I want to build a 50 ohm to 0.02 ohm transformer. It needs a 50 to 1 turns ratio. It takes a low A sub L core to make such a thing. I have a toroid with an A sub L of 75. That only needs
29 turns for my frequency of interest 500KHz to 1700kHz. I also have some potcores that I could gap to lower the A sub L.
What would make a better transformer a toroid or a gapped potcore?
I plan 50 turns with a 1 turn secondary.
What problems can I expect with a 1 turn secondary?
For a toroid, a turn is simple: A wire through the toroid hole is a turn, a wire outside is nothing.
IMHO, your real problem is on the 20 milliohm side to make the circuit connections so that you're not going to lose all of your signal in the connection resistances and inductances. This is regardless which way you build your transformer.
A bandwidth ratio of only three should be dead-easy, but the 20 mOhm impedance may spell trouble. The single-turn winding should be fat and compact. A single nH of stray inductance (!) or ten mOhms of resistance would ruin performance. I think I would implement it as several single-turn windings in parallel, distributed around a toroid. What will you connect to the 20mOhm side?
Why would you need low A_l? The usual rule of having the reactance of one winding four times the terminal impedance is only a minimum. There is no harm in going higher. There would be if you did it by using more turns though.
Use a toroid. Gapped cores are for energy storage, not for transformers.
A 20mOhm resistor. I'm replacing a tube osc, I'm not able to follow the coax back to the tube output on the schematic, so I don't even have a start to figure out the tube output impedance. I'm starting to think it is not low. But it is able to drive 1 amp through the 0.02 ohm resistor to develop
20mv.
My calculations tell me that with an AsubL of 75 it take 29 turns. This still doesn't get my a 50 to 1 turn ratio. If I use a core with an AsubL of 2340, I only need 5 turns, with that I can't get a 50 to 1 turns ratio.
I'll restate my problem, I want to substitute a more stable signal generator for the internal signal generator in my Boonton 260A Q meter. The osc. in the 260A drives a 0.02 ohm resistor. It actually does this through a piece of coax, so I doubt it really has a 0.02 ohm output impedance. Could be I'm chasing a spec. I don't need to. One advantage I have is I only want 4mv of drive across the 0.02 ohm resistor, also I can adjust the signal generator output to set the voltage regardless of transformer/connection losses. Thanks for the multiple single turn winding in parallel idea. Still open to suggestions, Thanks, Mikek
PS. Next project is an RF detector to measure the 4mv signal, I want a circuit to drive my multimeter. I suspect I'll need a preamp before the detector to get any linearity. I do have a scope I can use to get a useful calibration.
Your calculations show you that you need a _minimum_ of 29 turns, not _exactly_ 29 turns. 29 is less than 50, so if you feel bound and determined to maintain that ratio, use 50 turns and be happy.
What's probably a better way (and what's probably being done in the meter) is to not bother matching to that resistor, but rather to build an amplifier that can work into a low impedance (or whatever the impedance is when transformed by that coax and whatever is ahead of it), and just accept whatever inefficiencies result.
If all you want is for the thing to generally operate the way it does now but with a more stable oscillator, you may be best served by either turning the current oscillator into an amplifier stage to be driven by your oscillator, or injection-locking it.
It's all a matter of the core. I made a 1:20 step-up transformer with a 1-turn primary. The primary "wire" was a copper strip 1-inch wide. The secondary was 20 turns of a large litz wire, wound with mucho Kapton tape insulation. The 1-turn primary voltage was 500V, and the secondary voltage was 10kV. It was amazing to contemplate a foot of thick 1-inch-wide copper strip, holding off 500V eed-to-end. Worked great!
My transformer, with the one turn winding open, looks great.
10vpp in 200mvpp out exactly 50 to 1 ratio. However, when I load it with 0.02 ohms (a 15 in wire) it barely changes amplitude, and the primary amplitude also doesn't change. If 50 ohms was reflected back it would drop by 1/2. Shorting the secondary one turn with a pair of needle nose pliers drops the secondary voltage about 10%. Time to sleep on it. Later, Mikek
It has been for years a common practice in transistor RF amplifiers to make the single-turn windings of copper/brass tubing and threading the multiple-turn winding through the tube inside. This probably is like your paralleled windings.
In recent ARRL Handbooks there are fine examples (in 2014 edition on page 17.32).
I build 'wall current monitors' to measure the beam current in particle accelerators. Those are basically single-turn transformers very much like that: The secondary winding is a toroidal shell completely enveloping the core, with a small gap across which I pick-up the output signal. Accessorily, the secondary winding is also the vacuum chamber.
I also make hybrid transformers that basically create the sum and difference of two input signals on two separate outputs. One of the issues limiting performance at HF is the capacitance between the two input windings and the difference winding. I tried putting the difference winding on its own separate core, and then coupling the two cores with a single-turn winding threading both. This was in practice a piece of air core coax shorted at both ends, with the two cores inside. It worked, but it wasn't enough of an improvement to be worth the bother.
The problem with making a 1-turn winding on a toroid would be making it non-progressive.
As long as you have a turn in the plane of the toroid, you've got an external field and leakage inductance.
The tricks for making non-progressive winding do seem to involve a second turn going back around the toroid in the opposite sense (viewed from above the plane of toroid).
I fail to see how a one-turn winding that wraps around the entire core can be progressive. A multi-turn progressive winding, yes, but a single-turn winding has no current in the plane of the toroid. Making proper connections to such a winding may be challenging.
Make a bunch of 1-turn secondaries, each driving a twisted pair. Connect the pairs outside the plane of the toroid. Like the Space Needle.
I had dinner in the Space Needle once. The tables rotate around on a track to slowly to scan the view. If you lean your elbow on the window sill, it slowly creeps out from under you. If you go the the bathroom and then go again later, it's in a different direction.
I guess the Space Needle has low leakage inductance.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
Are you perhaps building an AM broadcast band to power line coupler? If so, how much RF power? If not, just ignore the following.
Way back on college daze, I helped build such a contrivance for the campus AM radio station. The problem was how to keep 60Hz AC from going back through the transformer and AM modulating the final stages. The fix was to use a ferrite toroid, which coupled the RF quite nicely, but none of the 60Hz. As I barely recall from 46 years ago, it was an ordinary ferrite toroid core about 2" OD. However, I don't recall the ferrite material used or number turns involved. No need for a pot core since the transformer was inside an aluminum box. I'll see if I can find some details (time permitting). Google for "carrier current AM transmitter"
Also, do you really need broadband for your unspecified application? Going from 0.5MHz to 1.7MHz without tuning is nice, but if your transmitter doesn't change frequency, an LC tuned arrangement (tapped coil or two capacitors) might be easier than a toroid.
--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
It seems that the OP is hacking an old Boonton Q meter, attempting to extend its measurement range, without the proper theoretical background to do it properly. Measuring higher Q's does not need (or tolerate) increased excitation to the DUT.
Pretty true, I'm a fish monger by trade.. The transformer was an attempt to use a different more stable osc. Abandoning that for now, in favor of getting my Q measurements done.
You're correct again, in "does not need increased excitation"
but I'm not increasing excitation, I'm lowering it.
I'm lowering the drive voltage from 20mv to 4 mv this changes the multiplier of the Q meter from 1 to 5. With that I can measure up to a Q of 1250.
If you would like to be helpful, I'd like to measure the low mv RF signal. I could use a design for a diode detector to measure down to
3.33mv with the output driving a DMM. I would like a low capacitance input, probably a fet input MPF102/2N3819 which I have, and an amp using
2N2222, 2N2907, 2n3904, or 2n3906 which I have. I also have 2n4401/4403.
1n34A will be the diode I'd have. Frequency range 500kHz to 1700kHz. I want it to run from 9V. FWIW, the source impedance is 0.02 ohms. I understand it will be non linear, that is fine, I can build up a cheat sheet of the 4 voltage setting I have interest in, 3.33mv, 4mv,
5mv and 6.66mv. I can use my scope to get enough accuracy to make the cheat sheet.
Out of 50 ohms, a pretty good match should be reasonable, up to whatever the electrical length of the primary winding is (at which point it looks like a transmission line over the secondary "ground plane").
Easy enough. This is a cross section. Imagine it's revolved around the axis on the right (so the gray core part is a torus, and everything else is a disk, cylinder or hole shape).
formatting link
Oh, the copper colored vertical sides connect to a plate shorting across the top, of course.
I'd show a picture of the finished thing, but it's pretty boring, like looking at a bank vault from the "concrete walls" side. :^)
Or if you want an inline structure (like a coaxial current transformer), you start with a pipe, slit it, then put a larger pipe on over the slit (with discs to short it to the first pipe, on either side of the slit). The core and windings go inside the space thus created.
In particle accelerators, the bunches are so small (sub-ns) that they can't afford any windings at all; transmission lines are simply connected in parallel across the gap!
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