Binocular Cores Question - RF Cafe Forums
Post subject: Binocular cores question Posted: Sun Oct
23, 2005 12:35 pm
Joined: Fri Sep 02,
2005 7:25 pm
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.
bifilar wire around each "outside" creates a 4:1 balun.
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!
Post subject: Posted:
Tue Oct 25, 2005 7:42 am
Sep 02, 2005 7:25 pm
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
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
I tried to convey why binoc cores are useful for a variety
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.
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
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
Joined: Fri Sep 02, 2005 7:25
Location: Hampshire UK
Darn.. this keeps
happening to me. That previous Guest post was me.
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
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
Well first of all I don't think you have been wrong
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
vs. "2 toriods" vs. "stems" vs. etc., but can't expain why that
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
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!
Post subject: Posted: Tue Nov 01,
2005 3:27 pm
Joined: Fri Sep 02, 2005
Location: Hampshire UK
I will be getting
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.
is just getting the most wire on the core that they can possibly
get with that size core, to maximize the inductance.
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
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.
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.
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.