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Choice of iron powder toriod?

Discussion in 'Electronic Components' started by Jeff, Oct 11, 2005.

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  1. Jeff

    Jeff Guest

    Im confused regarding the choice of iron powder toroid for a 7Mhz high pass
    filter. I need to use a small size (T68) toroid. For the freq range, some
    reference texts say T68-2 is the one, others say T68-6" grade. I see
    differing "best Q range" specs for the 6
    Have similar need for 20Mhz LPF, but assume "6" is really the choice there.
    Any suggestions as to the relative differences and reasons to choose between
    grades 2 and 6 for the HPF in T68 size?
  2. Tim Wescott

    Tim Wescott Guest

    As far as I know, 7MHz is on the border between using one mix or the
    other. You'll see different Q's because details of winding, wire choice
    and mounting will affect Q, as would use of a core from another
    manufacturer (I don't know if anyone other than MicroMetals uses that
    system, but if they did I wouldn't count on it being standardized).

    If you're not going to be hitting it with really high power and if you
    don't need it to be highly resonant I wouldn't worry too much. If
    you're going for an engineered solution then you should make a number of
    "identical" coils and test them; just make sure that the coils you make
    are representative of what you'll be manufacturing.
  3. Pooh Bear

    Pooh Bear Guest

    Having just been playing with Micrometals cores myself, I gather that -2 is
    indeed the 'best' for higher frequencies.

    The local distributor says it's stocked in greater depth too.

    Have you also looked at Magnetics Inc and Arnold ?

  4. Joerg

    Joerg Guest

    Not that this is an exact science but typically I switch from #2 to #6
    around 10MHz. But I never design with high Q because that can be a pain
    in production.

    Regards, Joerg
  5. Get a copy of Amidon's catalog which gives detailed suggestions for powdered
    iron and ferrite materials, based on many years of experience.

    Bill W0IYH
  6. Wes Stewart

    Wes Stewart Guest
  7. Jeff

    Jeff Guest

    Thanks for all the suggestions. Ive got some Amidon T68-6 toroids to wind
    up, and will test the completed filter on a spectrum analyser next week.
  8. Guest

    Been there, done that. In the practical case, what you can expect
    is a very slight change in insertion loss in the passband, perhaps
    a less sharp transition of attenuation at cutoff, depending on the
    type of highpass. It can be modeled in any SPICE analysis program
    with accuracy if you make a special model that adds a series R
    element computed from reactance divided by Q.

    For a highpass filter, the usual filter type has the inductor in
    shunt. In that configuration the inductor has a reactance
    directly proportional to frequency and will have little effect on
    the insertion loss in the passband. The equivalent series R due to
    Q won't matter much at 1/3 to 1/4 the cutoff frequency. It might
    matter on attenuation in the stopband region but the analysis on
    that is more difficult and a practical build-and-measure is the
    quicker way to go.

    For a lowpass filter, the usual configuration has inductors in
    series and there the Q of the inductors will affect insertion loss
    more. Those would use the lower frequency range for powder mix.

    In a highpass filter it is important that the capacitors have a
    minimum series inductance so that the 5x to 10x cutoff frequency
    isn't disturbed. Since capacitors are usually in series, their
    Q will effect the insertion loss. Fortunately, most capacitors
    will have a good Q up around 500+ and won't be a factor.

    Use what you have and measure the results (you've got a spectrum
    analyzer handy so that's taken care of).

    I made a fairly good, practical Synthesis-Analysis program for
    L-C filters that includes automatic modeling of (separately) all
    capacitor Qs and inductor Qs. I can attach that to private mail
    if you want it. Freeware. Proven by practical test comparison.

  9. Joerg

    Joerg Guest

    Hello William,
    Absolutely. I literally wore one of those catalogs to the point where
    you could see through several pages.

    Then there are the ARRL Handbook and their Antenna Book. Both well worth
    every penny.

    Regards, Joerg
  10. Hello Joerg, nice to meet you for the first time.

    Bill W0IYH
  11. Jeff

    Jeff Guest

    Hi Guys,
    Results of the build and test were that the 7Mhz HPF worked very well with
    the "6" grade cores, but the 20Mhz LPF was crap (it used #6 as well). It
    started to roll off at 20Mhz as expected, dropped smoothly to -10dB by the
    time it got to 40Mhz, then stayed at 10dB at all frequencies above 40Mhz.
    I suppose the core should be a grade #10 or #12 instead of #6? or perhaps
    just a normal air core?
  12. Then you haven't built the filter that you designed. The choice of core
    material will not affect the blow-by. If you measured the inductors at a low
    frequency, I would be pretty certain that you have way too much inductance
    at 20-40 MHz.

  13. Joerg

    Joerg Guest

    Hello Bill,
    Likewise. You name sounds very familiar, IIRC from a book about design
    of HF gear. Didn't you work at Collins in the good old days when they
    had mechanical filters in their gear?

    Regards, Joerg
  14. Yes. I started at Collins Radio engineering department in 1964 and retired
    from Rockwell Collins in 1990. For more info search Google for my name and
    my call sign. See QRZ.COM for W0IYH. See

    Bill W0IYH

  15. Did you ever work on their microwave receiver designs?
  16. Hello, Mike,

    I had a little experience with some military L-band RF design (JTIDS), but
    that is about all. I have also designed miniature lumped-element filters
    for the 3 GHz region. But most of my work and also my ham radio experience
    have been at HF. Also, I am mostly, but not entirely, an analog specialist,
    which has put me somewhat into the Jurassic Age.

    Bill W0IYH
  17. Joerg

    Joerg Guest

    Hello Bill,
    Not really. Newly minted engineers know remarkably little about analog
    techniques. Yet at the beginning and the end of circuits stuff usually
    needs to connect to the analog world. So don't be surprised when someone
    begs you to do just one more stint when you are past 90.

    Then again a SW engineer once told me that nothing is truly analog.
    There is always that smallest digital step, the quantum.

    Regards, Joerg

  18. There is no reason to apologize for being good with analog. ;-) I
    worked as a broadcast engineer during the time they announced the first
    memory chip, (1101) which was a slow, noisy 256 bit * 1 DRAM with very
    critical timing. I also did analog and digital work on the microwave
    equipment built at Microdyne, before L3-Com closed the Ocala plant.

    The reason I asked about the microwave equipment, I would like to
    meet the people who designed the C-band CATV receivers I had to maintain
    in the '80s. I always loved Collins equipment, till I ran into those
    radios. They had a horrible failure rate, and took over six months to
    have serviced by Collins. I think I still have a set of manuals for
    the fixed tuned, and the agile models. I started repairing them for
    United Video in 1982, and could return most of them to service the same
  19. Joerg

    Joerg Guest

    Hello Michael,
    Seems you started with digital stuff a year or two earlier than I did.
    My first RAM had a whopping 1024 bits. Not bytes, bits. 21...something,
    I could look it up since the device where its in still works.

    But their HF radios were quite reliable. Unfortunately at that time out
    of my budget range, and so were those nice mechanical filters :-(

    Regards, Joerg
  20. 2114? 1K * 4 bits?
    I still have an unused 2.1 KHz Collins 455 KHz mechanical filter I
    bought at the Dayton hamfest years ago. Its a little narrow for voice,
    but it would be good for CW.

    I still like Collins equipment, just not their poorly designed CATV
    equipment. I wish I could afford a R-390 and felt well enough to do a
    complete restoration. :(
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