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?
Thanks, Mikek
50 ohms to 0.02 ohm transformer
Started by ●November 3, 2016
Reply by ●November 3, 20162016-11-03
On 3.11.16 14:55, amdx wrote:> 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? > > Thanks, MikekFor 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. -- -TV
Reply by ●November 3, 20162016-11-03
On 2016-11-03 13:55, amdx wrote:> 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? > > Thanks, MikekA 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. Jeroen Belleman
Reply by ●November 3, 20162016-11-03
On 11/3/2016 8:58 AM, Jeroen Belleman wrote:> On 2016-11-03 13:55, amdx wrote: >> 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? >> >> Thanks, Mikek > > 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?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.> 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. >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.> Use a toroid. Gapped cores are for energy storage, > not for transformers. > > Jeroen BellemanI'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.
Reply by ●November 3, 20162016-11-03
On 11/3/2016 9:57 AM, amdx wrote:> On 11/3/2016 8:58 AM, Jeroen Belleman wrote: >> On 2016-11-03 13:55, amdx wrote: >>> 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? >>> >>> Thanks, Mikek >> >> 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? > > 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. >I meant to add there is a schematic on the last page of this pdf.> http://www.hparchive.com/Boonton/Boonton-Manual-260A.pdfMaybe you can see how the tube output gets to the coax. :-)> > >> 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. >> > > 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. > >> Use a toroid. Gapped cores are for energy storage, >> not for transformers. >> >> Jeroen Belleman > > 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. > >
Reply by ●November 3, 20162016-11-03
On Thu, 03 Nov 2016 09:57:09 -0500, amdx wrote:> On 11/3/2016 8:58 AM, Jeroen Belleman wrote: >> On 2016-11-03 13:55, amdx wrote: >>> 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? >>> >>> Thanks, Mikek >> >> 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? > > 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. > > > >> 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. >> >> > 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. > >> Use a toroid. Gapped cores are for energy storage, >> not for transformers. >> >> Jeroen Belleman > > 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. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
Reply by ●November 3, 20162016-11-03
On 11/3/2016 11:56 AM, Tim Wescott wrote:> On Thu, 03 Nov 2016 09:57:09 -0500, amdx wrote: > >> On 11/3/2016 8:58 AM, Jeroen Belleman wrote: >>> On 2016-11-03 13:55, amdx wrote: >>>> 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? >>>> >>>> Thanks, Mikek >>> >>> 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? >> >> 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. >> >> >> >>> 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. >>> >>> >> 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. >> >>> Use a toroid. Gapped cores are for energy storage, >>> not for transformers. >>> >>> Jeroen Belleman >> >> 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.Yes, I stated that in my first post.> > 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),I don't know, the schematic on the Last page of this pdf. It looks to me like the osc tube drives the 0.02 ohms directly. http://www.hparchive.com/Boonton/Boonton-Manual-260A.pdf>and justaccept 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. >That's beyond my capability, I have an HP 8640B that has a frequency lock. Boonton produced an accessory to sub in another signal generator, it was designed so you could lower the frequency of operation. It was a 500 ohm to 0.03 ohm coupling transformer.> http://www.angelfire.com/electronic/funwithtubes/images/100_2598-B.jpg> http://www.angelfire.com/electronic/funwithtubes/images/100_2597-B.jpgThanks for your help, Mikek
Reply by ●November 3, 20162016-11-03
amdx wrote...> > What problems can I expect with a 1 turn secondary?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! -- Thanks, - Win
Reply by ●November 3, 20162016-11-03
On 3.11.16 19:15, amdx wrote:> I don't know, the schematic on the Last page of this pdf. It looks to me > like the osc tube drives the 0.02 ohms directly. > http://www.hparchive.com/Boonton/Boonton-Manual-260A.pdfYou are not reading the schematic correctly. There is a transformer formed by the tuned circuit in the coil turret and its link coil. Besides, there is the impedance of the thermo-cross RF current meter on the way, as well. A transformer in the cable will only worsen your results. -- -TV
Reply by ●November 3, 20162016-11-03
On Thursday, November 3, 2016 at 1:37:50 PM UTC-4, Tauno Voipio wrote:> On 3.11.16 19:15, amdx wrote: > > > I don't know, the schematic on the Last page of this pdf. It looks to me > > like the osc tube drives the 0.02 ohms directly. > > http://www.hparchive.com/Boonton/Boonton-Manual-260A.pdf > > > You are not reading the schematic correctly. There is a transformer > formed by the tuned circuit in the coil turret and its link coil. > > Besides, there is the impedance of the thermo-cross RF current > meter on the way, as well. > > A transformer in the cable will only worsen your results. > > -- > > -TVright, I don't think the OP needs a 0.02 output Z, he simply needs to be able to drive some signal into 0.02 Ohms. m
