Bifilar Wound Balun Transformer

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.p
df

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, Sydney

"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_idG
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

--  
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

On Sat, 3 Nov 2012 18:50:54 -0500, "Tim Williams"


We do exactly that in a bunch of products, namely use the shield as a
primary winding and the inner as the fully isolated secondary of a
transformer. We do 1:1 and 1:2 (voltage step up) at levels from 5
volts to over 100.

https://dl.dropbox.com/u/53724080/Circuits/Xfmrs.JPG

This makes a transformer with very low leakage inductance, so we get
sub-ns rise times into a 50 ohm load.

https://dl.dropbox.com/u/53724080/Circuits/T760_pulses.jpg




  If  

Here's a hardline inverter:

https://dl.dropbox.com/u/53724080/Circuits/Coax_Inverter/MVC-229X.JPG

https://dl.dropbox.com/u/53724080/Circuits/Coax_Inverter/MVC-235X.JPG

https://dl.dropbox.com/u/53724080/Circuits/Coax_Inverter/MVC-232X.JPG

https://dl.dropbox.com/u/53724080/Circuits/Coax_Inverter/MVC-234X.JPG


Low frequency response sucks because it is, after all, a dead short at
DC. It gets better if you run the coax through a few ferrite cores.


--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

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

--  
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

On Sat, 3 Nov 2012 20:40:37 -0500, "Tim Williams"


But it works.  


--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

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

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

   Then you must have a hell of a time getting to Pluto to shit.

Yeah. Okay. Keep your mouth open.

After you imbecile.

Jamie

I'm an engineer. I don't need to understand it, I only need to make it
work. If a deep theoretical understanding of transmission-line
transformers is helpful, I might use it. But if an hour of
instinct-driven experimenting works, I'll go with that. My
mosfet-transmission-line output stage, which we've used thousands of
times, took about an hour of experimenting to design.

Some of the stuff that we do is so complex that closed-form solutions
are impossible, and serious simulation would cost way too much time
and money.  

In the electronic design business, we seldom really understand what
we're doing, at the first-principles level. We usually work further up
the abstraction stack. We usually buy parts, read data sheets, and
connect them up. It's actually unusual to *make* a part. [1]


I posted pics of actual isolating transformers made with micro-coax.
And some nice sub-ns-risetime 100 volt pulses that were pumped through
similar transformers. Why would a customer be turned off by something
that works?

A sub-ns rise time into a 50 ohm load implies equivalent leakage
inductance in the 10s of nH.  

[1] invite interesting tales of actually making components.


--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

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.


 > We usually work further up

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

How deep does your understanding go? Quantum mechanics? String theory?
Do you do closed-form Maxwell's Equations on every circuit? Or full EM
simulation? I bet you don't.


I'm not bragging. I wish I had the tools to fully understand or
simulate everything we do, down to the physics. Sometimes you do
whatever works.


"That they need to." Yes. Understanding *everything* about an IC would
be great, but we're not privvy to that information.


The pragamatism issue is real, and there are serious risks from not
fully understanding a system. But there we are, working with what we
have.

I can't see a lot of risk in designing a transmission-line transformer
by experiment. It should be very reproducible, and it is. A little
math can check for things like breakdown voltage, if that matters.


--  

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

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.

<snip>
<quoted from proceeding post>

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

--  
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

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