some experience with tuning real LC filters?

    Thanks,
    ---Joel
     
    Joel Kolstad, Aug 23, 2006
    #1
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  2. In message <>, dated Tue, 22 Aug 2006,
    Joel Kolstad <> writes
    >After the not-so-great bandpass filter I built last week, I built
    >another one this week using a capacitor-coupled resonator topology.
    >Happily, it works pretty much as designed! However, I did discover
    >that the "real world" method of Dishal tuning isn't quite so clean as
    >the literature would have you believe.


    Is this filter on just an open board? It's good to build a filter in a
    U-shaped metal channel that is a waveguide below cut-off at the highest
    frequency of interest. This attenuates radiated coupling, of course, but
    you may need to put metal partitions between the sections as well.
    --
    OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
    2006 is YMMVI- Your mileage may vary immensely.

    John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
     
    John Woodgate, Aug 23, 2006
    #2
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  3. Joel Kolstad

    Ian Guest

    "Joel Kolstad" <> wrote in message
    news:...
    > After the not-so-great bandpass filter I built last week, I built another
    > one this week using a capacitor-coupled resonator topology. Happily, it
    > works pretty much as designed! However, I did discover that the "real
    > world" method of Dishal tuning isn't quite so clean as the literature
    > would have you believe.
    >
    > Articles such as this one -->
    > http://www.johansontechnology.com/technicalnotes/dbf/ (which, ironically
    > enough, is trying to sell you on laser timming rather than trimmer caps,
    > which is what I used :) ) have, e.g., figures 3 and 4 where the first and
    > second resonator, respectively, are tuned. Well, I can get figure 3 just
    > fine, but when I go to the second resonator, the two peaks in figure 4 are
    > asymmetrical in amplitude, and I have to re-adjust the first resonator to
    > make them symmetrical again, kind of iterating between centering the
    > second resonator's dip and making the first resonators response symmetric.
    >
    > As you could imagine, this problem tends to compound itself as you move
    > from resonator to resonator. So, I'm curious:
    >
    > 1) Presumably the assymetries in one section arise due to the additional
    > parasitics present when you un-short the next section? Or perhaps they're
    > due to a little through-the-air coupling between sections? I'm making
    > another board that shields each resonator section to find out...
    > 2) I eventually was able to tune up the entire filter reasonably well by
    > deciding that I was only going to tweak the "current" section and the
    > immediately prior sections, getting things as centered and symmetrical as
    > possible (a somewhat iterative process). Trying to go through multiple
    > prior sections became as exercise in futility, and I rationalized that the
    > further away a section was, the less impacts the parasitics of the current
    > section had on it -- hence the idea of only considering the current
    > section and the one immediately prior. How's that sound for a tuning
    > procedure?
    >
    > I believe that all the plots in the Johanson paper were
    > computer-generated. Anyone have some experience with tuning real LC
    > filters?
    >
    > Thanks,
    > ---Joel
    >
    >

    I suspect you may be right about this being a simulation - I ran the circuit
    in LTSpice and got very close indeed to the Johanson plots.

    However, if your results were markedly different then I would suspect
    undesired coupling. John's suggestion sounds good, also if you are using
    chip inductors you could try orienting them so they are at 90 deg to their
    neighbours.

    I did find that the interconnecting track between sections can have an
    effect.

    Regards
    Ian
     
    Ian, Aug 23, 2006
    #3
  4. Joel Kolstad

    Joel Kolstad Guest

    Hi John,

    "John Woodgate" <> wrote in message
    news:...
    > Is this filter on just an open board?


    Yes, with a ground plane on the backside of the board. It's about 3" long by
    1/2" wide, with the signal flowing from one end to the other.

    > It's good to build a filter in a U-shaped metal channel that is a waveguide
    > below cut-off at the highest frequency of interest. This attenuates radiated
    > coupling, of course, but you may need to put metal partitions between the
    > sections as well.


    That's pretty much one of the things I'll be trying in the next few days
    (although I was still planning on a "straight" layout).

    ---Joel
     
    Joel Kolstad, Aug 23, 2006
    #4
  5. Joel Kolstad

    Joel Kolstad Guest

    Hi Ian,

    "Ian" <> wrote in message
    news:...
    > However, if your results were markedly different then I would suspect
    > undesired coupling.


    The amplitude imbalance was probably some 5dB or more, but "leveling" things
    requires all of about 5-15 degrees of tweaking on the trim caps.

    > John's suggestion sounds good, also if you are using
    > chip inductors you could try orienting them so they are at 90 deg to their
    > neighbours.


    They're air core ("spring") inductors. The board is ~3"x0.5", and the axis of
    the inductors is perpendicular to the long axis of the board; after shielding
    each section, I'll try rotating 90 degrees per section (I suppose I could also
    alternate board sides from section to section). Too bad no one (that I'm
    aware of) makes torodial air cores (or any core such that the unloaded Q is
    still, say, >100 at VHF/UHF)...

    > I did find that the interconnecting track between sections can have an
    > effect.


    Good to know I'm not alone. :)

    ---Joel
     
    Joel Kolstad, Aug 23, 2006
    #5
  6. In message <>, dated Wed, 23 Aug 2006,
    Joel Kolstad <> writes

    >Too bad no one (that I'm aware of) makes torodial air cores


    Plastic rings. Try your local low-cost 'everything' store.
    --
    OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
    2006 is YMMVI- Your mileage may vary immensely.

    John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
     
    John Woodgate, Aug 23, 2006
    #6
  7. Joel Kolstad

    Terry Given Guest

    John Woodgate wrote:
    > In message <>, dated Wed, 23 Aug 2006,
    > Joel Kolstad <> writes
    >
    >> Too bad no one (that I'm aware of) makes torodial air cores

    >
    >
    > Plastic rings. Try your local low-cost 'everything' store.


    micrometals also have a ur = 1 series of toroidal cores (aka formers)

    Cheers
    Terry
     
    Terry Given, Aug 24, 2006
    #7
  8. Joel Kolstad

    Phil Hobbs Guest

    Terry Given wrote:

    > John Woodgate wrote:
    >
    >> In message <>, dated Wed, 23 Aug
    >> 2006, Joel Kolstad <> writes
    >>
    >>> Too bad no one (that I'm aware of) makes torodial air cores

    >>
    >>
    >>
    >> Plastic rings. Try your local low-cost 'everything' store.

    >
    >
    > micrometals also have a ur = 1 series of toroidal cores (aka formers)
    >


    Note that using solid plastic cores will seriously degrade the tempco
    and distributed capacitance. The solid plastic will be strong enough to
    stretch the wire, so that the tempco will go up to twice the CTE of the
    plastic.

    Cheers,

    Phil Hobbs
     
    Phil Hobbs, Aug 24, 2006
    #8
  9. Joel Kolstad

    Joel Kolstad Guest

    Thanks for the suggestions on toroidal air (-like) cores. In my case I'm
    using coils that are about 1/4" long and 1/8" in diameter, so I don't think
    I'll be pursuing the dollar store ring option. :) Still, in the future for
    HF stuff, I like the idea!
     
    Joel Kolstad, Aug 24, 2006
    #9
  10. In message <>, dated Thu,
    24 Aug 2006, Phil Hobbs <> writes

    >Note that using solid plastic cores will seriously degrade the tempco
    >and distributed capacitance. The solid plastic will be strong enough
    >to stretch the wire, so that the tempco will go up to twice the CTE of
    >the plastic.


    I think that's a bit alarmist. I don't suppose the OP needs 0.1%
    resistance stability and military temperature range.
    --
    OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
    2006 is YMMVI- Your mileage may vary immensely.

    John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
     
    John Woodgate, Aug 24, 2006
    #10
  11. In message <>, dated Thu, 24 Aug 2006,
    Joel Kolstad <> writes
    >In my case I'm using coils that are about 1/4" long and 1/8" in
    >diameter, so I don't think I'll be pursuing the dollar store ring
    >option. :)


    No, use beads from your small daughter's toy necklace. Replace them with
    real pearls. (;-)
    --
    OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
    2006 is YMMVI- Your mileage may vary immensely.

    John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
     
    John Woodgate, Aug 24, 2006
    #11
  12. Joel Kolstad

    Joerg Guest

    Hello John,

    >
    >> Note that using solid plastic cores will seriously degrade the tempco
    >> and distributed capacitance. The solid plastic will be strong enough
    >> to stretch the wire, so that the tempco will go up to twice the CTE of
    >> the plastic.

    >
    > I think that's a bit alarmist. I don't suppose the OP needs 0.1%
    > resistance stability and military temperature range.



    I think Phil was looking more at the change in inductance. But it won't
    be a lot either since wire tends to spring back a little after winding,
    leaving some slack towards the PVC "core".

    Now if you ran this setup in L.A. and the smog gums up the PVC surface
    that's a whole 'nother matter.

    --
    Regards, Joerg

    http://www.analogconsultants.com
     
    Joerg, Aug 24, 2006
    #12
  13. Joel Kolstad

    Phil Hobbs Guest

    John Woodgate wrote:

    > In message <>, dated Thu,
    > 24 Aug 2006, Phil Hobbs <> writes
    >
    >> Note that using solid plastic cores will seriously degrade the tempco
    >> and distributed capacitance. The solid plastic will be strong enough
    >> to stretch the wire, so that the tempco will go up to twice the CTE of
    >> the plastic.

    >
    >
    > I think that's a bit alarmist. I don't suppose the OP needs 0.1%
    > resistance stability and military temperature range.


    The tempco of an air-core copper coil is approximately twice the CTE of
    the copper, because the inductance goes as r**2. If you wind the wire
    tightly on the plastic, the plastic now controls the thermal expansion
    because it stretches the wire as it heats up, and the tempco is then
    about twice the CTE of the plastic. The CTEs are:


    Copper: 16 ppm/K

    Polystyrene: 50-80 ppm/k

    Polypropylene: ~200 ppm/K

    Teflon: nonlinear, 125 to 1000 ppm/K

    So the typical tempco of an air-core coil is about 32 ppm/K, vs 400
    ppm/K if you wind it on polypropylene.

    Length effects may change the numbers somewhat, but the ratio will still
    be on the order of 10x.

    Cheers,

    Phil Hobbs
     
    Phil Hobbs, Aug 24, 2006
    #13
  14. In message <>, dated Thu,
    24 Aug 2006, Phil Hobbs <> writes

    >So the typical tempco of an air-core coil is about 32 ppm/K, vs 400
    >ppm/K if you wind it on polypropylene.


    OK, but you didn't say tempco of inductance and you still haven't
    explicitly. 400 ppm/K is still pretty low, even though it's 10 times
    what you get with an air core. And you only get 400 ppm/K if you wind it
    tightly.
    --
    OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
    2006 is YMMVI- Your mileage may vary immensely.

    John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
     
    John Woodgate, Aug 24, 2006
    #14
  15. In message <>, dated Thu, 24 Aug 2006,
    John Woodgate <> writes
    >In message <>, dated Thu,
    >24 Aug 2006, Phil Hobbs <> writes
    >
    >>So the typical tempco of an air-core coil is about 32 ppm/K, vs 400
    >>ppm/K if you wind it on polypropylene.

    >
    >OK, but you didn't say tempco of inductance and you still haven't
    >explicitly. 400 ppm/K is still pretty low, even though it's 10 times
    >what you get with an air core. And you only get 400 ppm/K if you wind
    >it tightly.


    I should have added, 'Bring back N750 capacitors!' (Ceramic capacitors
    from WWII to around 1970 had a tempco of capacitance of -750 ppm/K
    unless you paid more for something else). These COK things don't
    compensate.
    --
    OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
    2006 is YMMVI- Your mileage may vary immensely.

    John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
     
    John Woodgate, Aug 24, 2006
    #15
  16. Joel Kolstad

    Phil Hobbs Guest

    John Woodgate wrote:
    > In message <>, dated Thu,
    > 24 Aug 2006, Phil Hobbs <> writes
    >
    >> So the typical tempco of an air-core coil is about 32 ppm/K, vs 400
    >> ppm/K if you wind it on polypropylene.

    >
    >
    > OK, but you didn't say tempco of inductance and you still haven't
    > explicitly. 400 ppm/K is still pretty low, even though it's 10 times
    > what you get with an air core. And you only get 400 ppm/K if you wind it
    > tightly.


    Well, I think 400 ppm/K is pretty high if you're building filters or
    oscillators, don't you? At 30 MHz, that's 6 kHz per degree!

    And if you don't wind it pretty tightly, it'll squirm around unless
    you're using really heavy wire. If the wire is heavy enough, heating
    after winding will probably cause enough slump and shrinkage to relax
    the fit, which could be a reasonable technique for hand-winding. (Of
    course one could also melt the plastic core completely, but styrene
    monomer is not my favourite thing to breathe.)

    Anyway, this is all getting too ARRL Handbook-ish. Winding toroids on
    shower rings might be sort of fun in a weird way.

    Cheers,

    Phil Hobbs
     
    Phil Hobbs, Aug 24, 2006
    #16
  17. In message <>, dated Thu,
    24 Aug 2006, Phil Hobbs <> writes

    >Well, I think 400 ppm/K is pretty high if you're building filters or
    >oscillators, don't you? At 30 MHz, that's 6 kHz per degree!


    I meant to add the bit about N750 caps. And it depends entirely on what
    performance is wanted, whether the TC is acceptable or not.

    What are the CTE's of ferrites? Anisotropic, I expect.
    --
    OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
    2006 is YMMVI- Your mileage may vary immensely.

    John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
     
    John Woodgate, Aug 24, 2006
    #17
  18. Joel Kolstad

    Phil Hobbs Guest

    John Woodgate wrote:

    > In message <>, dated Thu,
    > 24 Aug 2006, Phil Hobbs <> writes
    >
    >> Well, I think 400 ppm/K is pretty high if you're building filters or
    >> oscillators, don't you? At 30 MHz, that's 6 kHz per degree!

    >
    >
    > I meant to add the bit about N750 caps. And it depends entirely on what
    > performance is wanted, whether the TC is acceptable or not.
    >
    > What are the CTE's of ferrites? Anisotropic, I expect.


    Since they're sintered powders, they should be pretty isotropic, I
    think. For gapped toroids, the gap will expand with temperature at the
    same rate as the core, and will usually dominate the reluctance, so the
    TC of inductance ought to be


    TC_L ~ (2*CTE) - CTE + TC_mu

    total xsection gap permeability.


    or CTE + TC_mu.

    For ungapped ones, there's no gap to expand, and the inductance should
    go as the volume, so TC_L ~ 3*CTE + CT_mu. I don't know how the CTE and
    TC_mu compare, but it's sort of interesting that for a CT_mu = 0, the
    TC_L of an ungapped toroid comes out to be triple that of a gapped one.

    Cheers,

    Phil Hobbs
     
    Phil Hobbs, Aug 24, 2006
    #18
  19. Joel Kolstad

    Joerg Guest

    Hello Phil,

    >>
    >> OK, but you didn't say tempco of inductance and you still haven't
    >> explicitly. 400 ppm/K is still pretty low, even though it's 10 times
    >> what you get with an air core. And you only get 400 ppm/K if you wind
    >> it tightly.

    >
    > Well, I think 400 ppm/K is pretty high if you're building filters or
    > oscillators, don't you? At 30 MHz, that's 6 kHz per degree!
    >
    > And if you don't wind it pretty tightly, it'll squirm around unless
    > you're using really heavy wire. ...



    Not to forget the microphonics when doing high-Q filters up there. Many
    of us have heard that telltale "boink" in the audio when tapping the
    chassis.

    --
    Regards, Joerg

    http://www.analogconsultants.com
     
    Joerg, Aug 24, 2006
    #19
  20. Joel Kolstad

    Terry Given Guest

    Phil Hobbs wrote:
    > John Woodgate wrote:
    >
    >> In message <>, dated Thu,
    >> 24 Aug 2006, Phil Hobbs <> writes
    >>
    >>> Note that using solid plastic cores will seriously degrade the tempco
    >>> and distributed capacitance. The solid plastic will be strong enough
    >>> to stretch the wire, so that the tempco will go up to twice the CTE
    >>> of the plastic.

    >>
    >>
    >>
    >> I think that's a bit alarmist. I don't suppose the OP needs 0.1%
    >> resistance stability and military temperature range.

    >
    >
    > The tempco of an air-core copper coil is approximately twice the CTE of
    > the copper, because the inductance goes as r**2. If you wind the wire
    > tightly on the plastic, the plastic now controls the thermal expansion
    > because it stretches the wire as it heats up, and the tempco is then
    > about twice the CTE of the plastic. The CTEs are:
    >
    >
    > Copper: 16 ppm/K
    >
    > Polystyrene: 50-80 ppm/k
    >
    > Polypropylene: ~200 ppm/K
    >
    > Teflon: nonlinear, 125 to 1000 ppm/K
    >
    > So the typical tempco of an air-core coil is about 32 ppm/K, vs 400
    > ppm/K if you wind it on polypropylene.
    >
    > Length effects may change the numbers somewhat, but the ratio will still
    > be on the order of 10x.
    >
    > Cheers,
    >
    > Phil Hobbs


    The micrometals "formers" are phenolic. I did a quick google, but CTE
    depends on the exact material, however I did find one type of material
    (X7101) whos CTE was close to Aluminium....

    Teflon would make a pretty crap former!

    Cheers
    Terry
     
    Terry Given, Aug 24, 2006
    #20
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