49:1 UnUn - Bifilar Winding  - Page 1
Forum section: electronics / rf-microwave

author: @rt
author_rank: Super Contributor
timestamp: September 28, 2023, 05:03:27 am
content: Hi GuysJust interested in confirming what my gut feelings dictate.The first couple of turns for this UnUn are commonly bifilar wound,then one of the primary ends exits the core, and the secondary continues to be wound after that.What, if any difference would there be if those first two turns were made with seperate wire, side by side, but not twisted together?What, if any difference would there be if the first two turns were made with asingle wire, and simply tapped after two turns like any other autotransformer?Thanks!

author: Hamelec
author_rank: Regular Contributor
timestamp: September 28, 2023, 08:00:08 am
content: All your versions will work with slight differences in losses and frequency behavior. Also the thickness of isolation and spacing of the windings is a topic. You can measure all of it with a VNA by using two and connecting it face to face.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 28, 2023, 10:19:53 am
content: Assuming the 1t is 50 ohms and the 7t is [stupid high impedance], nothing will really help you out here as no such transmission line impedance is possible (well, maybe with arbitrarily thin wire, but practically speaking). The best you can do, is to space the 7t winding very loosely (from anything nearby, itself, primary, core..), and, given that same limitation, place each turn near the primary.(And when I say "1t" and "7t", that's of course 1N and 7N for whatever N ratio the core needs for frequency and signal level.)Bandwidth is limited by stray capacitance and leakage inductance, which is to say, transmission line parameters of the secondary. Consider round microstrip over plane: suppose we used a single-turn foil winding for the primary, then that would be the ground plane (which happens to be slotted so that we can feed a signal into it and get transformer action). Even if we elevate that wire fairly far (say 10 wire diameters or more), it's still only 130-220Ω (depending on dielectric constant; you'd likely need tape to support it).If we allow the secondary to couple to itself (smaller pitch, say 1.1-3 wire diameters for imagination's sake), we end up with somewhere between a microstrip and helical waveguide structure, depending on the relative size of each (that is, of the primary to secondary diameters, and secondary pitch to height above primary). This can have much higher characteristic impedance, but the resonant frequency falls due to the slow-wave nature of the structure (it also becomes anharmonic* due to dispersion, not that that will be important for a transformer). The perspective shifts to something more like, here's a helical waveguide of 7N turns, with a primary winding slid inside of it, having some modest ratio of diameters. And then a core wrapped around everything to extend the LF range, making sure the core well clears the secondary.*That is, the resonant modes of the transmission line. Whereas you can imagine a coax TLT simply has resonances at N/2 or (2N+1)/4 wavelengths.Somewhere around there will be the absolute best you can do.Whether any of that is at all meaningful or useful for your case, I have no idea, you didn't mention frequency.Anyway, as to contrast with a more conventional say twisted-pair case, that gives a ~100Ω TL impedance for each pairing, and suppose you use 7 pairs and wire all the primary wires in parallel and all the secondaries in series: now for any given section, it has to push against all the TL segments above/below it. Since Zs ~ 2.5k and Zo ~ 100, that's a pretty huge capacitive load down to even 100/2.5k times the resonant frequency (i.e. the electrical length of the secondary). That's more motivation why I jumped straight into higher-Z examples.You also get fairly high Zo from block windings (basically, having the wire overlap itself, making successive turns as wire-pair TLs cascaded upon themselves, albeit rather crudely as turns go back and forth stacking up in layers), but bandwidth drops even more because Cp and LL are jointly increased. Perhaps with judicious wire spacing, and winding order (how the layers are arranged in the block), there is room for optimization; but that's a lot of effort versus simply laying down layers of wire.Case in point, I recall measuring a small common-mode choke (~10mH?) as Zo ~ 600Ω and Fo ~ 2MHz or something like that; with the split-bobbin winding, leakage is maximum, and with the multilayer block windings, capacitance is maximum too. So Fo is pretty low.Tim

author: Co6aka
author_rank: Unknown
timestamp: September 28, 2023, 11:58:05 am
content: Quote from: [member=98969]rt on September 28, 2023, 05:03:27 amThe difference is that joke contrasting theory with practice, and then combining both. If your goal is an actual transformer, build a bunch of prototypes and test fixtures and eventually you'll actually know what's the difference. I found this is the most practical method; last I heard everyone else is still theorizing and philosophizing, and arguing that my transformer can't possibly work like it does.

author: @rt
author_rank: Super Contributor
timestamp: September 28, 2023, 10:48:21 pm
content: ChatGTP actually gives a particular answer that favours the original 49:1 UnUn I didn’t consider, which sounds plausible.I guess there’s a reason the same thing would have been reproduced thousands of times.If you ask, don’t forget to provide the context that I did, that it’s a 49:1 UnUn.ChatGTP already consistently assumes it’s an impedance matcher for RF.I tried to delete this thread before it got started, but was unable. Thanks for the replies in any case!