EMRFD Message Archive 12107
Message Date From Subject 12107 2015-12-29 18:59:22 Ken Chase Toroid impedance transformers
I'm looking for guidelines to winding a toroid for impedance transformation.
Right now I'd like to transform 200 ohms to 50 ohms.
I already know that the impedance transform is the square of the winding ratio. So a 2:1 winding ratio is 4:1 impedance ratio.
But how many turns are required for each winding. Does the reactance of the windings play in the actual turns required?
12108 2015-12-29 20:19:34 kerrypwr Re: Toroid impedance transformers The inductive reactance of the low-Z winding (ie, one wire of a bifilar pair) should be somewhat greater than that Z; I have seen rules-of-thumb ranging from four times to ten times.
On that basis, the inductive reactance of the winding on the 50-ohm side would range from 4x50 (200 ohms) to 10x50 (500 ohms).
The basic bifilar 200 ohms - 50 ohms transformer often seen in EMRFD at the output of an amplifier has about ten turns bifilar on an FT37-43 toroid; that has an inductive reactance of about 560 ohms at 3 MHz.
For a wideband transformer the winding should be no longer than the length required to give good performance at the low-frequency end of the transformer's design range; the shortest possible length will ensure good performance at the high-frequency end, ie the widest-range broadband transformer.
12111 2015-12-30 07:07:11 Graham / KE9H Re: Toroid impedance transformers --- Graham / KE9HIf you have an antenna analyzer, you can use it to measure transformers with 50 Ohm windings, and learn a lot about what is going on.If you want the lowest operating frequency of the transformer to be 1.8 MHz, and the winding is supposed to operate at 50 Ohms, then the reactance of the winding at 1.8 Mhz should be at least 200 Ohms, or exhibit at least 18 milli-Henries inductance.So, by example:Ken:The "rule of thumb" is that you need the inductive reactance of each winding, at the lowest frequency of operation, to be greater than four times the operating impedance of the winding in the transformer.
12113 2015-12-30 09:36:58 Claudio Girardi Re: Toroid impedance transformers Hello,
when winding wideband transformers on ferrite cores it can be useful to use more turns than suggested by the simple "rule of thumb" as this can provide slightly lower losses. Then, depending whether the transformer is for a kW TX or a low-level preamp, this may be more or less important.
Some measurements for a 1:1 transformer are at http://www.qsl.net/in3otd/ham_radio/160m_transformers/160m_trafos.html
In this particular case, two turns provide (in theory*) more than 400 ohm of reactance but using three turns halves the losses (which are a fraction of a dB anyway).
73 de Claudio, IN3OTD
(* reactance should be computed using the material permeability at the operating frequency)
Data: 30-dic-2015 3.07 PM
Ogg: Re: [emrfd] Toroid impedance transformers
--- Graham / KE9HIf you have an antenna analyzer, you can use it to measure transformers with 50 Ohm windings, and learn a lot about what is going on.If you want the lowest operating frequency of the transformer to be 1.8 MHz, and the winding is supposed to operate at 50 Ohms, then the reactance of the winding at 1.8 Mhz should be at least 200 Ohms, or exhibit at least 18 milli-Henries inductance.So, by example:Ken:The "rule of thumb" is that you need the inductive reactance of each winding, at the lowest frequency of operation, to be greater than four times the operating impedance of the winding in the transformer.
12114 2015-12-30 09:53:53 Ken Chase Re: Toroid impedance transformers
I have a N3ZI DDS2 and claims to be 200 ohms impedance. I would like to use it as a source at 7 MHZ. Using the half voltage method to measure impedance at 7 MHZ, I come up with about 90 ohms. So to transform this 90 ohms to 50 ohms, 90:50 = 1.8:1 impedance ratio. Turns ratio is the root of 1.8 = 1.34.
If secondary is 10 turns(50 ohms) then primary is 13.4 turns or 13 turns. Correct?
12115 2015-12-30 12:29:48 kerrypwr Re: Toroid impedance transformers Ken; your description is of a conventional transformer.
Tx-line transformers usually work better, they are more broadband and have less loss, but, if you are only concerned with 7MHz, that doesn't matter much.
Your maths is correct; 1.8:1 turns ratio.
This is also the SWR in a 50-ohm system; 1.8:1 is not unduly high and might well be OK in your application, ie no transformer.
Another option that I have used in the two DDS generators that I have built is an output amplifier; I see that your N3ZI generator does not have one.
The amplifier could be a buffer, ie high-Z input, 50-ohm output; there are several schematics for that on the 'net and I have used this on occasion.
But, for my generators, I used a 50-ohm in & out transistor amplifier with about 15-20dB gain; this allows attenuation to further stabilise the output Z at 50 ohms.
The kind of amplifier used as a post-mixer amplifier (see EMRFD or this excellent design by Dave G4AON; http://s4.postimg.org/s8xtvrb71/G4_AON_Post_Mixer_Amp.gif ) will work very well.
Both my DDS kits came with an MMIC output amplifier that was obviously intended for gain & impedance stabilisation but the transistor amplifiers that I built to replace the MMICs gave much " cleaner" performance in each case.
If you do build a conventional 1.8:1 transformer, wind the shorter winding onto the centre of the larger winding as for a bifilar Tx line transformer; I have found that gives the best coupling. A winding on each side of the toroid will still work but there is more leakage inductance which will impair the operation of an "ideal" 1.8:1 transformer.
I think that 9 & 5 turns on a FT37-43 would do; this meets the 4x rule.
If you want more turns, something like 14 & 8 is close; there is wide latitude to vary the 1.8:1.
The downloadable MiniRing Core calculator will work-out the wire lengths; allow a little more for the short winding and wind from the centre of the twisted pair, one turn this way, one turn that way until you get your short winding on. Then unwind the extra length you twisted for that winding and continue with the larger winding until it's finished.
12116 2015-12-30 12:48:10 Dave Re: Toroid impedance transformers Claudo and Ken:
The low frequency will be enhanced with more turns, but the high frequency end can suffer. When the length of wire starts being an appreciable part of a wavelength, losses increase.
Dave - WB6DHW
12117 2015-12-30 13:50:01 kerrypwr Re: Toroid impedance transformers For information; the best reference on RF transformers that I know-of was published by Philips; app note ECO6907 covers transmission-line transformers whilst ECO7213 covers conventional transformers.
Both are available for download; Google will find them.
12118 2015-12-30 14:05:07 Lasse Moell Good Articles: Toroid impedance transformers For those who are utterly bored by sitting in front of the telly... I do have some good readings on wide band transformers :)Ruthroff: "Some Broadband Tansformers" IRE august 1959Pitzalis: "Broadband Transformer Design for RF Transistor Power Amplifiers" Unknown sourceMaddox: "Wide Band Balanced and Screened Transformers in the Range 0.1-200 Mc/s" Electronic Engineering nov 1957You do find them at:Have yet to locate the Guanella paper from Brown Boveri Review 1944, anyone?/Lasse SM5GLC30 december 2015 22:49:51 +01:00, skrev email@example.com [emrfd] :For information; the best reference on RF transformers that I know-of was published by Philips; app note ECO6907 covers transmission-line transformers whilst ECO7213 covers conventional transformers.Both are available for download; Google will find them.Kerry VK2TIL. 12119 2015-12-30 14:12:26 kerrypwr Re: Toroid impedance transformers The Guanella paper is in the Files section of this Group; look for WB6TPU and guanella.pdf.
12120 2015-12-30 14:15:09 Lasse Moell Re: Toroid impedance transformers Great!Cheers/Lasse SM5GLC30 december 2015 23:12:25 +01:00, skrev firstname.lastname@example.org [emrfd] :The Guanella paper is in the Files section of this Group; look for WB6TPU and guanella.pdf.Kerry VK2TIL. 12121 2015-12-30 16:45:57 Ronan McAllister Re: Toroid impedance transformers I am a real beginner here, the subject is very interesting!Of course papers by Ruthroff including "Some Broad-Band Transformers", the Guanella paper previously mentioned are WELL worth reading first.In an excellent article by Wes Hayward, W7ZOI “An Updated Universal QRP Transmitter”, Apr 2006 QST you will find interesting application of bifilar-wound wideband impedance-matching transformers in Fig. 1 and accompanying text, used in interstage impedance matching (between stages in RF PAs). If it's not on the EMRFD accompanying DVD (I don't have access to -- on the road) it may be elsewhere on the 'net.In one quick test of a preamp to driver RF stage, I used10 bifilar turns of #28 on FT37-43 to transform the approx. 200 ohms of the driven element (collector) to 50 ohms of the driven stage (base) ; I think you will see this is a pretty standard technique, but I've not had much of a chance to play with it in awhile.This may be a useful paper on impedance-matching wideband RF transformers: http://mwrf.com/components/designing-wideband-rf-impedance-transformers?page=173's,Ronan/KB6NHQ 12122 2015-12-30 17:30:04 kerrypwr Re: Toroid impedance transformers I've just realised that I made a silly mistake in my earlier post; the impedance ratio is 1:1.8 but, as Ken had already said, the turns ratio is 1:1.34.
A turns ratio of 6:8 will do; on a FT37-43 toroid the 6-turn winding will have an XL of 440 ohms at 7MHz.
12123 2015-12-30 22:24:35 Ken Chase Re: Toroid impedance transformers I used a 10 turn 50 ohm secondary to keep the high XL. But if a 8:6 turns ratio is OK, I can live with that.
I can expand what I am doing with the toroid trnsformer.
I am testing a RF preamp. It has an i/p 50 ohm impedance. The only 50 ohm o/p signal source I have, ready made, is a SI5351. But it's full of harmonics since it's a square wave.
I also have the N3ZI DDS2 but claims it is 200 ohm impedance. Using the half voltage test on the DDS at 7 MHZ, I measure 90 ohms. So I assumed I could wind a toroid to transform 90 ohms to 50 ohms.
According to the toroid calculator on KITSANDPARTS website for the FT37-43, 6 turns is 560 ohms XL. This is 10 x 50 ohms XL.
12124 2015-12-30 22:25:10 Ken Chase Re: Toroid impedance transformers Google searching on "An Updated Universal QRP Transmitter" pulls up ARRL first.
Wes uses a 10 turn bifilar FT37-43, but the connecting of the windings is important. So I guess there is 2 ways to use a toroid. A bifilar wound toroid or a turns ratio toroid, in my case a 8:6 on a FT37-43.
12125 2015-12-30 22:25:26 Ken Chase Re: Toroid impedance transformers I made a mistake in my last reply comparing Wes' bifilar toroid and winding a 8:6 toroid. Wes' toroid is for 200 ohm to 50 ohm, the 8:6 is for 90 ohm to 50 ohm
12126 2015-12-30 23:30:29 kerrypwr Re: Toroid impedance transformers Ken; yes, there are "transmission-line" transformers and there are "conventional" transformers.
Conventional transformers are like the common power-supply transformer; they have two separate windings that are coupled by flux in the magnetic-material core.
They are certainly used and useful at RF, particularly if the frequency range is not wide; they provide "odd" impedance transformations if that is required.
Tx-line transformers behave much like conventional transformers at low RF but, as frequency increases, the initial permeability (the u' component of complex permeability) of the ferrite (the property that gives inductance in a winding) reaches a certain frequency (Snoek's Limit) and, from there, drops fairly quickly; eventually it drops to unity at which point the winding inductance is the same as it would be if wound in-air or on plastic or the like.
(To see this phenomenon, download the Fair-Rite catalogue from their website and find the frequency -v- complex permeability curve for #61; the behaviour is very clear for this material. You will see fairly-constant u' up to a frequency where there is a "hump" (ferrimagnetic resonance) and, after the "hump", you will see that u' quickly drops to unity, equivalent to air).
But, as the frequency increases and the ferrite gradually "disappears", the length of the winding begins to approach a significant percentage of a wavelength; this is assisted, in the frequency region from ferrite full-effectiveness to the point where the ferrite "disappears", by the ferrite's effect on the velocity factor of the Tx line.
A Tx line that is not "short" in wavelength terms has the property of changing impedance; this property "takes-over", with some overlap, from the conventional-transformer behaviour and extends the operation of the impedance transform to higher frequencies.
That is mainly why a Tx-line transformer is inherently wider-band than a conventional transformer.
The (minor) downside to the Tx-line transformer is that it only provides "square" ratios like 1:1, 4:1, 9:1 etc; well, that's not entirely true as some other ratios can be achieved with interconnections that I think that Wes, in EMRFD (or it might be IRFD) refers-to as "diabolical". :)
I commend to you the Philips papers to which I referred earlier; they provide a very good, not over-mathematical, view of both kinds of RF transformers.
12127 2015-12-31 09:10:33 Ken Chase Re: Toroid impedance transformers Hi Kerry
This will be a read for after New Years day where I'll have some time to read and understand.
I wound a FT37-43 with a 8:6 winding and the impedance is very very close to 50 ohms. Yessssssssssss. Now I can use the N3ZI DDS
Thank you and everyone else that contributed to my topic and it was or is a great learning experience.
Happy New Year to you in VK land and to everyone.
12128 2015-12-31 13:06:42 Eamon Egan Re: Good Articles: Toroid impedance transformers VE2EGN, AB1NKEamon EganI have been wondering for a while how valid is the claim that the "transmission-line" aspect of these transformers plays an important role, beyond optimal magnetic coupling (which would be the case for a bifilar twisted pair, I think, regardless of its characteristic impedance). What difference does the characteristic impedance of the bifilar winding in such a transformer actually make, particularly at frequencies far below the point where its length equals a quarter wave?Thank you Lasse, particularly for the Ruthroff article which I hadn't seen before.He also claims that, besides the improved coupling (i.e. lower leakage inductance), the use of a wire pair forming a TL causes the inter-winding shunt capacitance to be absorbed into the TL's characteristic impedance (assuming this impedance is matched to the system impedance).
One of the things I am curious about is how much bi-filar transformers actually rely on the winding pairs acting like transmission lines. Ruthroff writes that a drop in performance can be seen at the frequency where the length inside the transformer reaches a quarter wave; of course, in practice, this won't happen at HF frequencies.
12129 2015-12-31 18:15:58 kerrypwr Re: Toroid impedance transformers I have had similar doubts in the past about the transmission-line effect and I have done some testing, although I don't regard it as conclusive at this stage.
I think that the transmission-line effect certainly exists and that it does assist the transformer operation at the high-frequency end just as the experts say; the problem is that the experts don't explain, at least in language that I understand, just why this is so.
I think that there are several things going-on.
The first is the effect of ferrite on the velocity factor of the line; I first noticed this when testing "string-of-beads" chokes. Depending upon the line and the ferrite, I have seen the 1/4-wavelength resonance frequency lowered by 10-20% with ferrite on the line. In practice this means that the physical length of a 1/4-wave line loaded with ferrite is somewhat shorter than the length of a 1/4-wave line in air.
Another thing to consider is that a 1/4-wave line is quite forgiving in its impedance-changing ability; whilst it is "perfect" at only one frequency, it is "not too bad" at frequencies quite some distance away. For instance the common coupled-line directional coupler, with its operation reliant upon a single 1/4-wave coupled line, is commonly used with tolerable error over a frequency range of up to an octave.
I also think that there is a wide frequency region at the upper end of the transformer's range where the two modes co-exist; for instance, the high (800) u' of #43 is maintained up to less than 1 MHz; by 10 MHz it is down to about 250 and, at 100 MHz, it is about 15. I think that, in this region where the material permeability is falling quickly, both modes of operation contribute.
12130 2015-12-31 19:28:31 John Re: Toroid impedance transformers A Tx-line transformer for 50 to 90 ohms is not TOO "diabolical".
A 6:8 ratio is also a 3:4 ratio. Twist 4 wires together to make a
section of quadarifilar wire. Using several turns of this on the core.
Separate and identify the winding ends, and wire all 4 of the windings
in series. the ends of this will be 90 ohms. Connect across 3 of the
windings for the 50 ohm side. I assume that 2 turns of the quadarifilar
wire through the core and 3 of the wires in series would be the same
reactance as 6 turns of a simple 6:8 transformer.
12131 2016-01-01 08:19:26 Phil Sittner Re: Toroid impedance transformers Kerry-
Jerry Sevick, W2FMI, wrote an entire book on the topic, "Transmission Line Transformers", published by the ARRL in 1987. Jerry presents the results of his experiments (done at Bell Labs) and presents efficiency numbers for most of the designs, and covers both toroid and rod designs. You would likely find it to be interesting reading if you're able to obtain a copy.
12132 2016-01-01 08:31:48 Bill Carver Re: Toroid impedance transformers 12133 2016-01-01 09:24:19 K5ESS Re: Toroid impedance transformers
I assume it’s the same book available from Amidon for $12 at http://www.amidoncorp.com/transmission-line-transformer-handbook/
12135 2016-01-01 09:27:56 Ken Chase Re: Toroid impedance transformers Hi John
I am very curious with your method. I'll have to give it a try.
12136 2016-01-01 09:28:13 Phil Sittner Re: Toroid impedance transformers Mike-
That appears to be the new, improved version. Thanks for the heads up.
12139 2016-01-01 11:05:20 Lasse Moell Re: Toroid impedance transformers The handbook for $12 seems to be focused on actual designs i.e. cookbook style... but Amdion do have the book"Transmission Line Transformers 4th Ed." listed for a mere $72.... have your pick :)/Lasse SM5GLC1 januari 2016 18:28:09 +01:00, skrev Phil Sittner email@example.com [emrfd] :Mike-That appears to be the new, improved version. Thanks for the heads up.PhilKD6RM 12140 2016-01-01 11:22:22 Bill Carver Re: Toroid impedance transformers 12141 2016-01-01 11:32:26 Mike Re: Toroid impedance transformers Here is a link to a .pdf of Jerry Sevick's "Transmission Line Transformers Handbook":
This book ("Transmission Line Transformers Handbook") is considerably lighter on theory than the "Transmission Line Transformers" book(s) by Sevick but provides a lot of examples, with construction details.
12142 2016-01-01 11:47:17 K5ESS Re: Toroid impedance transformers
I seemed to have created some confusion with my post. The $12 book is the “Transmission Line Transformers Handbook” not the one that was originally cited. They do have the 4th edition for $72 and the 5th edition for $84.50 (marked down from $99 Wow).
12145 2016-01-01 12:13:49 Bill Carver Re: Toroid impedance transformers Whew, OK. I did not see that was a cut-down book, so my post below compounded (or created) the confusion. That $12 book probably omits a lot of information.
12148 2016-01-01 19:50:07 firstname.lastname@example.org... Re: Toroid impedance transformers
one often dismisses the lowly air core transformers. there is quite some life in those.
the original bitx used a nylon tap washer as the output transformer. since then i have used RG178u on 2 in dia cores as baluns and 1:4 transformers. thet are capable of handling large power without saturating and they cost pennies.
they have two drawbacks. first, they need shielding. second is that they have interwinding capacitance. this capacitance is easily absorbed as a part of a low pass filter that may preceed or follow it.
12149 2016-01-01 23:03:58 kerrypwr Re: Toroid impedance transformers "Kerry; Jerry Sevick, W2FMI, wrote an entire book on the topic ..."
Thanks Phil; I have the 4th edition.
I greatly admire Sevick; a brilliant man who had a "down-to-earth" approach. I particularly liked the book chapter on his home-made test equipment; wonderful work.
I copied his bridge detector for similar applications in detection of tiny DC signals.
I also admire his patience & skill in making his own "special" transmission lines; that is remarkable "home-brewing".
12150 2016-01-02 04:29:47 Russell Shaw Re: Good Articles: Toroid impedance transformers 12154 2016-01-02 20:41:53 kerrypwr Re: Toroid impedance transformers Thanks Russell; that was helpful.
You have a knack for putting things into simple terms that I can understand. :)
12155 2016-01-02 23:55:31 Russell Shaw Re: Toroid impedance transformers