Broadband Bias tee? - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: June 11, 2020, 11:38:12 pm
content: How would you go about designing a bradband bias tee (this is one of the most interesting simple design problems andIve never been able to get it just right so far) with off the shelf components and nothing too esoteric as far as PCB material or other costly unobtaniums Would a network analyzer like the nanoVNA 2 or basic be helpful in getting it right?

author: TheMG
author_rank: Frequent Contributor
timestamp: June 12, 2020, 12:20:05 am
content: How broadband does it need to be (lowest and highest frequency)?It's tricky because real-world components have parasitic capacitance and inductance, which means the same capacitor or inductor won't work well over a very wide range of frequencies.To tackle this issue, the common solution is to cascade several different inductors/capacitors in the bias tee, starting with ones suitable for handling the higher frequencies and working your way down.This will require trial and error, or a lot of simulation/modelling if you know the real-world specs of the inductors and capacitors you want to use. The more wideband you want it to be, the more complicated this will get and the more cascaded components you will need.

author: cdev
author_rank: Super Contributor
timestamp: June 12, 2020, 01:31:00 am
content: Right, thats been my experience, placement matters a lot too. Ive used a combination of L, C and Rs.. also ferrite beads, and yes its been trial and error and placement matters a lot too. especially for inductors.I only need it to work above maybe 45 MHz up to around ~6000 LOL... (Thats in a perfect world..) I would be satisfied if it worked from half that for now. I have quite a collection now, both store bought and home made.Quote from: TheMG on June 12, 2020, 12:20:05 amActually, as of a couple of days ago I have a new weapon in making inductors some clearnail polish which should allow me to make some conical inductors that stay in one place. Conical inductors are supposed to be the best for broadband bias tees.

author: radiolistener
author_rank: Super Contributor
timestamp: June 12, 2020, 05:31:22 am
content: Quote from: cdev on June 12, 2020, 01:31:00 amyou can use Chinese bias tee, something like that:https://www.aliexpress.com/item/32900337275.htmlit has nice shielding, you can easily desolder connectors, make modification and solder connectors again. I performed that at least 4 timesPower supply is applied through 1 nF feedthrough capacitor.It has some loss above 3 GHz and response is not flat, but it worksAnd if you want you can redesign PCB for better response...

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 12, 2020, 05:40:45 am
content: Hi,There is a bare-naked bias tee.It has better pictures.Regards,Jay_Diddy_B

author: radiolistener
author_rank: Super Contributor
timestamp: June 12, 2020, 05:43:10 am
content: Quote from: Jay_Diddy_B on June 12, 2020, 05:40:45 amThis is the same bias-tee PCB as I proposed above, but with no shielding.I have one with metal shielding enclosure, bought from the link that I proposed above. But I used discount coupon and bought during the sale event, so it cost me something about 8 USD.Here is high res photo:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 12, 2020, 01:45:56 pm
content: In general, for lumped elements, you will have some parallel equivalent RLC for inductors, and series RLC for capacitors. Series branches also have stray capacitance to ground (and parallel branches have stray inductance between them).When you chain inductors in series, each one resonates with the capacitance around it. When the inductors are equal, the resonances aren't quite equal, depending on stray capacitance. For example if you cut out ground under a chain of inductors, the ones closer to the signal and ground will have more capacitance than those in the middle, and you get a group of resonances, with impedance and bandwidth not much more than one alone.If you use a tapered chain, you can have the smallest inductor resonate with the capacitance of the next larger one, and so on. To keep the network impedance high and flat, the resonances must be dampened, and the damping resistance equals the resonant impedance sqrt(L/C). (The resistance might be in parallel with one or the other element, or implemented as an R+C to ground.)Supposedly, conical inductors do this by design, so are often promoted for the purpose. I don't know that they're really necessary. They look neat, if nothing else.Here's a commercial example:https://www.seventransistorlabs.com/Images/Picotronics/Note the use of (geometrically) tapered values, holding a more or less constant resistance (100s to low kohms). Instead of a conical inductor it seems they've gone with a solenoid contacting the signal path.In any case, if a single chip inductor will do the job -- by all means do it!Tim

author: cdev
author_rank: Super Contributor
timestamp: June 12, 2020, 03:23:43 pm
content: Quote from: T3sl4co1l on June 12, 2020, 01:45:56 pmThank you for this breakdown, thats one of the things that I have been the most curious about, what the design process that went into them was broken down.A cheap nanoVNA seems like a good investment if I want to learn how to make my own biastees.The photo of the PPS biastee is very interesting. The inductor thats unraveled, is that the first inductor that goes into the signal path accidentally unwound? (the point on the schematic thats undefined)?------------Radiolistener-This looks like a great deal, even if one was only going to buy it for its nice case-And as you say, its a great platform for experimentation. Can one really just buy one for $13 ??I have been making the easiest cases imaginable for my RF things (cardboard + copper tape) its embarassing.Quote from: radiolistener on June 12, 2020, 05:31:22 amThank you!

author: cdev
author_rank: Super Contributor
timestamp: June 12, 2020, 07:46:52 pm
content: Quote from: T3sl4co1l on June 12, 2020, 01:45:56 pmCan a modern day VNA act like an old style grid dip meter to ascertain the resonances of "areas" of a circuit separately from the entire circuit? That seems to me as if it would be very useful when trying to design a bias-tee.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 13, 2020, 09:33:03 am
content: Quote from: cdev on June 12, 2020, 03:23:43 pmYes, it seems to be a coil up to and contacting the middle (signal) conductor. Not clear how far the ferrite rod extends into that.Note the ferrite also provides some loading, it might be NiZn ferrite which would be acceptably lossy in the GHz to help dampen resonant modes of the solenoid (or should I say helical waveguide).The teardown and photos are due to John Larkin; I just rehosted them in case he or Dropbox feels like moving them around again.QuotePah, pick up some 2-sided copper clad and build beautiful, strong and well-shielded enclosures like the pros.https://www.seventransistorlabs.com/Images/GPIBSerial1.jpgCopper tape isn't bad, in and of itself; just invest in a firmer substrate. G10 is the mechanical version of FR4, just as available from the usual suspects. Or phenolic, if you're feeling old school...I've built power switching sections out of alternating layers of copper and polyimide tape before, with excellent performance. The adhesive isn't the best at heat transfer, but it's good enough it's a pain to solder, for one! Main downside is it's hard to cut precise shapes with a craft blade; you want to make a lot of clearance, say when opening holes to lower layers or whatever. And it quickly gets lumpy and uneven, as bubbles and solder joints and kinks pile up in the stack. Probably don't need more than a few layers anyway, can't get too bad.Also a horrible pain to service, if you should ever need to rewire or repair it; but perhaps still a slight improvement over the rigid laminate kind.Quote from: cdev on June 12, 2020, 07:46:52 pmCertainly!A GDM is a crude, qualitative instrument, sure the frequency might be about right, but it can't tell you anything about impedance, and not much about bandwidth/Q (I think?). A VNA tells you, quantitively, the whole story: real and imaginary impedance and gain at frequency.On the bias tee, you'll see very slight errors in insertion loss, with dips due to resonances in the series impedance. Maybe fractional dB. More substantial will be the return loss, as an infinite series impedance has no reflection so normally the return loss tends towards infinity. An impedance of say 250 ohms, acting in parallel with a 50 ohm terminated line (25 ohm Thevenin impedance as seen by the bias tee), gives a 10% reflection or 20dB return loss (which would be quite poor for precision work).Tim

author: cdev
author_rank: Super Contributor
timestamp: June 13, 2020, 02:38:24 pm
content: Quote from: T3sl4co1l on June 13, 2020, 09:33:03 amMy problem has been in getting the size of the pieces of PCB that I cut exactly the right size. I use a pair of tinsnips to cut it. (I have a paper cutter and have tried that but the paper cutter gets a lot of use and use for cutting PCBs doesnt seem good for it. My wife is an artist and she uses the paper cutter a lot so I dont want to ruin it for her.If I could cut PCB material to precise squares then I could use it much more properly. My main issue is that with RF construction, its the inside surface that needs to make contact with the sides of the PCB a fair amount of the time. For example, a device where I want the two sides of the PCB to also be RF shielded from one another, say ons side is RF, the other power and control. In order for the device to work well I now make a box out of stiff thin cardboard squares , apply copper tape to both sides of them, the inside and outside surfaces, using cardboard I can get exactly the right size and cutting the cardboard into squares (not bending it) means I can have nice sharp corners. Then I can use components like a feed through capacitor or an SMA connector fairly normally and get a decent electrical connection to both the inner and outer box. (it actually ends up being a double shielded box.) Cardboard coated with copper tape is also solderable to (little dabs, not more) . This works fairly well, for low power devices or passive things like filters. At least it keeps them out of harms way. But this is not suitable for anything that has even a remote possibility of needing to be waterproof or banged around, because its flimsy mechanically. Which is why I only make small boxes.----I'd like to see a picture of this. I can see copper tape and polyamide tape being very useful in a filtering context because of the capacitance. It might act much like a feed through capacitor.Quote from: T3sl4co1l on June 13, 2020, 09:33:03 amI am thinking of getting a NanoVNA. Right now everything I do is just about seat of my pants, and I'm just fooling around. With a VNA suddenly I'll have tons more data about whats going on, so I think that will be a good investment, I'll suddenly be able to make devices which I can verify as working, which will be a big plus. Its insanely cool that suddenly I'll be able to make this huge leap ahead.It would seem to me that using a VNA like a grid dip meter would probably be possible, as they do have a built in siggen, I could also build a modern one myself I suppose..Maybe with plug in oscillators for different ranges, just like they used to make them. I have a bunch of analog meter movements lying around and even a nice Pomona box pre cut for one.---- I have come questions for you about misc. stuff in your images directory. You made a bowtie antenna and balun. That looks very handy because of the easy rotativity. Do you use that for off the air TV or receiving ham radio horixontal polarization, also how does the balun work? I have a bunch of magnetic materials at this point. Are those sleeves #43 iron powder or ferrite - For VHF/UHF baluns I usually use #61 when I can - very small binocular cores ones work the best for receiving. What is the design frequency use for your bowtie?I'm thinking of putting a small white hook and a short piece of monofilamint line with a fish swivel and loop so I can attach antenas to my ceiling as needed and stow them out of sight the rest of the time.I'd like to make something exactly what you have there to see if I can receive more hams on the VHF/UHF horizontal polarization /SSB/CW The RTLSDR is truly a crappy receiver for receiving really weak signals, especially on 6 meters where the birdies are really bad. 2 meters is pretty bad too. Although on the higher FM parts of 2M its semi acceptable.It's almost hopeless. It may also be my QTH, its very noisy and it seems as if the noise is coming from someplace out of my control nearby. Maybe its light ballasts in the big industrial building near me or something. the worst noise seems to come and go.Anyway, I am desperate to improve this. Baluns decople the antenna from the feedline and are really essential. I like the way that is set up. So I am going to try to see how that works.Also your baluns. I am interested in things like that. Thank you for making it available.Any ideas on making better baluns or antennas, I am very interested in.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 13, 2020, 11:21:05 pm
content: Quote from: cdev on June 13, 2020, 02:38:24 pmYeah get two shears, the one will get useless in short order...I use tin snips most of the time, works fine. Scribe the line, measure twice cut once and all that. Watch and guide it carefully during the cut, it's easy to wander.When it needs to fit tightly (like the above picture: it's an enclosure held with only two screws, with two clamshells that slide onto a chassis in the middle), cut slightly oversized, and use SiC sandpaper on a flat plate to grind it straight and to size. Tedious, but it works.Another example, this with slots on both end pieces so the whole thing is held with just one screw:https://www.seventransistorlabs.com/Images/Reverb2.jpgThe pushbuttons and MAX232 (tinned board, bottom right) were the hardest to position I think, followed closely by the four slots...Sawed cuts are fine as well. Saws leave smoother cuts than shears, and if you cut straight, not much material needs sanding to get a flat, clean end.Power tools are excellent of course; normal sawblades can be used, but better life is had with fine toothed carbide blades or even abrasive wheels. The dust and smell are obvious downsides. If the tool is compatible, water can be used to keep the dust down (consider a tile saw?).Files work too of course, but don't use any sharp files that you use for metals, especially slippery metals like brass. A few touches to FR4 and it'll be too dull to cut brass again...QuoteAnd you want to solder both seams if possible, to get extra shielding? And it may not be feasible to solder either, in part or in full, for various reasons?Edges can be lapped over with foil. If you don't have copper foil handy (which you do; copper tape works for this, obviously conductive adhesive being a huge bonus, if not a necessity), and don't feel like ordering any, you can peel it off PCBs with a bit of heat, and sand the (usually oxidized) face clean so it accepts solder.You can always make multiple walls, just soldering one side of each. If they're like nested boxes, this makes things far easier to maintain, too!Putting circuitry on both sides of a proto is already a bit of a construction nightmare. I prefer to build on single sides and stack things up as needed. Fly wires connect between boards, so they can be unfolded for maintenance. Or using connectors even, so they can be completely unplugged.If I need something to fit tightly, I can try to optimize it, but I'd much rather just not, or order a proper PCB instead.The most optimizing I think I've done is this,https://www.seventransistorlabs.com/Images/CukHack5.jpgwhich has a board on the bottom with some power transistors and diodes, which connect to one side of the inductor; then on top of that, the smaller board with all the capacitors on it, and power in/out wires. The gate driver board floats in the middle. Control sits in the top enclosure half, with double stick tape. (The power board has a thermal pad to help dissipate power to the enclosure, and a bit of copper tape lining the enclosure to further help spread the heat. It's an ABS enclosure, not much dissipation at all, but it's something.)Obviously, it's wide open from an RF standpoint. If it were a diecast enclosure, it would kick a fair amount of ass, and some additional filtering on the power leads would make it completely quiet. I wouldn't want to do a copper clad enclosure for this, partly because I already have the enclosure, and it wouldn't be as strong.QuoteYeah, that's not bad electrically; the substrate is just not very good mechanically. And if the adhesive doesn't make (or allow) very good electrical contact, you've just made an RF mummy full of seams, which will take so many dabs of solder to stitch up... (another good reason to keep it small).QuoteDoesn't look like much, because, well, it is what it is, plus there's stuff on top...https://www.seventransistorlabs.com/Images/Inv1kW_Deck1.jpgThe TO-220s are mounted on copper tabs (heat spreaders), and brown electrolytics are sitting on top of some area. You can see a strip, and the blue wire, carrying power up to the inverter sections from bottom-left. The transformers (wrapped with yellow tape) are constructed in the same way, the primary in layers of copper foil tape, with the secondary (200V) wound on top. Topology is push-pull, so each pair of transistors connects to opposite sides of the primary, and the CT goes to the supply (with local ceramics SMT'd to the tape, and the electrolytics standing on top). There are up to five layers, four of which are relevant: the PCB is 2-sided copper clad, of course I'm not making use of the bottom side so we can ignore that. PCB top is GND, then (in what order I don't remember exactly) the three primary terminals, in foil tape.Control board is to the left, on wires. Should've made them with connectors, honestly.Side view:https://www.seventransistorlabs.com/Images/Inv1kW_Deck2_1.jpgStill can't really see the right side transformer, it's in shadow here, but suffice it to say there are two down there. It's a duplicated circuit (ultimately to make +/-200V for a mains inverter on the top deck(s)). Can also see the control board mounted (facing down), with a DC-DC module on top of it. The module, and HV output, are all on plugs so they can be tested separately.This whole thing is going to be enormous, about 8 x 5 x 6". It's made for serviceability. A commercial version would be a fraction of the size. I've fought myself plenty of times trying to squeeze more things into already congested layouts, I'm not doing it again.QuoteHaven't used one myself but the results others have posted here are impressive. Seems like it's comparable or perhaps even better than a classic boatanchor HP something or other, the VNAs with sticker pricesstarting at$20k y'know? (Give or take range I suppose, the big iron going to some GHz usually?)QuoteHeh, it is actually quite rotatable at the moment, I've got it hanging from the ceiling on that wire and I can just reach over and twist the cable and spin it around... fun to watch different stations rise and fall as it rotates.I don't do much radio at all so that's about the extent of it, watching signals on the spec and doing a few experiments here or there.It's just long enough to pick up FM BCB, for which I've made a tube radio that receives quite nicely:https://www.seventransistorlabs.com/Images/FMRadio2.jpghttps://www.seventransistorlabs.com/Images/FMRadio3.jpgSeems to be reasonably wideband, which is nice. I see much of the TV and cell UHF bands, I think some aviation (possibly ADS, I forget?) at higher frequencies. (My spec is only 1.8GHz so that's about all I care to look for, anyway.)The balun is simply a 1:2 TLT. Twisted pair (~100 ohm Zo), two in parallel giving 50 ohm Zo at the feedpoint. They are equal lengths. One is ground to ground, signal to bal+; no ferrite bead needed but I used FBs symmetrically anyway. The other is ground to bal-, signal to GND; i.e., inverting, and a full, whatever core impedance, is required to give it useful bandwidth. I think, the beads are Laird #28 LF/wideband, and the binoc is #61 (maybe #43; it's NiZn in any case). Which is more than adequate for the >100MHz response it gets. So as a 1:2, the feedpoint is 200 ohms, which, eh, more or less right. I'm not transmitting so I don't care if the SWR is moderately ass.QuoteSure, easy enough to make; I'd recommend against making it from wire like I did, I was hoping it was as simple as tacking the crossings with solder, but wire this loose is rather finicky, and rather floppy too for that matter, hence the crossing wires (which probably do something for or against reception, but without a reference antenna or background to measure it against, I have no idea). About half the joints are twisted in place.This is actually a good place to use your favorite cardboard and foil tape method -- super easy to make something big and usefully shaped. Could even use tinfoil and adhesive spray (or double stick tape) for most of the area, saving cost; then make a good crimped joint to copper foil tape, and use that to carefully shape the vertex, and solder the feedline to it. (The root is where all the high frequency response happens; if you want it flat to some GHz, you need clean geometry on the same length scales.)Obviously, you'll need to go a lot bigger to reach 6m, or you might go with a range of dipoles (like the multi-element kind so it's effectively overlapping bands, or maybe a short log periodic) which might fold up better than a huge foiled-cardboard book will. Or a tuner.QuoteTell me about it; I've got some low-VHF conducted noise in my lab here, and I don't know from what, if it's something of mine here, or a neighbor, conducted or induced; it's conducted locally anyway, and is pretty annoying any time I'm probing something with the scope, when that something is also grounded to my PC. It's literally conducted, if I lift a ground anywhere it goes away. Argh...Looks and sounds like some shit SMPS or charger with no filtering.There's not usually much switching noise above 100MHz, so I don't know what your situation is about exactly, but who knows; computers without cases, SMPSs pushing higher and higher frequencies; radio (ab)users, licensed and unlicensed; even just old fashioned distribution line corona.QuoteI'm quite fond of TLT theory, and I wind transformers in that way whenever possible. Like the DC-DC in the picture above, the small ferrite transformer and bobbin that you only see the top of,https://www.seventransistorlabs.com/Images/Inv1kW_DCDC.jpgit's wound with (salvaged) triple-insulated wire in twisted pairs. The primary pairs are wired in parallel. It's also 24:12+12+12 but the secondary pairs are all 1:1 right, so the primary has its own pair which is wired in series, making it a combined 2:1 and 1:1:1:1 TLT. Circuit is just an ordinary flyback converter. Barely needs any snubbing at all, thanks to the very low leakage of this design!Another flyback with a similar aim:https://www.seventransistorlabs.com/Images/DCDC_800V.jpg12V in, 100-800V out (adjustable). Primary is foil, secondary is two layers of however many turns, wired in series, with the CT taken out to a pin. Secondary is rectified as +/-400V, and the "-400" is simply grounded so it's a unipolar output.The transmission line structure can be considered as a very low impedance TL for the primary (the foil over itself, and over the secondary), which is probably dominated by the stray wiring really; and the secondary is two TLs with respect to the primary (which looks like ground plane), of whatever length the turns are. Because the secondary is a complementary pair, their displacement currents cancel out, giving no common mode noise from the secondary! The primary still gives full CM noise of course, which is "merely" 80Vpp.Not that CM noise matters much on this common-ground application, but it's one less mode to have to worry about, and which will inevitably get into the output, demanding more filtering than otherwise.The windup looks like so:https://www.seventransistorlabs.com/Images/DCDC_800V_FoilWindup.jpgThe layers alternate P:S:P:S:P, so the S's are trapped between planes of P foil. This gives a fairly low impedance, probably 60-100 ohms, for a given turn of the secondary. Pretty low for a 400V output (it dominates as stray capacitance), but the winding length is also kept short so I don't have to worry too much about it.I forget if I measured leakage inductance; it's not in the note below the figure, obviously. Those notes are ungapped and gapped primary magnetizing inductance.It should be around, let's see, mean turn length is about 68mm so 15 turns is about 1m of wire, about 0.6uH per half of the secondary. OTOH, primary strays are probably around 20nH, or 4.5uH secondary referred, so, like I said, guess which one is dominant...I think TLTs may be most interesting in switching applications, as the impedances and ratios can be all over the place -- in radio you're mainly matching small signals over modest ratios, in the range of say 20 to 1000 ohms. OTOH, SMPS don't have to worry too too much about wide bandwidths, but that is perhaps changing these days. But also you don't get many applications where it can be done at all; there are just too many ratios and values of transformers to possibly supply them COTS, so there aren't many opportunities to apply theory.At best you can practice it with custom windups (but probably not with twisted pairs like I did above, as the labor will be costly), or planar transformers. Which are fantastic, but are nigh impossible to wind with high impedances due to the broadside-facing nature of planar stackups, and the dielectric constant of FR4. That makes them attractive for LV applications at various power levels, and mains applications at high power levels.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: June 14, 2020, 11:35:26 pm
content: a shitty tile saw mangles the copper. Those things from harbor freight are not good for PCBs.A good power tool for this I think is a dremel saw-max run with a guide over a trench (very high speed). And you might be interested in this, since you can hook it to a vacuumhttps://www.toolnut.com/milwaukee-2522-21xc-m12-fuel-3-compact-cut-off-tool-4ah-kit.html?utm_source=google&utm_medium=shopping&gclid=EAIaIQobChMI6o-X6ryC6gIVCm-GCh3HCAFfEAQYASABEgKp7_D_BwE

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 15, 2020, 05:52:50 am
content: Keep in mind that the glass fibers in FR4 and similar are absolutely horrible for your lungs and skin. If possible, I would do the cutting while having a steady stream of water or cutting oil running over the cutting tool, as well as doing it outside. Wear a respirator or face mask.

author: coppercone2
author_rank: Super Contributor
timestamp: June 15, 2020, 06:19:12 am
content: fucking dealing with crackers, this is why dead bug is niceabout as pleasant to work with as eating wheat thins. are you supposed to dip those in grease to make them palletable?there is this for water sawinghttps://www.amazon.com/Proxxon-37172-Micro-Band-Saw/dp/B001D6PJAQ/ref=cm_cr_arp_d_product_top?ie=UTF8I have a water cooled saw, a giant one for cutting stones (14 inch), its not appropriate for crackers. Is there a reasonably sized one for working with crackers with water that does not mangle copper?otherwise I recommend vacuum extractor and dust mask

author: cdev
author_rank: Super Contributor
timestamp: June 15, 2020, 11:36:06 am
content: Yes, I have a P-100 mask and I use it obsessively when doing stuff like that.Long before COVID-19 I was the guy who used a HEPA filter mask to mow the lawn, even.. I hate drilling or sawing FR4, thats why I usually use the tinsnips. Less dust. Will try to improve the accuracy, that is a good idea. All my filters, LNAs really are small.really small. Sometimes I use batronix extruded boxes from ebay.Quote from: coppercone2 on June 15, 2020, 06:19:12 am

author: cdev
author_rank: Super Contributor
timestamp: June 15, 2020, 12:01:52 pm
content: I wanted to ask you if you knew of some reference on TLT, which II am guessing means transmission line transformer. Also some link so I can try building a 1:2 TLT for VHF/UHF A 1;2 transformer iis one balun I dont have and it sounds ideal for a bowtie or similar. Ive got a bunch of small balun cores and want to make some new baluns to have around. for fooling around. I'll use thin enameled wire, its likely. A good balun makes all the difference for reception. Have a bunch of off the shelf CATV baluns, which are potted so I cant see whats inside, but I doubt if they are very good as their performance leaves a lot to be desired. I have others that I've made with Cat5/6 twisted pair cable wire which work better.Sometimes I make baluns out of #43 material too. Also going to try tomake my tunable varactor loop work on 6M so I can finally hear some 6M ham activity.My attempts to do thus using my RTLSDR so far have been totally futile.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 15, 2020, 11:53:55 pm
content: http://home.earthlink.net/~christrask/TraskTLTTutorial.pdfhttp://www.introni.it/pdf/Amidon%20-%20Transmission%20Line%20Transformers%20Handbook.pdfhttp://k5tra.net/TechFiles/Transformers.pdfOh this looks great, a whole book:http://dl.booktolearn.com/ebooks2/engineering/electrical/9781891121975_sevicks_transmission_line_transformers_ccda.pdfFamiliarize yourself with the ideal transmission line model: a TL is two ports with no ground reference or CM impedance between each other, and connected only by the delayed voltage and current sent from one to the other according to its Zo and length.A zero-length ideal TL, is an ideal (DC-light) 1:1 isolation transformer.We approximate the ideal TL by putting cores on coax, twisted pair or other shapes of TL. The core means we get good performance at mid frequencies, and suffer an LF cutoff, as we expect, since it's a transformer after all.With the ability to put TL ports at any CM voltage, it's trivial to connect ports in series-parallel combinations, at will. Transformation ratios are an immediate and obvious application!The Guanella type offers the best bandwidth. Each transformer port is composed of a series-parallel combination of TL ports. All TLs are the same length. The series-parallel combinations at the two transformer ports are inverse. Cores are placed on the TLs corresponding to their voltage drops, or wound on a common core with respective numbers of turns. Downsides: complex ratios are... tedious to compose; and you only get autoformers.This is okay, as it's often advantageous to use an autoformer to compose a ratio, then use a 1:1 isolation transformer on one side of it, and perhaps a 1:1 CMC to further improve its isolation or balance, than to do everything at once with a ratio isolation transformer that performs poorly in comparison.The TL length nulls out, so the bandwidth is limited not by electrical length, but by length matching, common mode errors (Z_cm is not ideal infinite, but depends on proximity between TLs, the cores and core material, and dielectrics), how much layout area the port connections take up, and the cross section of the TLs themselves (since the signal should propagate in TEM0 mode).Impedance of free space being what it is, Zcm >> 300 ohms isn't feasible, hence transformers over 600 or 1200 ohms are somewhere between arbitrarily hard, or actually impossible. (In that case, we can't have a matched impedance transformer, and use a mismatched design -- where Cp or LL dominates -- and we probably won't care about mismatched delays, so a simpler design is possible.)The Ruthroff type allows loops, giving unequal propagation delay, and so a cutoff corresponding to electrical length. Complex ratios are easier to compose.This can be seen as a refinement of conventional transformer winding: consider a transformer wound on a bobbin, consisting of a single layer each for primary and secondary. If the layers have equal numbers of turns, then each turn sees the image current of the corresponding turn in the opposite layer and it can be considered a edge-wound twin lead TLT, 1:1 ratio. If the turns are unequal but the pitches are equal, then the extra turns extending beyond the one winding are unsupported and take their image current from whatever's left; the impedance in that location is higher, giving much more leakage than expected. We can model this as a Ruthroff having a higher-Z TL stacked on top of the 1:1 section. If the turns are unequal and the widths are equal, we can imagine one winding acting as a ground plane for the other, but in reality their velocities won't match; one winding simply has more wire length than the other. It's some inbetween case. The distributed mismatch probably performs better than the unsupported case. (I don't know offhand how you'd compute that.)Note that leakage and capacitance are simply the LF equivalents of TL parameters Zo and length. A higher bandwidth transformer always has shorter wire length.Interestingly, this means the LL and Cp of a Guanella type can be anomalously high compared to its bandwidth -- multiple wavelengths can propagate through it simultaneously. This is only consequential when the overall delay matters, or when (ab)using it at a mismatched impedance, where one of LL or Cp dominates.An application where delay matters, is a push-pull output stage. We want the two outputs (MOSFET drains, say) to drive each other as synchronously as possible; any delay costs bandwidth, and we're already limited by device capacitances, which are acting in parallel. We might use a CT winding to couple both outputs to each other, and provide bias. The CT winding can be a 1:1 inverting transformer, incurring one TL delay between ports. Then we can use any other transformer to couple to the load: an isolation transformer or balun, whatever.What you can't do, is just an isolation transformer: its load current serves to pull the drains in the same direction, opposing balance entirely. And a balun isn't as preferable, because it incurs two TL delays (or more, for an odd ratio Ruthroff type).So for best CM rejection, you might opt to design for both, i.e. use a TLT CT choke for balancing and bias, plus a balun for additional balancing and output coupling.With this covered, now it's interesting to look at real power amp designs. Often you'll see a binoc core with one coax hairpin through it, the vertex of which is grounded (well, B+, whatever), the two ends of the shield going to the two drains, and the center conductor going to the output. Well, I ask you: where's the balance in that? It's two RFCs with an isolation transformer strung between them!So the minimum viable, actual transformer, using binoc cores, is one full turn for each half. Just as you'd use an EE core. Obvious enough, right, but once just one counter-example has been published, the ignorance keeps spreading -- "it must be right, they did it here", y'know?Tim

author: cdev
author_rank: Super Contributor
timestamp: June 16, 2020, 03:01:06 am
content: Well, this explains a lot of the designs Ive seen. I intend to bring this to my erstwhile kitchen table tomorrow with a bunch of balun cores and toroids and bars and my sig gen and see if I can figure this out in a more hands on manner. Time to get to sleep soon. **Thank you!**UPDATE: I still have not had the quality time to set up to do this but I have not forgotten it.It really sucks not having a good area set up to do electronics for some time, its just insanely hot upstairs and its too humid in the basement this time of year even with a dehumidifier, and the moldiness makes me ill.

author: KE5FX
author_rank: Super Contributor
timestamp: June 16, 2020, 04:43:35 am
content: Jeff, K6JCA has some really nice content on TLTs, baluns, and related hardware. Just ran across his site today, in fact:http://k6jca.blogspot.com/Note the categories in the list at right.

author: cdev
author_rank: Super Contributor
timestamp: June 18, 2020, 12:35:48 pm
content: This is a great site.Quote from: KE5FX on June 16, 2020, 04:43:35 amSo much info..