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Question on Q of toroid coil

Discussion in 'Electronic Design' started by Dave, Dec 1, 2006.

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  1. Dave

    Dave Guest

    Just got an Extech LCR meter, and it displays the Q of an inductor at the
    same time as the inductance. Only it says that the coil I wound has a Q of
    ..132, when the Amidon tech manual inidicates it should be somewhere around
    110 or higher. That is at several MHz, however, not the 1KHz that the meter
    tests at. Could this be the explanation for the differing values? Or does
    ..132 somehow translate to a Q of 132? Anyone with any ideas is sought for
    possible answers. This coil is currently in a tank circuit that I *think*
    should tune from 4 to 10.5 MHz, only it doesn't seem to, and I am looking
    for ideas as to why. If it matters, the coil is 62 turns of #36 wire on an
    Amidon T-37-2 toroid form. Meter indicates it is 14 microhenries.


  2. Fred Bartoli

    Fred Bartoli Guest

    Dave a écrit :
    Q is the ratio of circulating energy to losses.
    Q = L w/Rs

    With Rs the equivalent series resistance.
    Rs accounts for core loss, and copper loss and have a minimum value at
    low frequency, so your Q will vanish at low freq.

    Also Rs varies quite a lot with frequency, so its almost impossible to
    tell the HF value from the LF one.
  3. Eeyore

    Eeyore Guest

    Yes. Very much so. A Q figure is related to the frequency.

  4. Q is XL / R, so for a higher frequency you will get a better Q.

    BUT: core losses and skin effect are represented by R and they will
    rise somewhat, too. Above the self resonance frequency, parasitic C will dominate
    and X will dwindle, so Q will not rise forever.

    Anyway, your LCR meter is probably OK.

    regards, Gerhard
  5. john jardine

    john jardine Guest

    (Nice meters those Extechs').
    The 14uH will be correct but the reported Q value is worthless (unless
    you're working at 1kHz) and would still be as worthless if tested at 10kHz
    100kHz etc.
    The coil -must- be Q tested at the frequencies it will be used at (or within
    5% of) and this means getting hold of, or making, a Q-meter. These are rare
    beasts nowadays but mandatory if you want to make tuned circuits and
    filters that do the job.
    Any Q prediction or guess tends to be wasted effort. There's just too many
    physical and frequency related variables at work together. If you're after
    good hi Q coils (ie >100) then it needs approaching from a craft skill POV
    and is not something that can be calculated 'up front'.
    (foregoing said by someone who confidently assumed the magnificent set of
    coils he'd spent ages carefully putting together, would have final Q's of oh
    "at least 200" but measured out at Q=15 :(
  6. Phil Allison

    Phil Allison Guest

    "Fred Bartoli"

    ** Huh ???

    Rather big non sequitur there.

    ** The rather dramatic variation of " Lw" with frequency seems to have
    gone for lunch.

    ........ Phil
  7. Eeyore

    Eeyore Guest

    LOL !

    Well spotted that man.

  8. Dave

    Dave Guest

    Huh. A Q meter. This interests me. Any ideas on where I could find info
    on building one of those? (I have a lot more time than money, and it does
    sound interesting...)

    Thanks for the info.

  9. They turn up on eBay from time to time. They sell for less than the parts.
  10. Fred Bartoli

    Fred Bartoli Guest

    Phil Allison a écrit :
    Phil I know you can do better. Maybe my wording wasn't the best but you
    also know I'm not english native and I know you know the answer, so...

    Anyway, when Lw tends to zero and Rs has to be > Rcopper (which happens
    to be finite) Q obviously tends to 0.
    No non sequitur at all.
    Since when the ratio of a known quantity (Lw) to an unknown one (Esr)
    gives a known value?
  11. Phil Allison

    Phil Allison Guest

    "Fred Bartoli"

    ** Yawn - what a pathetic, self serving insult.

    ** Squirm, squirm squirm .......

    I'd like to be, under the sea

    In an octopus's garden, with you ........

    ** Since when does caulk taste just as good as cheese - Fred ??

    Fishing up such a putrid red herring from the depths of the ocean could
    impress only those congenitally out of their depth.

    Only aging hacks play to that audience.

    You can do much better.

    ........ Phil
  12. PeteS

    PeteS Guest

    You can measure the Q of the coil by driving it at the frequency of
    interest and measuring the phase angle between input and output.

    Q = arctan phase angle


  13. Phil Allison

    Phil Allison Guest


    ** You can also tell who is a SPACE ALIEN by first sucking out his entire
    brain and putting it through in a blender.

    Aliens brains have a thicker consistency ....

    But not as thick as some.

    ....... Phil
  14. PeteS

    PeteS Guest

    That should have read:

    Q = arctan (90 - phase angle) = arccotan phase angle.


  15. Fred Bartoli

    Fred Bartoli Guest

    Phil Allison a écrit :
    Don't know, but I know places where cheese tastes as good as caulk.
  16. john jardine

    john jardine Guest

    This one is a nice design but needs an external signal generator.
  17. Dave

    Dave Guest

    Aah. Yes, I found that last night thanks to Google. Also two others, one
    of which I have the pay for. Am looking at options. But thank you very
    much for the referrence. I'm saving that as a favorite, thinking I'm going
    back there.

    Take it easy...

  18. Dave

    Dave Guest

    Now, *this* is really interesting. But how would you accurately measure the
    two phase angles? Please share whatever information you have on using this
    method of determining Q.


  19. amdx

    amdx Guest

    Hi Dave, You can use an oscilloscope and a signal generator to get a
    good idea of Q. Do you have an oscilloscope and signal generator?
    If so, I'll post the method I have used.
  20. Dave

    Dave Guest

    Hello Mike,

    I have an Oscope, but no RF signal source. Am now planning on correcting
    this second-place deficiency however, so please feel free to post your
    information. I'll save it, and use it as soon as I can. Knowledge is never

    I am planning on building a Q meter, probably the Poor Man's Q-Meter from
    Silicon Chip Online at

    For this project I also need a signal source, so it is on the agenda.

    Thanks much,

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