Forums

50 ohms to 0.02 ohm transformer

Started by amdx November 3, 2016
On Thu, 3 Nov 2016 15:12:53 +0200, Tauno Voipio
<tauno.voipio@notused.fi.invalid> wrote:

>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, Mikek > >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.
Leakage inductance can matter. It would be interesting to make a toroidal transformer where the 1-turn winding wraps around the entire core. Visualize that! It can be approximated by paralleling multiple 1t windings. http://www.spaceneedle.com/wp-content/uploads/2013/08/view-from-heli-250x211.jpg -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
"amdx" <nojunk@knology.net> wrote in message 
news:nvfrar$295$1@dont-email.me...
> 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 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. >> > > 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.jpg > > Thanks for your help, Mikek > >
The coax is driven from the winding connected to pins 1 &2 of the turret. That winding couples RF from the plate winding connected to pins 4 & 8. The coax feeds the series load of the thermocouple and the 0.02 ohm resistor. Regards, Tom
On 11/3/2016 12:37 PM, 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.
Thermo-cross? Your looking at the thermocouple? It heats and the voltage developed drives the 'Multiply Q By' meter. My ohmmeter reads .1 ohm when I measure just the 0.02 ohm resistor or the series thermocouple and 0.02 ohm resistor. So the thermocouple has very low resistance.
> > A transformer in the cable will only worsen your results. >
I see the coil (winding) on pin 1 and 2 of the turret. I don't see what it links to. Can you point out the other winding that makes a transformer? Thanks, Mikek
On 11/3/2016 12:37 PM, 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. >
I'm can not disputing that; I see the B+ (342V) connected to R121 (220 ohm) and going to the unmarked variable inductor, then to pin 4 and the anode of the tube. But I don't know how you know the are magnetically coupled? It seems to make sense, I just would never have picked that up. Mikek
> 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. >
On Thu, 03 Nov 2016 13:31:03 -0500, amdx wrote:

> On 11/3/2016 12:37 PM, 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. >> >> > I'm can not disputing that; > I see the B+ (342V) connected to R121 (220 ohm) and going to the > unmarked variable inductor, then to pin 4 and the anode of the tube. > But I don't know how you know the are magnetically coupled? > It seems to make sense, I just would never have picked that up.
It is magnetically coupled because (A) that's the only way it'd work, and (B), it's toobs, and that's how toob radio stuff works. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
On Thu, 03 Nov 2016 19:37:44 +0200, 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.
Oh lordy, I saw that days ago and didn't catch it. Andy: the DC resistance of that thermocouple unit is going to be pretty close to the RF resistance, and I'll bet it's around 50 ohms -- or at least a hell of a lot closer than 20m-ohms. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com I'm looking for work -- see my website!
On 11/3/2016 2:39 PM, Tim Wescott wrote:
> On Thu, 03 Nov 2016 19:37:44 +0200, 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. > > Oh lordy, I saw that days ago and didn't catch it. > > Andy: the DC resistance of that thermocouple unit is going to be pretty > close to the RF resistance, and I'll bet it's around 50 ohms -- or at > least a hell of a lot closer than 20m-ohms. >
No it's very low ohm. My ohmmeter reads .1 ohm when I measure just the 0.02 ohm resistor or when I measure the thermocouple and 0.02 ohm resistor in series. So the thermocouple has very low resistance. Mikek
On 11/3/2016 12:55 PM, John Larkin wrote:
> On Thu, 3 Nov 2016 15:12:53 +0200, Tauno Voipio > <tauno.voipio@notused.fi.invalid> wrote: > >> 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, Mikek >> >> 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. > > Leakage inductance can matter. > > It would be interesting to make a toroidal transformer where the > 1-turn winding wraps around the entire core. Visualize that! It can be > approximated by paralleling multiple 1t windings. > > http://www.spaceneedle.com/wp-content/uploads/2013/08/view-from-heli-250x211.jpg > >
Hows this John,
> https://www.dropbox.com/s/6klg2v2awamchb5/2001-12-31%2023.00.00-122.jpg?dl=0
> https://www.dropbox.com/s/3gmnukf4u7kawx3/2001-12-31%2023.00.00-120.jpg?dl=0
I haven't tested it yet. Mikek
On Thursday, November 3, 2016 at 2:52:49 PM UTC-7, amdx wrote:
> On 11/3/2016 2:39 PM, Tim Wescott wrote: > > On Thu, 03 Nov 2016 19:37:44 +0200, 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. > > No it's very low ohm. > > My ohmmeter reads .1 ohm when I measure just the 0.02 ohm resistor or > when I measure the thermocouple and 0.02 ohm resistor in series. So the > thermocouple has very low resistance.
Er... but this is about something in the 1.9 MHz range, so there's a bit of a problem in using a DC measurement. Skin depth is about 0.05 millimeters in copper at that frequency. 1mm wire, 100 mm long, is 12 milliohms at 1.9 MHz, and only 2 miliohms at DC. <http://chemandy.com/calculators/round-wire-ac-resistance-calculator.htm>
On 11/3/2016 4:56 PM, amdx wrote:
> On 11/3/2016 12:55 PM, John Larkin wrote: >> On Thu, 3 Nov 2016 15:12:53 +0200, Tauno Voipio >> <tauno.voipio@notused.fi.invalid> wrote: >> >>> 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, Mikek >>> >>> 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. >> >> Leakage inductance can matter. >> >> It would be interesting to make a toroidal transformer where the >> 1-turn winding wraps around the entire core. Visualize that! It can be >> approximated by paralleling multiple 1t windings. >> >> http://www.spaceneedle.com/wp-content/uploads/2013/08/view-from-heli-250x211.jpg >> >> >> > > > Hows this John, > >> https://www.dropbox.com/s/6klg2v2awamchb5/2001-12-31%2023.00.00-122.jpg?dl=0 >> > >> https://www.dropbox.com/s/3gmnukf4u7kawx3/2001-12-31%2023.00.00-120.jpg?dl=0 >> > > I haven't tested it yet. > > Mikek
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