Do any free tools allow you to model RF transformers? - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: January 26, 2016, 07:54:53 pm
content: Basically I want to be able to model transformers - which cannot be done with RFSIM99.

author: Kelbit
author_rank: Regular Contributor
timestamp: January 27, 2016, 06:52:05 am
content: Transmission line or flux-coupled transformer? EM simulation or component simulation?

author: cdev
author_rank: Super Contributor
timestamp: January 27, 2016, 08:05:47 pm
content: Well, its a toroid core with three windings. Two small and one large. The small ones are the supposed to be 50-or-75-ohm input and output and the larger winding (have tried six to 20 turns, currently using #2 ferrite material..) is a tuned circuit.

author: G0HZU
author_rank: Super Contributor
timestamp: January 27, 2016, 10:34:35 pm
content: It's a long time since I used RFSIM99 but I think it has a transformer model built in?I've forgotten how well this model works but have you tried stacking several transformers in RFSIM99 to make up an (ideal or physical) equivalent for your three winding transformer? Maybe it won't allow this but in theory you should be able to make up something that resembles your wanted transformer in terms of basic RF performance.Otherwise, have a look at something like the Simetrix SPICE simulator. The free version is only limited in circuit size and you can select various transformer options including the number of secondaries, the various ratios and also the primary inductance and the coupling factor(s). These are basic models but you can add a few parasitics if you choose.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2016, 10:40:56 pm
content: You can make some basic estimates using wave-of-the-hands transmission line theory, but the construction has to be described very precisely (pictures, drawings, winding schedule..?).BTW, ferrites do not come in numbers such as #2: that's a Micrometals designation for mu=10 powdered iron.To clear up confusion, check outwww.micrometals.comlisting of power and RF materials, andwww.fair-rite.com's listing of ferrite materials. (There are oodles of other materials, but these two manufacturers, or their designations at least, seem to be the most commonly seen in RF work.)Tim

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: January 27, 2016, 11:02:47 pm
content: Answer is ... NO. Unfortunately NO.There are tools but results do not match. This guy have shared some details about his attempts for example:

author: G0HZU
author_rank: Super Contributor
timestamp: January 28, 2016, 12:44:52 am
content: QuoteNo model is perfect but even RFSIM99 can model (as in linear, small signal model) some simple transformers quite well if you import the model as a 2 port device. i.e. measure the transformer using test gear and then import your 2 port model to RFSIM99. I think you can do the same with the free version of Simetrix although I've not tried this on the free version. The model should be useful but it will obviously be limited to the range of frequencies the test gear measured it over and also it will depend on the quality of the test gear and the competence/experience of the person using it. But you can extract a decent model in a few seconds with a modern VNA for example.Better simulators (expensive ones) can handle 'n port' models for n>2 allowing decent (linear) modelling of more complicated transformers.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: January 28, 2016, 01:23:36 am
content: Problem is that there are no good models for ferrite core materials that work in RF and microwave frequency range. And that is what guy in the video was pointing out.If any manufacturer provides models for their cores that work above let's say 300 MHz I would be curious to check them out. I think it is just like a cure for a rare disease - luck of commercial interest, means no models ...Just found this video- it looks like AWR Office has not only ideal transformer models.

author: G0HZU
author_rank: Super Contributor
timestamp: January 28, 2016, 02:05:08 am
content: If you only need a linear model of the transformer and you have access to a VNA you can take an n port model of the transformer. Any supplied data for frequency behaviour of the core material from the manufacturer becomes irrelevant as does knowledge of the number of turns. The VNA will just spit out an n port model regardless. It doesn't need to know anything about what sits between the ports. It doesn't need to see a data sheet or be told what core material is used in the device under test. It will just measure it and spit out the model.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 28, 2016, 11:09:32 pm
content: Quote from: Alexei.Polkhanov on January 28, 2016, 01:23:36 amWell, how much does that matter? ...And why should it?Would be useful for fine tuning, and high efficiency circuits, I suppose. You're basically asking about an impedance much higher than the circuit impedance, and if it's very lossy or what.Transformers -- as such -- are designed for maximum impedance, so at any frequency where they matter, you simply take the low frequency inductance, give or take.If you're doing coupled inductors, that's different, but you rarely if at all use ferrite for that. Powdered iron is much more predictable, I guess.Regarding core modeling, if you really must know detail that fine, then, even if the material were linear, a complete circuit-level model requires EM analysis of the core geometry itself. Induced magnetic fields, and accompanying EMF, propagate into the bulk of the core, at quite a low velocity (because e_r is modest and mu_r is huge), and reflect around, leading to standing waves at quite modest frequencies. Loss also yields a diffusion and skin effect phenomenon, which dampens and broadens the impedance peak and, obviously, leads to a largely resistive component above "cutoff".So, in a nutshell, that's why models of cores are almost completely absent, and wouldn't be very useful anyway, unfortunately...And, if you were ever wondering about mu', mu'' vs. F curves (commonly provided for core materialsof a given size!), yes indeed, they are only representative of the cores measured.This is why you can have a 16mm toroid in #43 ferrite with a |mu| peak around 10MHz, or a ferrite bead-on-lead with a |Z| peak of 100MHz, or a ceramic chip peaking at 500MHz! (Not that |Z| is the same as |mu|, but it implies that the |mu| equivalent isn't dropping at -20dB/dec above the rolloff point, which is interesting behavior, and useful of course for ferrite beads.)The ultimate frequency limit of ferromagnetism is actually some GHz, where the spin dipoles can't respond any faster.Tim

author: cdev
author_rank: Super Contributor
timestamp: January 28, 2016, 11:43:39 pm
content: It doesn't matter that much. I guess I am just trying to gain a deeper understanding of what's going on faster. Shortening the feedback loop between trial and error..I enjoy working with RFSIM99 to make a filter and it certainly saves money too..Does the information provided by magnetic material manufacturers adequately describe their products so that something like that can be adequately simulated, with commercial software? Or is there still a missing something?

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: January 29, 2016, 12:01:06 am
content: This is a link to a page for AWR Microwave Office documentation returned by Google:https://awrcorp.com/download/faq/english/docs/Elements/XFERTL1.htm. In a model that they use properties of the core (mu) are "embedded" in "inductance per turn" If I understand right.T3sl4co1l mentioned #43 ferrite for example, it has max frequency at 10MHz according to specification. Yet I can often find a balun with same core designed for 1GHz range or mixer for 2.5GHz. How? I mean how do they design it? Which models or formulas they used? Whoever makes this cores provides a model that is useful for 99% of their customers and 0-10 MHz data is enough for them. Remaining 1% are those that make mixers for example at 3GHz and they will have to use their own equipment to get data for their own models etc. etc. This is not only happen for cores - operational amplifiers with very low input current - same story. Companies like Modelitics (http://www.modelithics.com/products.asp) try to feel the void for some components.Core manufacturers often do not provide specifications for their cores in RF/Microwave frequency range. They can be found from alternative sources only. Try going to Medelitics site and type "ferrite" in search window.

author: G0HZU
author_rank: Super Contributor
timestamp: January 29, 2016, 12:51:05 am
content: You can use a VNA to measure a core based coil (or transformer if you have enough analysis ports) up to very high frequencies and then extract an s parameter based model. Obviously, this only provides a small signal/linear model but this is often good enough for many purposesYou mentioned #43 material and by pure coincidence I did do a few measurements of a simple 10 turn inductor on a FT37-43 core a few days ago on another forum linked below.http://www.vintage-radio.net/forum/showthread.php?t=122842This is only a crude 1 port model but I can measure up to 4 ports very quickly with this VNA and higher than this if I allow lots of time. However, I've not tried going beyond 4 ports with this VNA.But you can see I attached the raw 1 port models for the new and old versions of 10 turn FT37-43 toroids at the bottom of post #7 in that thread on the other forum.If anyone wants to see a more complicated model of a proper FT37-43 transformer I could measure one over the weekend? Maybe a 2 port model and also a 3 or 4 port model of a simple twin winding on the core? I can't measure below 300kHz with this VNA but it can go up to many GHz so how about I show a model up to 250MHz?The two port model could be used in RFSIM99 but it can't import 3 or 4 port models. But you can use these models in AWR MWOffice if you have a demo or student version.Or you could just go to the MiniCircuits websiteand download their n port models for a few of their RF transformers. The VNA that generated these models at MiniCircuits doesn't need to know ANYTHING about the core material used or the windings. VNA's can't see to read datasheets or see the core material or look at the winding info but you can still extract a valid small signal AC model right up into the UHF region using an n port VNA. The proof is in these MiniCircuits models.In the link below you can find a 5 port s parameter model of a ADT4-1WT transformer that has 5 connections that goes up to very high frequency.http://194.75.38.69/MCLStore/ModelInfoDisplay?14540276433740.5860461671361954Obviously, you need a decent RF simulator that can cope with 5 port models but you get the idea

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: January 29, 2016, 02:11:27 am
content: G0HZU of course everyone can use VNA and get S-param set but why? S-parameter set is not a model it is just a data set that will turn a transformer into a black box. Can you change number of turns in primary winding of your black box, NO, it is a black box. Can you change size of core, NO - it is a black box! Essentially it will be of very limited use.If you have a model you can attach number of turns or any other parameter of a model to a variable and then simulate it 100s of times with different values. Then you peek the optimum values for your design - that is why we need a useful MODEL, ideally one that models physics of the transformer as close as possible.

author: G0HZU
author_rank: Super Contributor
timestamp: January 29, 2016, 02:16:13 am
content: I can take a fresh 4 port model of the transformer and import it into the RF simulator (via GPIB) as a new model about as quickly as you can alter the winding by hand. Just a few mouse clicks... maybe just a few seconds... How quick does it need to be for each modification to the transformer?

author: uncle_bob
author_rank: Unknown
timestamp: January 29, 2016, 02:29:25 am
content: HiThere are some very different things being tossed around here:1) The design of an RF transformer. For that the answer is no, you can not pop up a program and come up with one that does "this" and "that".2) The analysis of a circuit with an RF transformer. For that the answer is most certainly yes. There are a wide range of ways to do that to just about any level of precision you require. There are numerous papers on modeling the various odd things that turn up in RF transformers.3) The automatic translation of physical design details to circuit models. This is the reverse of the design process. The same limitation applies. There is no perfect formula for doing this. There are just guidelines and then you test to see how close you are.Depending which of the three questions is being asked, the answer will (of course) be different.It's not actually very different with a *lot* of RF components. Even something as simple as a coil has many of the same limitations.Bob

author: G0HZU
author_rank: Super Contributor
timestamp: January 29, 2016, 02:36:32 am
content: QuoteI would normally prefer to use the s parameter models from MiniCircuits or my own VNA based models for any critical RF design using a transformer. Especially at the concept stage. These transformer models are useful for predicting isolation, amplifier stability if it uses the transformer, frequency response, return loss etc etc.A good example would be a transformer based directional coupler. How would you initially test such a coupler for performance into various test loads across a wide bandwidth? I would initially do it with a VNA measured 4 port model on a simulator. This would quickly show if the coupler is a suitable contender for further analysis. It would be much quicker than doing it all by hand and I can't think of any other way I'd rather model this coupler.Can you suggest a more suitable method to model it?

author: G0HZU
author_rank: Super Contributor
timestamp: January 29, 2016, 02:53:11 am
content: QuoteAgreedQuoteYup. Modelling components and circuits effectively* is one of my passions and I've been doing this stuff for many years now.* Sometimes the component model can be a very crude one. When it comes to transformers I've often used simple/ideal transformer models and sometimes I've added parasitics. For more critical stuff I often use s parameters. It just depends on how good the model needs to be for a given task.

author: cdev
author_rank: Super Contributor
timestamp: January 29, 2016, 05:53:33 am
content: I'm trying to get rid of noise and improve reception of weak radio signals on HF with a simple, easily adjustable - selective preselector.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 29, 2016, 07:49:07 am
content: So would that be like, a matching transformer for various parts of the HF band, to a high Q tank coil? Thus, the matched resistance might potentially be quite low (indeed, induction heating coils in the 20-400kHz range typically place in the 50mohm range, so this is not unusual from my experience!), needing a transformer?Nice thing about that is, if the ratio is large, you should attempt to design it to use only one secondary turn. This may require a stack of cores, to keep the impedance (or saturation flux) high enough (probably not, for HF use, but for those induction heaters delivering real power, it's typical), and then whatever turns make up the primary, are wound on the core first. Now, a single turn secondary, for a toroidal core, means using two flat ring (disk with a hole missing) shaped pieces of metal, and two cylinder/hoop pieces, one that fits outside the core, and one inside. Solder them all together, except that you leave a gap all the way around one joint rather than closing the seam: this seam instead connects to, say, both layers of a PCB enclosing the outer hoop. Thus, for example, the top layer connects to the outer hoop, which connects to the top disk, which connects to the inner hoop, which connects to the bottom disk and the bottom of the PCB.The cross section might look like this:Not that you have to use PCB or anything.Of course for a straight core (like you'd get from a CRT flyback transformer), you'd use a split cylinder for the one-turn secondary, and put the primary inside that.As for physics, the voltage on the secondary is basically zero relative to any voltage developed by the primary, so it looks like a solid shield all the way around. Which means the primary winding exhibits a characteristic impedance against it (and against the core, but the core isn't usually very conductive, so can be ignored). Which means, when you apply voltage to the winding, it will take [electrical length] for a wave to propagate down, charge the line, and reflect back. So if you need to meet a constraint like 1/10 lambda at 30MHz, you need a total winding length less than 1m.Doing an entire transformer design, based on winding length, isn't a very well defined problem, so you'll probably want to just shop around and look for cores that have enough inductance or impedance/turn^2 to fit the problem, and a small enough cross section so that the winding length isn't exceeded. Small, fat toroids are preferable here, as they have more inductivity (of course, you need to be able to fit all the turns through the center hole, plus a tube for the secondary winding path).And speaking of impedance, guess what? Naturally, you want that impedance (of the primary wire, inside the secondary 'shield') matched or at least similar to the circuit impedance.If you can't get a high enough ratio with just one transformer, it might be reasonable to cascade two. You can save on winding length this way, extending the upper frequency limit; the total inductance drops though, raising the LF cutoff somewhat.If you need only modest ratios, but a wide frequency range, you'll probably need to use a multifilar design instead. You'll want to twist together a number of wires, wind the combination around the core, then connect the strands appropriately (series-parallel combinations). The time/length constraint is on the total length of the winding, which includes adding up all series segments together.The arrangement of wires, before winding them on the core, should be appropriate for the circuit impedances being used. (You can use an arbitrary TL calculator, like ATLC, to determine the approximate impedances of combinations of wires.) The low frequency explanation is: you want the wires closely coupled, to minimize leakage inductance; but if this causes parasitic capacitance to be too great, HF performance will suffer just as badly. A compromise is needed; and this corresponds to a matched impedance for the windings. Indeed, for higher impedances (say 200-300 ohms), it is necessary to space the wires apart additionally (twisted pair doesn't go much over 120 ohms, for common insulation sizes and materials).You can also build an unlimited-HF-bandwidth type autoformer, by observing that transmission line ports can be connected in series and parallel, just like resistors; and that, if you hang enough ferrite beads on a transmission line, the common mode current (between the two ports) goes arbitrarily low, so the two ends are free to be connected into arbitrary series-parallel combinations.Tim

author: cdev
author_rank: Super Contributor
timestamp: January 29, 2016, 11:13:53 pm
content: Basically, its exactly that, a tuned tank circuit connected to a transformer. Its a resonant circuit at the frequency you're listening to (or sending out at, the two are the same) but a black hole for RF (ideally) at higher and lower frequencies.I'm trying to visualize the additional material around the toroid, I am guessing you're suggesting that I make a shell of sorts with a particular structure using PCB - so to speak by putting the PCB - etched with the appropriate pattern, inside - but not fully, the electromagnetic field?I have been using multiple toroids. I think that perhaps a balun core would be better but am waiting for them to arrive in the mail, all of my current balun cores are tied up at the moment(I made a clone of the quite useful Elecraft switchable 1:1 - 1:4 balun with two of them and it works great!)

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: January 29, 2016, 11:19:12 pm
content: cdev you never mentioned what is your frequency range?

author: cdev
author_rank: Super Contributor
timestamp: January 29, 2016, 11:42:09 pm
content: It would be ideal if I could get it to work from 1.8 to 29.7 MHz (the range covered by my receiver)Using multiple stranded windings and switching them into several different, but matched combinations might work well (to change the effective ratio of the coil) .My home-made copy of the newer Elecraft balun does something like that (it only uses a very few turns, surprisingly) and works quite well, as a broadband, moderately impedance-switchable balun, (i.e sometimes 1:1 works better, sometimes 4:1 works better.)However, what I am trying to do is learn how to model these kinds of transitions - with the practical use of making better radio related hardware, a spectrum of hardware. I do do stuff with VHF-UHF but thats not what I am talking about with this specific device right now. I have had better luck than I was expecting with VHF/UHF experimentation in the past (low noise amplifiers for receive) Ive done a fair share of that so I would like to learn skilsl which would be applicable to a wide range of frequencies, however right now I'm talking about a specific HF tool.I've tried a bunch of different kinds of preselectors but this specific design looks potentially quite good if I can get decent performance out of it across this basically 20-1 frequency range.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: January 29, 2016, 11:59:32 pm
content: Well, I should have asked the question firstbecause I started thinking about this problem from assumptions that you are talking about 1-6GHz hence "RF". Life is soooo much easier below 200MHz!

author: uncle_bob
author_rank: Unknown
timestamp: January 30, 2016, 01:58:44 am
content: HiSo really we are really looking for is a cookbook?That certainly was not on my list of items at all.As noted in other posts. If you are looking for a cookbook, it's best to spell out what you are after in pretty full detail.Bob

author: G0HZU
author_rank: Super Contributor
timestamp: January 30, 2016, 02:35:48 am
content: Like I say earlier in the thread If you just want basic n:1 transformers you can model these in RFSIM99 already because it has a crude transformer model.However, I can't remember if the transformer model is any good...But it is a fairly simple task to hand write a 2 port model for a (close to ideal) 4:1 Z transformer and RFSIM99 can cope with 2 port s parameter files.eg here's about the simplest hand produced model I can think of for a 4:1 unbalanced to unbalanced transformer covering 100kHz to 30MHz. It's not a very sophisticated model but it will probably work very well over 2-30MHz when compared to a well designed TLT for this frequency range if you are just going to put a simple series tank between the two transformers. Port 1 is the low impedance port of the transformer. The transformer is almost ideal over the whole frequency range. In reality, a typical TLT will roll off down around 2MHz and will have some loss at higher frequencies. You can see that I've added a small amount of loss in the model, where the 0.795 should ideally be 0.8. There should also be some departure from ideal phase shift in the model but I've left the angles at 0deg and 180deg. This will probably just affect the tuning accuracy of the peak of the series tuned circuit a little bit up in the higher parts of the band.# MHZ S MA R 500.1 0.6 -180 0.795 0 0.795 0 0.601 030 0.6 -180 0.792 0 0.792 0 0.601 0It's only a starting point but I suspect that this model will be good enough for some early work in RFSIM99 for a 4:1 transformer. You could probably use it even if you cascaded two of the models to get a 16:1 transformer but it would probably be wise to produce a more sophisticated model for this, especially if you want to model a typical TLT down near 2MHz. The model above is too close to ideal down at 2MHz but it is probably going ot be very useful over most of 2-30MHz.I've put the completed s2p file in a zip attachment below and you should be able to import this into RFSMI99 as a 2 port s parameter file. Try importing two of them and then place a series LC tank in between them and simulate it. It should show a preselector response.

author: cdev
author_rank: Super Contributor
timestamp: January 30, 2016, 03:32:58 am
content: Quote from: uncle_bob on January 30, 2016, 01:58:44 amI edited my last post to make it clear what I would like to do, basically I am just beginning with this, would like to get my ham ticket and get much more serious about QRP, because Ive found I really enjoy making some radio thing work.Its very relaxing for me. So i hope for a deeper understanding. I don't have any illusions about ever making it a professional anything, however, I enjoy things a lot more when i know a bit about what's happening with something - at least to get over the hump at the beginning, deeper knowledge means more of a flow experience - so its more fun.

author: uncle_bob
author_rank: Unknown
timestamp: January 30, 2016, 05:34:19 pm
content: Quote from: cdev on January 30, 2016, 03:32:58 amHiOk, well, there are indeed specific designs that have been worked out already to handle most standard matching chores that you run into. There also are pretty good articles showing what these designs are and how they apply. The gotcha is that one set works at microwaves, another set at VHF, yet another at HF. If you are working below HF, the set of things you do changes a couple of times before you get down to audio. Digging into each of them is fun and exciting stuff. You pretty much need to define a frequency band before you start. Toa lesser extent you also need to decide on power level.Bob