The article here says type 43 NiZn ferrite's resistive impedance exceeds it's reactance at about 2MHz
but I think that's a misprint, looks more like 15MHz:
The data-sheet is here:
I have a bunch of FT37-43s ferrite toroids on hand and I need a 100uH common-mode choke for about 3 MHz, the calculator says about 30 turns should give me that but I'm unclear if this is an appropriate material
You can work that out. Sixteen turns on a toroid sounds like a single layer winding, where the parallel capacitance tend to be around 1pF, which would make the coil parallel resonant at about 8MHz (very roughly).
As soon as you start stacking up layers of windings, life gets more difficult and you have to starting thinking about partitioning the winding into successive multilayer chunks, where there isn't a lot of voltage difference within any one chunk.
The material itself doesn't have reactance. It does have a bulk resistivity, but that isn't directly related to core loss effects, though lossy materials can often be identified by their low bulk resistance.
"It was my understanding the that the point where u' and u" cross on apermeability curve is where the material Q = 1."
- Refers to a graph constructed using measurements performed on a predifined core shape and turns count, as comparative reference information for the core material in question. The graphical contents would change with a different core shape or winding configuration - shifting this specific 'Q' point, without altering the material composition of the core.
Another common graph is for the single-turn bead, which attempts to reduce winding effects on measurement of the loss characteristic. Each bead size and shape will still have its own curves, without alteration in the core material type.
Anyone can stipulate that a Q factor exists for the relation between any two measurable quantities. providing that its definition is presented within the scope of the work.
In a non-resonant circuit, the term has little meaning, as losses in the core are (non-linearly) dependent on the amplitude of the peak flux excursion, and many other inter-related physical factors of the core and winding.
I've tested some commercial AC-line CM chokes. I was surprised by how little normal-mode DC current it takes to saturate the ones I tried. They seem to assume that all AC loads are exactly DC balanced.
John Larkin Highland Technology, Inc
Science teaches us to doubt.
Graphs ARE available for parts under that specific condition, and for that use.
The OP, however is refering to a graph for 43 matl, and is only applicable to a 17mmOD /10mmID / 6mmH test speciment with an unknown number of turna. Fairite, the mfr of 43 matl, doesn't even bother to offer this part size for sale. It is a lab test specimen.
The size of the test specimen can vary from one matl data sheet to the next. None are necessarily commercially-sized parts, though some manufacturers do use a recognizable part number.
For core material there IS a 'relative loss factor' which is specified as a maximum limit value at a test frequency. There ARE graphs for that - check Magnetics Inc matl data.