Help making a wideband transformer - Page 1
Forum section: electronics / rf-microwave

author: floobydust
author_rank: Super Contributor
timestamp: November 24, 2016, 05:26:01 am
content: I'm looking at making a wideband transformer and need suggestions for core material and shape.50ohms and 0.01MHz to 30-50MHz, flat response -1dB if possible. It's to be used for low-level signals to a spectrum analyzer (more below).For choosing a material,MicrometalsorAmidonhave materials with the same #'s? Micrometals mix no. 15 is rated 0.15-3MHz and Amidon's material 15 is rated 0.1-2MHZ resonant but they mention using it to 100Mhz. So I'm a little confused choosing a material, let alone core shape. Any guidance is appreciated.The lovelySirenza Microdevices LF-428is 0.01-50MHz -1dB but $432! and I'd need two. The transformers/balun goes on the output of a LISN and is used to add/subtract RF on mains line+neutral lines, to view either common-mode or differential-mode noise. Mentioned in Henry Ott's EMC book.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2016, 02:26:57 pm
content: You want ferrite, not powdered iron -- you're making a transformer (which doesn't store energy), not an inductor (which does). Energy storage is inverse with equivalent permeability, so a low-mu (powder) core, or a gapped core, stores energy, and a high-mu, ungapped core stores little.You also want a good CMRR (or DMRR, as the case may be, I suppose!), for which you may want a common mode choke in series with your transformer.Wideband transformers are super simple: take a transmission line and stack cores on it (or loop it through a bunch of times)!If you need an isolation transformer, then you have the most difficult application -- whereas a CMC or autoformer can be straight through (no matching up of delayed waves, everything arrives at the same time -- bandwidth not limited by electrical length), you only get flat bandwidth up to a cutoff frequency determined by electrical length of the transmission line.And, near that cutoff frequency, common mode coupling becomes comparable to differential mode coupling. So it can be worthwhile to add a CMC to tame that.So, for 50MHz, you need strictly under, uh... if 50MHz is 1/4 wave, then 200MHz is 1 wave, which is 5ns, which is about a meter of coax, or a little more of twisted pair.You might be able to get away with more, but I doubt it'll stay within 3dB if you push near 1/2 wavelength. 1/4 might not even be quite good enough for your 1dB spec.The transmission line impedance should be the circuit impedance.Now you need to loop enough of that around a core, such that the (transmission line's common mode) inductance is enough to satisfy the 10kHz spec. -1dB at 50 ohms is 460 ohms, at 10kHz is 7.3mH. Quite a lot!You maximize inductance by using the highest mu possible (to cover the frequency range, you'll probably use a combined stack of "wideband" ferrite and nanocrystalline strip), on a fairly large core. You won't be using the fewest turns possible (which would be 1 turn around an impractically massive core!*), but you will be on the low side (probably on the order of 10 turns).FYI, Amidon's powder coresareMicrometals, as far as I know, and their ferrites are Fair-Rite (numbers) and Magnetics Inc. (single letter). The "wideband" material of choice would be a #43 or so.*Magnetic fields propagate through core materials, just as waves go through anything else; the velocity is proportional to 1/sqrt(mu). A very thick core (like ~cm across) won't even "feel" a modest frequency wave (~MHz) through its full cross-section -- result? Effective area (and therefore inductivity) drops with rising frequency! So, a compromise between core size and turns count is desirable, particularly for very high-mu materials.It's also nice to use pot cores, because the winding length is minimal and the inductivity (inductance/turn^2) is high. Might not be high enough, though.Nanocrystalline cores are stripwound, so, toroid or cut (C core) shapes only.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 24, 2016, 02:29:39 pm
content: Also, relevant:http://seventransistorlabs.com/Precompliance/index.htmlNo pictures of the coil construction, but that wouldn't be very educational anyway.. just some wire around a core.Tim

author: snarkysparky
author_rank: Frequent Contributor
timestamp: November 24, 2016, 03:15:22 pm
content: Another reason it's so expensive is that it is hard to do for an application like this. The bandwidth and leakage specs along with capacitive coupling between windings and a dozen other parameters relevant for a measuring instrument application.A decent introduction article.http://www.engr.colostate.edu/ECE562/98lectures/l34.pdfI think I would try to find one made close to your specifications.

author: David Hess
author_rank: Super Contributor
timestamp: November 25, 2016, 01:20:13 am
content: Take a look at how broadband HF transformers for amateur applications are constructed using binocular cores or separate cores in a binocular configuration. They commonly cover 1.8 to 30 MHz or 50 MHz at high power but the input coupling transformers are smaller.

author: floobydust
author_rank: Super Contributor
timestamp: November 25, 2016, 04:17:55 am
content: Choosing a core material from datasheets is very difficultThank you for theClifton Labs link, he has great frequency response plots and a wealth of informationI searched more andFair-Rite 73 (MnZn ferrite)or Laird/Steward 35 core material look good for 10kHz-50MHz.Carbonyl iron-powders can do RF work, so I was originally looking at that:http://www.micrometals.com/appnotes/appnotedownloads/ipc4hqi.pdf"Iron powder is a core material well suited for high Q stable inductors to be used in the 100kHz to 200MHz frequency range."From Clifton Labs tests, he measured 5kHz-50MHz at -3dB for many ferrite parts... I am going to try aLaird/Steward 35T0501-10H corewith 14T bifilar #26AWG to get 200ohms magnetizing impedance for the 50ohm application. 14T is ~578uH for 200ohms at 55kHz by my math. Hmmm maybe Mr. Clifton measured down to 5kHz by driving with a lower source impedance? His results seems a bit too good.The binocular Fair-Rite core 2873000202http://toroids.info/BN-73-202.phpalso looks pretty good.I understand an EMC certification-grade transformer is individually made and tested, probably why they are very expensive.For a bit of slop and where accuracy is not exact, I'll try the homemade part. -1dB is a goal but a $0.30 core gotta try it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 25, 2016, 11:38:46 am
content: So I laid out the complete solution that meets spec, but you're okay with less?Tim

author: floobydust
author_rank: Super Contributor
timestamp: November 26, 2016, 02:20:17 am
content: Tim, I had trouble understanding some of your postYou are suggesting using two core materials in a stack? That's a great idea it sounds like the only way to get big bandwidth.I did not know anything aboutnanocrystallinematerial. Tapewound only cores, in strips and I haven't seen it before. Seems really good for high mu.So I'm trying to visualize the "sandwich" - "wideband ferrite" and a tape wound core, two different (for example) toroids stacked?But I got lost thinking wavelength or transmission line for transformer design. I mostly use the core's AL value and inductance/Z target at the lowest frequency as a starting point.It looked like I would start 1/4 wavelength with say 1m coax/twisted pair, but is that physically large? Most core apertures are only a few mm-cm.I do need isolation between primary/secondary.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 26, 2016, 01:51:15 pm
content: Quote from: floobydust on November 26, 2016, 02:20:17 amYes. The permeability of a core drops with increasing frequency, just as the gain of a low-pass filter drops with increasing frequency (well, "just" is subject to material variations, with some falling faster than others).So you can very easily have a modest-mu, but high frequency, material surpassing the permeability of a high-mu, low frequency material. Two combined means the best of both worlds!QuoteYup. Here's a listing of easily found parts:http://www.mouser.com/Vacuumschmelze/Passive-Components/EMI-Filters-EMI-Suppression/EMI-RFI-Suppressors-Ferrites/Ferrite-Toroids-Ferrite-Rings/_/N-bw7t9?P=1yyvu4f(Magnetec is also German, and may well be a rebrand of VAC material.)The permeability starts to drop at a mere 10kHz, which is where they measure the inductivity. So you will not have an inductor (i.e., impedance proportional to frequency) above that frequency -- still, figures like 40uH/t^2 are hard to beat, and despite the drop in inductance, the impedance (which is really what we're after, here) remains quite high into the 10s of MHz.QuoteThe fundamental problem with the textbook introduction of a transformer is, youneverput the windings on opposite arms of the core. Because that's not where the waves are travelling. Waves don't magically jump from one winding to the other!Flux is fine at DC, but not AC. It should be a huge clue that waves matter, because the winding length (that is, the actual wire used) is comparable to the wavelength, in the high frequency limit!I would much rather see the transmission line model taught: a transmission line is a pair of pins at one end, and a pair of pins at the other. Each end exhibits a characteristic impedance, and each end communicates with the other through a propagation delay.Transmission lines are easier to analyze with matched impedances in the time domain, whereas capacitors and inductors yield to frequency-domain analysis. It may well be easier to work with transmission lines, intuitively speaking -- the waveforms are what you see on the oscilloscope, in the time domain. You don't even have to understand Fourier analysis.And that's all you're doing here -- you're using the impedance of a transmission line to couple one side to the other. Since you need isolation, you're doing it "sideways", so to speak: each end of the transmission line (plus and minus) connects to input (plus and plus) and output (minus and minus). Consider a coax cable: connect the shield to input (say), and core to output.For very short time scales, each TL pair looks like a resistor of the characteristic impedance. Apply a voltage step, on the primary side, and that voltage is dropped across a loop of four resistances: the source resistance, the first TL pair, the load, and the second TL pair. (At low frequencies, the TL pairs communicate with little phase shift, and their voltage drops cancel out, thus giving low insertion loss. The cutoff frequency is where the phase shift between the two ports changes that significantly.)I would recommend twisted pair for an isolation transformer -- there's no reason for the primary or secondary to be asymmetrical (it's a transformer, it works equally well both ways), so the winding might as well be, too.Real cables have common mode impedance as well, but we can make reality approximate theory by wrapping the cable around a magnetic core. This gives a very high impedance between ends of the transmission line. That approximation breaks down at low frequency, giving the usual constraint (inductance and number of turns).If you don't need any low frequency range (say, a bandwidth of around an octave), you don't need a core at all! The transmission line does the job on its own. These are common enough in RF applications, where the bandwidth doesn't need to be much. (Also common are 1/4 wave transformers, which are resonant, so even less bandwidth. That's a different thing though -- harnessing the power of waves directly, not just transmitting them with good matching.)QuoteWell, 1m is the maximum, but you'll probably need it. Also, the transmission line's cross section is arbitrary -- you could use high power hardline, but that would be... nevermind.Thin coax, or twisted pair, is going to be the most common choice.You can't go so thin that the insertion loss (which will vary with frequency) blows out your flatness spec either. So, DCR << 1 ohm would seem to be a good idea. Still, that gives you anything from 36 gauge and down.No, it probably won't fit on a binoc core, but those aren't the best choice for wideband transformers. They're best for small signal applications of modest to high bandwidth (or, often, of narrow bandwidth, but just so you don't have to care about what it is).Tim

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: November 26, 2016, 02:36:10 pm
content: I'm wanting learn this also , thanks JohnAnother source of info , Steve hangs out on ham radio forums group .http://www.karinya.net/g3txq/chokes/

author: G0HZU
author_rank: Super Contributor
timestamp: November 28, 2016, 08:25:34 pm
content: The intended (EMC) application looks a bit dodgy but normally you would try for a winding length of an eighth of a wavelength or less for a basic wideband 1:1 50R isolation transformer like this. If you tried using a 50R tline that was a quarter wavelength long for the isolation transformer then I think you are doomed to failure on VSWR and insertion loss at 50MHz. I think it is dead even from a basic analysis. Note that when aiming to the required electrical length you have to take into account the velocity factor of the tline and even a twisted pair tline will be quite slow.The nanocrystalline materials look interesting and it would be interesting to see how they would perform here compared to the traditional ferrite materials. I do have some design experience of making wideband transformers from VLF thru to VHF but it was a long time ago now. I have wound transformers for use in bridges for basic measurement gear and the old school approach here would be to model this as a sideways transmission line transformer and then use a lumped model to simulate the LF effect of the core. Often a simple parallel RL can be used here. I would use the classic T35 (MnZn) material here on a core maybe 2cm or so across and try and get <= an eighth wave at 50MHz on it using twisted pair windings. I'd experiment using more than one winding in parallel to get the Zo down and the result of an ultra crude (ballpark) model is given below that uses two windings in parallel.This assumes maybe 17 or 18 turns (twisted wire pairs maybe 35-40cm long?) on the core with Al of over 3000. The T35 material is fairly low loss and would be OK for small signal stuff up to VHF. To improve the performance of the transformer I'd recommend fitting a simple current balun ahead of it and this won't cost much in additional insertion loss. But the plot below is just for the isolation transformer section on its own. This is a simulation not a real measurement and it uses a low Zo transmission line and an RL network to provide the response below. I would consider this to be a realistic benchmark and it should be possible to improve on this if you don't mind adding more complexity.I'm tempted to order and have a play with some of the nanocrystalline ferrites. I've not used these although I have a lot of experience with the traditional MnZn and NiZn ferrites.

author: G0HZU
author_rank: Super Contributor
timestamp: November 29, 2016, 09:57:43 pm
content: I managed to find an old T35 core at work today but it's quite small. Only 12.5mm OD. I managed to cram 17 turns of (two sets of) parallel twisted pairs onto it and measured it on a VNA across 300kHz to 100MHz and also on a Racal true rms meter from VLF up to 1MHz. My VNA only goes down to 300kHz so I used a sig gen and the Racal meter to do the low frequency end.I tested it as an isolation transformer and the results were pretty much as the simulation. I didn't get quite the same Zo for the twisted pairs as the simulation but the 1dB bandwidth was 8kHz through to about 90MHz with about 0.2dB midband loss. The electrical length for the Tline was an eighth wave at just under 70MHz.I think theses ferrites can still be purchased today for very little cost. But I think a slightly bigger one would be easier to work with. I think this one was a K44X35 core if that helps. But it must be 20-25 years old because that's the last time I tried making a VLF-VHF wideband transformer like this at work. The distributor name on the bag was Electrovalue and I still have their old catalogues from back then.