Source of three-winding signal transformers - Page 1
Forum section: electronics / projects

author: matthuszagh
author_rank: Frequent Contributor
timestamp: November 21, 2022, 04:04:51 pm
content: I'm replicating a design (low phase noise 10 MHz buffer, first schematic here:http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html) that uses a three-winding signal transformer. It's possible to modify this design without too much difficulty to use a typical two-winding transformer and blocking cap, but the simulations indicate modestly better performance using the the three-winding transformer. However, while two-winding signal transformers are easy to find, I'm having trouble finding three-winding varieties. I typically use coilcraft or minicircuits for this sort of thing, but haven't found much there. Is there a better source for this? What do other people do?

author: Terry Bites
author_rank: Super Contributor
timestamp: November 21, 2022, 10:10:34 pm
content: For one to a few off: Get a torroidal or binoucular core and some magnet wire.Dip it in laquer or epoxy.An Al value of about 100 would be about right.https://www.tdk-electronics.tdk.com/en/529426/products/product-catalog/ferrites-and-accessories/ring-double-aperture-cores.

author: planet12
author_rank: Regular Contributor
timestamp: November 22, 2022, 03:09:42 am
content: As above, for small quantities/prototyping, DIY is perfectly feasible.A couple of good starting links:http://ludens.cl/Electron/Magnet.html(best introductory text I've found)https://web.archive.org/web/20150107073523/http://wiki.4hv.org/index.php/Gate_drive_transformer(archive.org link as the real site is returning blank pages)https://sound-au.com/xfmr.htm(more about power/audio coupling transformers, but good nonetheless)

author: jonpaul
author_rank: Super Contributor
timestamp: November 22, 2022, 07:25:18 am
content: Micrometals powdered iron or TDK/EPCOS/Fair Rite ferrite toroid.LF cut and Zo determine # turns for Lpri.Chosse material appropriate for freq bands.small (10..15 mm ) toroid, 3 legnths of magnet wire, wind trifilar.Ask the designer for the transformer used.Jon

author: matthuszagh
author_rank: Frequent Contributor
timestamp: November 24, 2022, 04:48:14 pm
content: Quote from: jonpaul on November 22, 2022, 07:25:18 amHow do I wind trifilar when the wires are of different lengths? Do I not want each individual wire to cover most of the toroid?

author: jonpaul
author_rank: Super Contributor
timestamp: November 24, 2022, 06:35:17 pm
content: depending on the ratio, you wind several wires twisted to gather, and connect windings to achieve desired ratios.See how amateurs wind balunsJon

author: TimFox
author_rank: Super Contributor
timestamp: November 24, 2022, 06:38:03 pm
content: Have you looked aat COTS RF transformers from MCL?https://www.minicircuits.com/WebStore/Transformers.html

author: matthuszagh
author_rank: Frequent Contributor
timestamp: November 24, 2022, 11:40:42 pm
content: Quote from: TimFox on November 24, 2022, 06:38:03 pmI have, but I only see 1:1:1 and 1:1:2. I need to 1:4:16.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 25, 2022, 08:04:31 am
content: Quote from: matthuszagh on November 24, 2022, 11:40:42 pmYour first schematic showsL2 = 1 uHL3 = 4 uHL1 = 16 uHAs Ln = N^2 * LbaseUsing L2 as a baseL2: L3 ratio N is sqrt(4/1) = 2L2: L1 ratio N is sqrt(16/1) =4So the transformer is 1:2:4, using L2 as the base.This transformer has DC in L1 and L2.So the initial Al value may not be accurate on ungapped cores. A gapped core with bobbin may be a preferable prototype.The core selection, firstly frequency, then Al to get a practical number of turns on L2 and L3Looking at a TDK, there is a pot core P26/16 material K1, Al = 100 nH.That would needL2 = 3 or 4 TurnsL3 = 6 or 8 TurnsL1 = 12 or 16 turnsCheck my figures before using!I would initially try winding them concentrically with interwinding insulation of Kapton over each.First L2, then L1 then L3Testing prototype to check inductances with DC is not easy.The way I would do it, is add a 16T test winding on top and feed that with equivalent I_collector_dc from a higher voltage bench supply via two high value resistors.A pair of chokes would be better but more involved.

author: jonpaul
author_rank: Super Contributor
timestamp: November 25, 2022, 08:39:09 am
content: Hello againTo build this buffer,your transformer ratios are wrong.no the trsf is 1:2:4The inductance is proportional to the SQUARE of the turns.http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html"The Buffer amplifier depicted below uses transformer(1:4:2 turns ratio)"Schematic has 1uH, 4uH, 16 uH!(poorly drawn as L1,2,3 are transformer windings)Take 7 strands of wire, tag the ends.Twist togather, wind on toroid coreL1 = 16 uH = 4 turns = 4 wires in seriesL2 = 1 uH = 1 turnL3 = 4 uH = 2 wires in series.Should be a few minutes of work after the wire and core are selected.Suggest to ask ko4bb for pix of the trsf and more info re core, etc.These Coilcraft WB trsf are tapped one may work if you use the taps.https://www.coilcraft.com/getmedia/a455fe28-d328-4aef-ae99-abd4ca49066a/wb.pdfBon Chance!JonPS: If this is for a low nose clock buffer eg TCXO/GPDSO, 10 MHz house/ref clock, we found the Leo Bodnar GPDSO at 10 MHz and max power can easily drive 4..12 50 Ohm loads as REF clock.We use the Mini Circuits BNC 50 Ohm splitters in tandem. No added nose, totally passive!

author: matthuszagh
author_rank: Frequent Contributor
timestamp: November 26, 2022, 04:42:40 pm
content: Quote from: mag_therm on November 25, 2022, 08:04:31 amQuote from: jonpaul on November 25, 2022, 08:39:09 amAh, yeah that's a bit of a careless mistake, thanks for correcting me.Quote from: jonpaul on November 25, 2022, 08:39:09 amThanks! This seems like a good method and is the information I was missing.Quote from: jonpaul on November 25, 2022, 08:39:09 amUnfortunately I don't think that will work. None of the transformer winding leads can be shorted together. Otherwise these would have been my first choice (and is what I'll use if I get rid of the third winding and use the blocking cap).Quote from: jonpaul on November 25, 2022, 08:39:09 amI've heard good things about the Leo Bodnar GPSDO, but the OCXO here is fixed (HP 10811A). There's a potential problem with that setup though, which is that a passive splitter provides very little reverse isolation. That makes injection locking possible, which I need to avoid. It also doesn't help if you need more than a few copies of the signal, but distribution is a bit of a separate issue.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 26, 2022, 06:04:41 pm
content: The designer's transformer details don't seem to be available.He states 30dB of reverse isolation for circuit 1.For improved isolation I think it best to avoid using the bifilar winding method for the output winding called L3.If there are say 12 turns on a bobbin with average layer radius of 12 to 15mm, then a bifilar L3 might be in contact withL1 and L2 for a conductor length of 500 to 600 mm.I calculated a bifilar inter-winding capacitance of ~25 pF. : At 10 MHz, -j680 Ohm, At 20 MHz, -j340 Ohm, At 30 MHz, -j260 OhmThere is a 2nd harmonic in the collector voltage being corrected by negative feedback in the emitter.There could also be 3rd harmonic from the transformer core.I think it is OK to wind L1 and L2 bifilar, but wind L3 as a separated winding on top to minimize the mutual capacitance.There is a way to add a grounded foil shield between L2 and L2.(I recall doing it once on a pot core I think, requiring skilled hand work by the transformer winders.)My buffer has no transformer, output is just an emitter follower.Here is a trace (0017) showing harmonics with fundamantal at 14 MHz, 2nd is -43dB and 3rd is about -60dB.Actually the harmonics are about same as in source sig gen.https://app.box.com/s/ehu052fv6of2rlsqdqutdeo34c0v9b6sHere is the buffer reverse isolation switched on (0018), with sig gen feeding output, turned up to 0dbm and FFT at top (-0dBV)Reverse isolation of fundamental is about -50dBhttps://app.box.com/s/sy2vihchmbpwuivaibor3a59lirh76reHere is circuit of my Fet-Bi buffer (posted before)https://app.box.com/s/74j8faqyjbk1xsjudkw7ngy9s4q5jnyi

author: matthuszagh
author_rank: Frequent Contributor
timestamp: November 26, 2022, 07:49:18 pm
content: Quote from: matthuszagh on November 26, 2022, 04:42:40 pmActually, I think I was right. Minicircuits gives an impedance ratio, not a turns ratio, so it should be 1:4:16.

author: matthuszagh
author_rank: Frequent Contributor
timestamp: November 26, 2022, 07:59:38 pm
content: Quote from: mag_therm on November 26, 2022, 06:04:41 pmI hadn't considered the inter-winding capacitance and it's effect on reverse isolation. Thanks for pointing that out. Anyway, that does sound a bit tricky. I may just build this without the third winding and with a blocking cap instead. The simulations for this still look really good and someone else built it this way and achieved good results.Quote from: mag_therm on November 26, 2022, 06:04:41 pmThanks for sharing!

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 26, 2022, 08:08:06 pm
content: Actually, I think I was right. Minicircuits gives an impedance ratio, not a turns ratio, so it should be 1:4:16.Yes, You were always 100% right.But if you can't buy one and need to wind one...

author: matthuszagh
author_rank: Frequent Contributor
timestamp: November 27, 2022, 04:19:05 pm
content: I ran some simulations to see the effect of interwinding capacitance and the reverse isolation really isn't affected at all, and is only barely affected with grossly exaggerated values. Interwinding capacitance between the emitter and collector (L1 and L2) can kill the gain but for 25pF, the effect isn't noticeable at 10MHz where this will operate. If I wanted to use this at higher frequencies, e.g., 100MHz, it would matter though.I emailed the original designer. It's been a while since he built this, but he did wind it himself and thinks it was with a binocular core he had lying around. I got the sense that he didn't feel the transformer selection was too critical here, just to make sure the core doesn't saturate.I'll probably go back to the earlier plan of winding my own on a suitable toroid. I'm tempted to use Litz wire too to minimize the resistance at higher frequencies. I'll post back with results.

author: jonpaul
author_rank: Super Contributor
timestamp: November 27, 2022, 04:56:21 pm
content: Bonjour Matt,The fine HP TCXO like 10811A are very well designed, and have an internal buffer.Most of the instrument clock réf inputs you will drive are 50 Ohms and also buffered.VSWR should be 1..1.5.Hence I do not think a passive splitter will cause injection locking from RL reflection.Why not just try a simple resistive Y splitter and see if the HP10811A output frequency is affected by the loads. Finally, suggesting that you check HP spec and schematics.Bon chanceJon