Bifilar Wound Balun Transformer

I am learning a bit about antenna design and one of the references I
found talks about coupling the antenna to the feedline with a bifilar
wound balun transformer. I dug a bit and although I found any number
of references talking about bifilar wound baluns, none explained clearly
why it is important to be bifilar wound.

Any pointers?

Rick

 

Ah, a picture is worth a thousand words. I finally found a page that
shows a bifilar balun in the application circuit I would be using it
with and it makes perfect sense now. Well, mostly. The circuit is
single ended to differential coupling.

I get why the thing is wired up the way it is, I suppose it is important
to use a bifilar winding to keep the two windings as identical as possible.

Actually, I've looked at too many pages. I had two pages mixed up. I
see the one that showed a toroidal core matching transformer is not the
same page as the one that said to bifilar wind the balun. Seems the
first one is a transformer like I'm used to seeing, but the bifilar
wound balun is used in a different way that can't match impedance over
the range I believe the toroid is doing.

It's pretty amazing how many web pages there are that cover in such
detail so many highly specialized topics! And most of these are hobby
pages!!!

Rick

 

A balun is actually a transmission line transformer.

http://en.wikipedia.org/wiki/Balun

The twisted pair that constitutes the bifilar winding is a
transmission line, with a particular characteristic impedance which
depends on the diameter of the wire involved and the thickness and
natire of its insulation.

IIRR a twisted pair twisted out of enamel-insulated transformer wire
has characteristic impedance in the ball-park of 120R.

Google throws up a few tutorial papers

http://home.earthlink.net/~christrask/TraskTLTTutorial.pdf

http://www.highfrequencyelectronics.com/Archives/Jan06/HFE0106_TraskPart2.pdf

Transmission line transformers keep on working to much higher
frequencies than conventional transformers - the inter-winding
capacitance becomes part of the transmission line rather than a simple
parasitic load - and in fact only start falling over when the
wavelength of the frequency being transmitted approaches the length of
the winding.

And - for John Larkin's benefit - this is electronics.

 

"Bill Sloman"

A balun is actually a transmission line transformer.

http://en.wikipedia.org/wiki/Balun

The twisted pair that constitutes the bifilar winding is a
transmission line, with a particular characteristic impedance which
depends on the diameter of the wire involved and the thickness and
natire of its insulation.

IIRR a twisted pair twisted out of enamel-insulated transformer wire
has characteristic impedance in the ball-park of 120R.


** For clarity, it needs to be said that twisting of a pair of parallel
wires in incidental to their operation as a transmission line. Twisting
merely serves to eliminate radiation and pick up of external EM fields.

A "bifilar wound " transformer may well have no twisting of the wires at
all, but simply has them laid side by side in smooth layers.


.... Phil

 

Not a necessary construction method; a balun is just a transformer with
tapping such that it inverts one side.

I got closer to 30 ohms last I measured a pair. Enamel is a whole lot
thinner than extruded jacketing. It's going to be even lower in a
piled-up winding due to the crowding.

The low frequency way to think of it: your leakage inductance is almost
exactly the inductance of the windings as a transmission line.

If you take a piece of twisted pair 1m long, it'll have maybe 0.5uH
inductance (measured at one end of the pair, shorting the far end, at a
frequency well below the electrical length of the line). If you wind it
up onto a form with an air core (making a bifilar solenoid, say), the
self-inductance of each winding might be a few uH, while the inductance
between wires remains the same (it's lower, if anything). Note that you
can measure this leakage two ways: terminus shorted (as a transmission
line) or secondary shorted (transformer leakage). The difference is, you
test P1-S1 and short P2-S2, or test P1-P2 and short S1-S2.

Now if you insert a permeable core, inductance goes way up (into the mH,
perhaps), and coupling coefficient likewise goes up (some fraction less
than 1.0). But leakage remains fairly constant.

Leakage depends almost entirely on winding construction. Self-inductance
depends on the windings and core. Coupling coefficient is the factor
relating the two.

(Yes, you can make a transformer that specifically depends on core
geometry, not just winding construction. An example would be two coils at
right angles, with a core snaked through each. Without the core, they
have zero mutual inductance (infinite leakage). With the core, it's
nonzero. I'm more interested in applications where you actually give a
damn about performance in the first place. )

The important thing about transmission line transformers is to forget
about using them as transformers. Use them as transmission lines! If you
put a few loops of coax on a core and drive the shield (calling the shield
the primary, P1-P2), you can't expect any useful kind of behavior from
that, because the shield carries all sorts of crazy currents, depending on
how it's looped through, and which turns it's adjacent to, etc. If
instead you drive the transmission line from one end (P1-S1), you'll get
the same signal out (P2-S2), delayed, except the core allows you
common-mode voltage. You could flip the terminal end around (S2-P2), and
get an inverted signal!
http://www.picosecond.com/product/product.asp?prod_id=47
That's more or less what they do here. The shield necessarily does still
carry a signal (the act of flipping the terminals forces the output
voltage onto the shield anyway), but this occurs "after" the signal
propagated through, and what you do with the shield is now an open
variable -- you could loop it through a whole bunch of ferrite beads,
damping out any oscillations.

It follows that you can create any ratio by connecting transmission lines
in parallel, looping them through a core (it doesn't even matter that the
same core is used, it's just a common mode choke now!), and connecting any
desired series-parallel combination on input and output sides to set the
desired impedance and ratio.

The dirty secret of transmission line transformers is, they aren't at all
interested in reducing leakage inductance, or capacitance, or anything
like that. It's just a big common-mode choke that lets you pipe signals
from wherever to wherever else.

Tim

 

In the sense that the original source of the name was as a contraction
of "balanced to unbalanced transformer".

The wikipea article makes it fairly clear that one should understand
it as a transmission line transformer. As Phil Alison correctly points
out, you don't actually have to twist the wires together to make them
into a transmission line, though twisting them is a mechanism which
does keep the pair close together.

How thick was the wire? The thickness of the enamel is more or less
independent of the copper gauge, and the impedances is going to be
appreciablyb higher for 40# gauge wire than for 10# gauge.

Most of the field is confined between the two wires of the pair. I
wouldn't think that adjacent wires would make much difference.

<snip>

This is wrong. I've certainly used them as 1:1 isolating transformers
and they worked fine.

There is some interesting literature on creating integer ratio
transmission line transformers, and if you are clever enough I'm
fairly sure that you can create non-integer ratios - I think there's a
famous paper on the subject. There are also a lot of ways of getting
it wrong.

That's certainly one way of using them.

 

Except that, as I said, the leakage is not particularly low. One gets
better performance in that regard from, say, copper foil pairs (which,
ultimately, is still doing the same thing, but with a low impedance
symmetrical stripline, not 50 ohm coax). Which is often done in power
circuitry. But "very low leakage" is not what you're going for, so it's
best not to claim that's what you're doing.

Tim

 

Wow, that's a lot of reading. Thanks.

Rick

 

See, this is the sort of stuff that, if I were a potential customer,
would turn me off to doing business with you. Geeze, if I am talking to
someone about what is going on in a system and they say to me, "but it
works", I would think they didn't understand it at all.

Do you not see how your posts make you look?

Rick

 

Anybody know how to accurately model a transmission line transformer in
Spice, taking into account core properties?

 

Fred Abse a écrit :

For a simple one, just as it is:
use a TLine/RLC tline and between the 2 ""shield/ref plane" connections
you just tie the magnetizing inductance, with maybe your core model
(losses, non linearities, hysteresis,...)

 

John Larkin a écrit :

If you want to accurately model a coax cable you need two TLines. One
modeling the center/shield transmission line, and a second one to model
the shield WRT to "space".

The "standard" perfect transformer is composed of a vcvs to transport
voltage to the secondary and a CCCS to reflect the secondary current
back to the primary, and a 0 voltage source to probe it.
It is much less computationally demanding than the Tline which has to
maintain history.

 

Well, apparently you're not.

Oops! One potential customer lost! Damn, John, this will put you out of
business.


Geeze, if I am talking to

Maybe the foot is on the other shoe. Maybe you didn't understand it at all.

And you understand how your posts look? That's curious.

 

Those that don't know shit, should shit elsewhere ! Get it?

The doctor made a mistake when you were born, they disposed the best
part that came out of your mother, the afterbirth.

Jamie

 

Amazing. There are times when a line is drawn and a designer says, "I
understand this well enough", but the way you say it comes off like an
amateur. I have spent a lot of time in my career fixing systems
designed by people who obviously "only needed to make it work", but then
it stopped working for some unknown reason.

That is scary. I find a lot of people like that though. I just thought
they were posers. I've never heard any of them brag about it.

Yeah, that's what everyone does, but when they connect those parts,
typically they understand everything about them and how to connect them
that they need to.

I'm talking about the statements you make that sound like they are from
someone with no level of understanding.

I shouldn't be posting about this. It is clear that you understand
completely what you are saying and I expect you understand how it makes
you appear. So sorry for bothering you with this.

Rick

 

At least you actually do something, not like a good many here that
would like to make people think otherwise.

I spend more time at actually experimenting with what works the best
instead of fighting with PC software that only gets it close but not
good enough.

I just love those that talk shit and most likely hardly even touch a
piece of equipment. When they do I am sure they're all thumbs and
fingers with it and most likely end up getting some one else to do it
for them and take all the credit for it.

Those guilty of this need not to step forward, I already know who
most of you are.

Jamie

 

I had a guy tell me a cheap scope (he always buys the most expensive
equipment) I got for him was broken-- turns out the brightness control
(or whatever you call it on a digital scope) was turned down. Same guy
claimed an expensive SRS bridge with 0.1% accuracy was giving 10%
error on a reading-- turned out he was using 100Hz to measure a
tens-of-pF cap and it was performing well within spec according to the
manual. He has written peer-reviewed papers on these things.. sad.

It's a rare person that can get all the theory right and the practice
right- they deserve to be well-rewarded.


Best regards,
Spehro Pefhany

 

As a physicist, I can affirm that. Others may vary.

Doesn't count -- even the string theorists don't understand the stuff. ;-)

And you don't?

I do on every single board I make. Not closed-form, but open-form
approximation, qualitative accuracy. Implemented in wetware, too. Works
very well.

Well, if you really wished, you'd buy the entire Ansoft suite and *do*
it -- but I'm guessing that wish isn't as unconditional as it was phrased.
In actuality, you don't care at all, and are more than happy enough
guessing. Which again illustrates your inconsistent self-representation.

Are you aware that ~20nH is ~86mm of 50 ohm, 0.67c coax?

Assuming the headers pictured are 0.1" centers, the cores are roughly T37
size ferrites, a bit thicker than usual. I get 14mm for the length of a
single turn on a regular T37, so it might be closer to 18mm per turn,
maybe 20mm with coax thickness. That's 60mm total length, or 14nH. The
soldered connections and board traces have almost as much, depending on if
there's a ground plane just out of sight or not. But by then it's not
mutual, which is all the more reason it's not LL you're supposing about.

Actual performance will show helical resonator action starting around
1GHz, which is what the under-hump on your leading edge comes from. And
probably other nasties if you tested it with a ps generator rather than
the "sub-ns" this particular device produces.

Tim

 

I learned a little in chemistry classes... very little.

My understanding is not that they don't understand it, it just doesn't
predict anything different from existing quantum theory.

Have you solved the Schrödinger wave equation for any of your systems.
Only then will I call you a real engineer

Hey, want to help me design a LF shielded loop antenna from coax? It
sounds like it would be right down your alley! I don't know nothing
about birthin' no babies, Ms. Scarlett! But it looks like I'm going to
have to learn...

Rick

 

Notwithstanding Feynman's quote, "nobody really understands QM", that's
more accurately the problem, as I also understand it.

Sure! My work is done:
http://vk1od.net/antenna/shieldedloop/
Well, maybe not *my* work, but... helpful nonetheless. Lots of excellent
analysis on his website.

Tim