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I'm building a filter from a kit with some given (microscopic pre-soldered SMT) capacitors and 2 toroids. It looks like the notch frequency is a bit too low with 10 wire turns around a T37-2 iron powder toroid and a bit too high with 9 turns. The windings have all been wound tightly and evenly spaced.

Is there some variation in toroid winding method that could get the inductance closer to halfway between 9 and 10 turns?

Possible alterations are winding the toroid with uneven turn spacing (turns scrunched to one side or the other, or to the top away from the PCB or to the bottom closer to the PCB. Winding with fatter wire or thinner wire. Winding with a twisted wire pair soldered together at the ends. Winding all the turns looser (by using a spacer during winding). Winding some of the turns loose and some turns tight (all on one side or alternating?).

Which of those strategies might I try to tune my filter? (without throwing away the parts and PCB and starting over with a different design...)

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  • \$\begingroup\$ Either change the capacitors, or change the inductor's core. \$\endgroup\$
    – Hearth
    Commented Jun 23, 2022 at 4:58
  • \$\begingroup\$ Are there toroids that come with a range of such small differences in permeability? Or can a toroid be machined to change its wound inductance by a small amount? (a fraction of one turn delta) The tiny SMT capacitors are too small for me to even see, much less change. \$\endgroup\$
    – hotpaw2
    Commented Jun 23, 2022 at 7:29
  • \$\begingroup\$ You can try perhaps coupling" a new "winding" with "some" load (capacitor or resistor). Simulate first to see what happens. It is better if you know the schematic of the filter. \$\endgroup\$
    – Antonio51
    Commented Jun 23, 2022 at 8:56
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    \$\begingroup\$ @hotpaw2 I think you are missing hearth's point. Use a core with less change in inductance per turn. Why are you stuck on 9-10 turns? Or put another way, why don't you disclose what you are doing and why and what your design process looks like? The answer will fall out from those details soon enough. \$\endgroup\$
    – jonk
    Commented Jun 23, 2022 at 8:57
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    \$\begingroup\$ What is preventing you from doing 9.5 turns? \$\endgroup\$
    – evildemonic
    Commented Jun 23, 2022 at 17:28

2 Answers 2

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The position of turns matters. How much, depends on the mu_r of the core.

Micrometals has some reference material on the matter:
https://s3.amazonaws.com/micrometals-production/filer_public/7e/d0/7ed096a0-fe6e-4df1-9da9-e129c1ee73d2/q_curve_catalog_issue_h.pdf

[this is a current link from their website, but it doesn't look too permanent; anyone got a better one?]

Specifically the diagram on page 4, but you may find the surrounding information helpful as well.

If your toroids are bi-colored, you may be able to identify them -- most manufacturers seem to follow Micrometals' color scheme, or indeed are produced by/for them -- and then find specs. Typical performance can then be found here, or in related documents on their website.

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    \$\begingroup\$ Moving the turns might be a good solution. In figure D on page 4 of the Micrometals documentation, iec-international.com/micrometals/micrometals/downloads/… , it shows that, for a small number of turns, scrunching the turns together to use only one third of the toroid can increase the inductance by over 50%, with lesser increases of inductance for lesser scrunching of the turns, compared against the inductance from evenly spaced turns. \$\endgroup\$
    – hotpaw2
    Commented Jun 23, 2022 at 17:20
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It looks like the notch frequency is a bit too low with 10 wire turns around the toroid and a bit too high with 9 turns.

Without a schematic and some pieces of information, I can just guess this.
Here is an example of what can be done in my particular circuit.

Just add a coupled inductor (1 turn or 2) with a capacitor and adjust the value of this capacitor until the notch slip at the right place ...
NB: the right curve is for the "lowest" value of the capacitor.
Increasing the value "slip" the notch to "lower" frequency.

enter image description here

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  • \$\begingroup\$ I like this idea because one could glue a tiny trimmer capacitor nearby with a one turn coupling, and (non-conductive) screwdriver tune out some of the component variation without desoldering anything. \$\endgroup\$
    – hotpaw2
    Commented Jun 23, 2022 at 22:33

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