Ferrite materials are commonly used for transformers for kHz and low MHz frequencies. Some vendors of RF products do however provide small transformers using small binocular cores (it looks like ferrite - could also be iron powder..) which have low insertion loss all the way up to 2 GHz or more. Does anyone know which kind of core mix is typically/can be used in such transformers?
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1\$\begingroup\$ What difference does it make unless you are planning on making your own ferrite material? Please explain. \$\endgroup\$– Andy akaCommented Dec 4, 2021 at 10:24
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\$\begingroup\$ It makes a difference if I am going to wind my own transformer with a magnetic core for 1 GHz. But the question is mainly out of curiosity. There is a lot to be found on the Internet about core types for lower frequencies: laminated steel for < a couple of hundred Hz, MnZn ferrites for kHz, NiZn ferrites or some iron powdered cores for low MHz and so on, but hard to find anything for very high frequencies. \$\endgroup\$– rubundCommented Dec 4, 2021 at 12:02
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\$\begingroup\$ Well, if you are not going to make your own ferrite material, then you are left with what vendors can supply so, I'll ask again; why do you need to know the core mix? \$\endgroup\$– Andy akaCommented Dec 4, 2021 at 12:07
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1\$\begingroup\$ I have not found any vendors who sell cores alone for this purpose - only ready-made transformers. Of course I can buy such a transformer, and rip out the winding and rewind it, but it would be nice to know what to look for if I want to buy the core alone. \$\endgroup\$– rubundCommented Dec 4, 2021 at 12:13
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1\$\begingroup\$ I am aware of that and I have been looking over quite a few datasheets, and none of them seem to offer any performance well above a few hundred MHz. Either the permeability drops towards 1, or the loss increases way too much to make them usable. It would be nice to get a hint where to look. \$\endgroup\$– rubundCommented Dec 4, 2021 at 12:25
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2 Answers
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Does anyone know which kind of core mix is typically/can be used in such transformers?
Ferrite core vendors supply separate material data sheets for their cores. Just look at those to understand the high frequency characteristics. The material data sheets are unspecific to core size (and shape) and reasons will be apparent when you look at a few. It keeps things easier to analyse. The actual core data sheets will be specific to core shapes and sizes and, they will quote the relevant materials used i.e. you get a cross reference to the material in the core data sheet. This links everything together nicely IMHO.
Ok, I think I got your point now. The transformer vendor actually provides a material declaration.. I found it now. I thought you were talking about the core vendors. Thanks, this could be an answer to the question.
Done! Any further help on specific cores/materials, just leave a comment.
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\$\begingroup\$ Micrometals.com for iron powder cores, Mag-inc.com for alloy powders like muMetal, permalloy and many others, ArnoldMagnetics, FerroxCube and several others for ferrites. Distributors like Digikey sell the bare cores (E-cores, Toroids, and Rods (solenoid) shapes) to wind your own chokes and transformers. \$\endgroup\$ Commented Dec 4, 2021 at 13:34
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\$\begingroup\$ I did not exactly find a material data sheet at the transformer vendor's website, but I found a "RoHS Material Declaration" which presents the material content of the core separately. That helps pointing in the right direction. Actually one of the cores for 100-1000 Mhz has 96% iron, 3.5% binder resin, so I assume an iron powder one. Another core for 2-1100 MHz has 73% ferrite, 15% zinc oxide and 15% manganese tetraoxide - so some kind of ferrite core. A third one for 500-2500 MHz has a 100% "phenolic" core - sounds to me like a non-magnetic core only used for keeping the winding fixed. \$\endgroup\$– rubundCommented Dec 4, 2021 at 18:24
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\$\begingroup\$ @rubund if we are done here (and you aren't prepared to share links to the core set you my be considering) then maybe you should take the 2 minute tour to understand the motivation behind people giving their free time to help others. \$\endgroup\$– Andy akaCommented Dec 4, 2021 at 18:26
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\$\begingroup\$ @Andyaka I've been looking at Mini-Circuits' transformers, i.e. minicircuits.com/pdfs/TC4-11+.pdf I avoided pointing to specific vendors to keep the question technical only. I know at least it is some kind of ferrite now - thanks to you :) I am active on other StackExchange sites, so I am not sure where you are heading with your comment - other than that there is a bit much chit-chat in the comments here. I would though like there to be an answer specifically saying what cores are used at GHz - and why. I may have gathered enough info to be able to write such an answer myself soon. \$\endgroup\$– rubundCommented Dec 4, 2021 at 19:22
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In the GHz range: transformers, filters, diplexers, combiners use air as a coupling media.
The external metal case plays a key role in the design.
Many filters are layied up on a RF PCB and then enclosed in a metal case.
I've never seen commercially available magnetic cores working in the GHz range.
Magnetic dipoles of ordinary materials like soft ferrite are too slow to change orientation in space in the nano second range.
Maybe in some lab they are working on an advanced material working in the GHz range.
answered Dec 4, 2021 at 13:53
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\$\begingroup\$ mini-circuits have a whole bunch of small signal RF transformers using magnetic cores with low insertion loss into the GHz range. \$\endgroup\$– rubundCommented Dec 4, 2021 at 15:42
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\$\begingroup\$ Maybe not for transformers, but ferrites have a number of uses in the RF discipline, up to and including 8 GHz. \$\endgroup\$– SteveShCommented Dec 4, 2021 at 16:21
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\$\begingroup\$ Phase shifters (see AWACS, B1-B), circulators. See magsmx.com/links/MAG_Backgrnd_Products_2016.pdf \$\endgroup\$– SteveShCommented Dec 4, 2021 at 17:24
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\$\begingroup\$ What air coupling? It might be into the phase shifter (like from a waveguide), but not in the interior. Do a search for "rotary field phase shifter", and look at the paper by Charles Boyd. \$\endgroup\$– SteveShCommented Dec 4, 2021 at 19:39