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Binocular cores question - RF Cafe Forums

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Post subject: Binocular cores question Posted: Sun Oct 23, 2005 12:35 pm


Joined: Fri Sep 02, 2005 7:25 pm

Posts: 34

Location: Hampshire UK

I have been reading what I can find about the ways of winding two-hole flat toroidals (binocular) RF ferrite cores as widely used in baluns, transformer-type hybrids, etc. There are many types, but a basic feature leaves me confused.

You can visualise a binocular as two toroids stood together. Therefore a winding which goes through a hole, and back down the other is a turn. Windings like this on the "stem" I can understand. Two such windings will couple strongly, and can give a nice flat component with input at one end, and output at the other. Is just a small version of a regular mains power transformer, morphed to be a bit flatter.

It is when I see windings that go around the outside that I start to become unglued. A wire through a hole, that goes outside around one limb, then back though the same hole is not going to much couple to another similar winding on the other "toroid".

Aside from use as a high power common mode choke where the "stem" is expected to saturate, does anyone know where there is a clear explanation of how these things operate?

Thanks much if you can help



Post subject: Posted: Mon Oct 24, 2005 3:57 am

I design mixers for a living and use binoc cores alot for the baluns.

Winding bifilar wire around each "outside" creates a 4:1 balun.

Similarly, it is a good way to create directional couplers.



Post subject: Binocular coresPosted: Mon Oct 24, 2005 11:38 am

Binocular cores are most often used as part of transmission line transformers.

Baluns (as in the reply from the previous poster) are usually transmission line transformers. The classic book on the subject is by Jerry Sevick, and is titled "Transmission Line Transformers" and published by Noble.

Transmission line transformers do not work like regular transformers, by having magnetic flux linking two windings, but by adding series impedance to common-mode transmission modes. If this doesn't make sense, you really should "read the book" - it will be an eye-opener!

Good Luck!



Post subject: Posted: Tue Oct 25, 2005 7:42 am


Joined: Fri Sep 02, 2005 7:25 pm

Posts: 34

Location: Hampshire UK

Thanks for the replies, and for the book reference Gerry.

I have appreciated that if you wind (say) a toroid with primary one side, secondary the other, well separated, you get a transformer that works up to the magnetic limits of the core material (nickel / manganese / iron / ferrite mixes).

If instead, you use a bifilar, or twisted pair as a transmission line in the windings, the capacitive coupling takes over where the magnetics run out, giving a very wideband performance limited maybe by the losses of the material at frequencies where "air-only" would maybe have been better.

The use as mixer transformers is different. I can find analyses for all sorts of classic transformer setups including (at HF to UHF) hybrid couplers/splitters (magic tee!), directional couplers, baluns, etc. It is when we get to the actual physical construction of the examples that I find this "gap" in the rationale. I can see the recipes. Both kinds of winding type are there, sometimes used within one device!

Do I take it then that any turns that loop round the outside increase the inductance in that route by the effect of one toroid turns worth, but with a limited coupling into the other half that leaks across the "stem"? Windings that share the "stem" will couple very strongly, and also capacitively, being as they drive a common part of the magnetic circuit, and are close together, I take that as given.

I understand what you say about 4:1 baluns. I do not see that the use of a binocular core is fundamentally special to it. I could also construct that 4:1 balun on a single (one-hole) toroid. I can make the same balun somewhat better on a binocular core, putting both windings on the shared central stem. Provided both (say bifilar) wires take the same route together wherever they go, I will get the much the same balun, even if I wind some turns around any outer limbs. I do question why do this, when it loses the advantage of enclosing more of the turns in a twin-toroid strapped-together scheme? What happens if you let separated circuits use these uncoupled routes is not so clear.

What I hoped to tease out was a clear explanation of how/why to exploit the "outer" limb route when designing windings. I wanted what motivates it as a choice. Maybe Gerry's book reference will provide it without getting me in even deeper into the hard sums!



Post subject: Posted: Tue Oct 25, 2005 5:08 pm


Pehaps I misunderstood your question. I definetly under estimated your knowledge of transformers, baluns etc.

I tried to convey why binoc cores are useful for a variety of circuits.

I used the example of a 4:1 transmission line transformer. You noted it could be done with 2 wires around the "stem". This would create an unbalanced to unbalanced 4:! transformer. Whereas a bifilar wire around each utside", a unbalanced to balanced 4:1 transformer could be created. Whether it be use in a mixer or not is not important.

To make a 4:1 unbalanced to balanced by wrapping the wires around just the stem would require you to use quadfilar. This degrades perfomance greatly due to excess C and common moding.

Of course you may lose something because the winding are not totally enclosed whithin the core, but it's not significant in alot of application.

In fact it sometimes inhibits high end frequency performance because of added shunt C.

I have a feeling I still didn't answer you question, but I guess my point is for some applications maximum coupling is less important than versitilty, balance etc.



Post subject: Posted: Wed Oct 26, 2005 8:39 pm

Gerry - that book by Sevick is expensive! Yet I think it is likely worth it because I have dug up other Sevick articles on broadband transmission line transformers. He is clearly an expert.

Given what you say about the 4:1 balun example, I think I may have run across a (possibly cultural) difference in what I thought it was. Allow also that I just might still be ignorant about some. Hell, there are so many of them!

I know of two very different 4:1 baluns builds. The first is made on one core, which can be a toroid, or the stem of a binocular core. The essential feature is that it is an autotransformer connection where the end of one winding connects to the start of the next. One winding is energised relative to the grounded centre. The balanced output is right across the two windings. ie. one leg of the balanced output shares a connection with the "hot" AC input.

I guess this what you would have called an unbalanced to unbalanced 4:1 thing, but many folk have used it as an antenna feed balun.

The other kind is a full-blown two-transformer affair which is made using either two separate cores, or, just might be made by winding each transformer on the outer limbs of a binocular core. The winding connections are such that the balanced side now has each leg connecting to a winding end of its own, without any sharing. (I won't even try a full description in words of the "series crossed over" primary connection)!

I guess this is maybe what you have in mind when you think of a "proper" unbal - to - balanced device.

My point here is that the binocular core in this case is incidental. It is only two cores (maybe conveniently) stood next to each other. Even so, if you connect transformers together in this fashion, there are some paths formed that do take currents around the stem. Moreover, when the windings are excited as transmission lines, you get a whole raft of useful devices that do not behave as simple transformers. This is the point I ran out of smarts, prompting the original question.

Hmm.. maybe Sevick's book can be had on Ebay... yeah!



Post subject: Posted: Thu Oct 27, 2005 6:35 am


Joined: Fri Sep 02, 2005 7:25 pm

Posts: 34

Location: Hampshire UK

Darn.. this keeps happening to me. That previous Guest post was me.

Further to the theme Jerry Trask untangles exactly what we have been talking about in ..http://www.home.earthlink.net/~christrask/Trask4to1Balun.pdf

Yet - the question about what motivates a "on the limbs" winding remains..

A 45MHz to 2.5GHz matching transformer (yeah - that good!) from EPCOS is a simple autotransformer connection, with no bells and whistles unless there is some coupled circuit artistry in there that nobody explicitly articulates.


Maybe stem windings are only for low frequency work. About halfway down this page is a great picture of one used for a ham http://www.isp.ca/ve3nh/rxant.htm

More elaborate versions of this, complete with brass tube shields going right through the holes are used in VHF power amplifiers. The frequency coverage of these transformers is clearly way beyond where the core magnetics frequency limit is - or am I wrong about that as well?

Am I just being obtuse? Am I questioning what others find obvious? Is this getting into the area of "proprietary black art"? What is it that motivates a designer to make some types on the stem, other types being two transformers, one each on its own limb, and yet other types being one transformer with each winding on its own limb?

Also - if this is getting too esoteric, I guess I can abandon this thread. There are sexier subjects on this forum than the finer points of binocular cores.



Post subject: Posted: Thu Oct 27, 2005 12:11 pm

Well first of all I don't think you have been wrong about anything...yet!:)

Maybe I'm wierd, but I'd much rather talk about transformers than alot of the other topics in this forum. The problem is that there are so many applications an variables to transformers that it's difficults to discuss without visual aides.

For example, I can think of at least 6 dirrerent ways to make a 4:1 transformer, but It would take to long to describe how each one is constructed. My boss is already mad at me as it is.

Anyway, I guess I never answered your question. Being the empirical designer that I am, I've found pro's and con's to using

"binocs" vs. "2 toriods" vs. "stems" vs. etc., but can't expain why that is.

Good luck Graham, let me know if you figure it out.



Post subject: Binocular coresPosted: Thu Oct 27, 2005 12:21 pm

A few comments:

1. Sevick's book shows up for about $39 on www.bookfinder.com - that's not too bad, given what tech books go for these days. It's also available in a reprint edition from Noble Publishing.

2. Extreme bandwidth is characteristic of transmission-line transformers. There's nothing proprietary or "black art" about it - again, Sevick's book and articles explain it clearly. While I couldn't get the Epcos link to work correctly, I'd bet money that they're not "just" an autotransformer, but have transmission-line functionality.

3. The reason for extreme bandwidth is that the performance at the high end is not determined by the magnetic aspect of the device, but by the transmission line imperfections. The magnetic path only affects the low-frequency end.

4. The ideal magnetic core surrounds the windings entirely. Obviously, the wires have to get in and out. The VE3NH picture shows the use of a binocular core as a way to get a good low-impedance path for the magnetic flux. The same idea is used in pot cores at lower frequencies. Unless I miss my guess, the brass tubing you've seen is actually part of a winding.

5. Chris Trask's site is a good one - Chris is a good guy to know about!

Good Luck!



Post subject: Posted: Tue Nov 01, 2005 3:27 pm


Joined: Fri Sep 02, 2005 7:25 pm

Posts: 34

Location: Hampshire UK

I will be getting the book.

In this, I have trawled hard for some clear explanation. I have got used to the fact that you can re-draw the 4:1 transformer schematic three different ways, one to emphasise the connection analogy to a half-wave balun, the second as a traditional 4:1 transmission-line transformer, and yet again as a conventional auto-transformer.

However many papers I dig up, however acedemic the analyses, the bit that is always missing is the motivation for the sort of thing you see on mini-circuits homepage. (Miniature Bias Tees 20 to 6000 MHz as seen in RF Design Magazine).


They cannot have decided to hang the wire in there just any old way it would fit!

I am thinking that Gerry's empirical design methods are used far and wide, and in more places than the big companies selling simulation software would readily admit is so.



Post subject: Binocular coresPosted: Sun Nov 06, 2005 3:46 pm

Ummm, the Mini-circuits Bias Tee has almost nothing to do with transformers and baluns at all - they're just getting the most inductance they can with the core in question. It's a single-winding inductor in series with a physically-separated small microstrip inductance, so that self-resonance of the large inductor won't kill the performance of the bias tee.

Mini-circuits is just getting the most wire on the core that they can possibly get with that size core, to maximize the inductance.

Schematics don't tell you all there is to know about a part - sometimes you have to look at the current flows and the magnetic fields.

The essence of transmission line transformers is the separate handling of differential mode and common mode waves on the transmission line.

The differential current generates (essentially) no external magnetic field, because the currents are traveling in opposite directions almost on top of each other - the magnetic fields cancel each other out.

On the other hand, the common-mode current (the current going the same direction through the two wires of the transmission line) does generate an external magnetic field - which can be handled with an external magnetic circuit.

A conventional transformer, or an autotransformer, uses wires which are not part of a transmission line, so there's no differential current at all. So all (desired) coupling is via magnetic fields.

It's not the capacitive coupling between the conductors "taking over" from magnetic coupling in a transmission-line transformer, it's decoupling the common-mode current.

By the way, Sevick published a series of articles in QST magazine on the subject, too.

Good Luck!

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