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
r
A balun is actually a transmission line transformer.
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
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.
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!
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
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.
This makes a transformer with very low leakage inductance, so we get sub-ns rise times into a 50 ohm load.
If
Here's a hardline inverter:
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
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.
be even lower in a
Most of the field is confined between the two wires of the pair. I wouldn't think that adjacent wires would make much difference.
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This is wrong. I've certainly used them as 1:1 isolating transformers and they worked fine.
yThere 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.
sThat's certainly one way of using them.
-- Bill Sloman, Sydney
If you're connecting to an antenna, this might be helpful:
A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing Quoting: The primary function of most baluns, at least in our ham stations, is to minimize the interaction of our antennas with the transmission lines that connect them to our radios. There's more to baluns than just impedance matching.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...
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
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
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?
--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
(Richard Feynman)
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,...)
-- Thanks, Fred.
What's interesting about the LT Spice transmission lines is that they have no common-mode DC continuity between ends. They act as if there is an ideal 1:1 isolation transformer in the circuit.
That sort of makes sense, since, say, the outer conductor of a coax has its own complex impedance against the universe, and Spice elects to not model that. Still, it can throw you if you don't know about it, and make baluns seem to work much better than they will in real life.
So a txline can make a handy 1:1 ideal transformer. Unlike a 1:1 VCVS, it's bidirectional and the output loads the input.
-- 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
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.
-- Thanks, Fred.
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.
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
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
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