Hi, Since it seems a useful thing to have, I am making a wideband current probe, ~10-1000MHz, like one of these: <http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure42.jpg> Every home should have one. It's from the article: <http://www.interferencetechnology.com/the-hf-current-probe-theory-and-application/> 1) They suggest an electrostatic shield made from copper tape, "with a small gap around the inside of the toroid". Well this turns out to be a PITA, I have shredded finger tips from copper cuts and am pausing for thought. What about just using very thin coax instead, and using it's shield as the - uh - shield? 2) The output frequency response of a commercial probe is supposed to look nice and smooth like this: <http://www.fischercc.com/ViewProductGroup.aspx?productgroupid=127> <http://www.fischercc.com/productfiles/DS%20F-51%20Rev--RLSE_cd51.pdf> But he is measuring more like this for both the home-made and commercial: <http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure9.jpg> Is there a better way to do this measurement? Needless to say I am a neophyte at RF, but do have some kit available (thanks, ebay). -- John Devereux
Homemade Wideband Current Transformer
Started by ●April 10, 2013
Reply by ●April 10, 20132013-04-10
On Wed, 10 Apr 2013 21:44:52 +0100, John Devereux <john@devereux.me.uk> wrote:> >Hi, > >Since it seems a useful thing to have, I am making a wideband current >probe, ~10-1000MHz, like one of these: > ><http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure42.jpg> > >Every home should have one. It's from the article: > ><http://www.interferencetechnology.com/the-hf-current-probe-theory-and-application/> > >1) They suggest an electrostatic shield made from copper tape, "with a >small gap around the inside of the toroid". Well this turns out to be a >PITA, I have shredded finger tips from copper cuts and am pausing for >thought. > >What about just using very thin coax instead, and using it's shield as >the - uh - shield? > > >2) The output frequency response of a commercial probe is supposed to >look nice and smooth like this: > ><http://www.fischercc.com/ViewProductGroup.aspx?productgroupid=127> ><http://www.fischercc.com/productfiles/DS%20F-51%20Rev--RLSE_cd51.pdf> > >But he is measuring more like this for both the home-made and >commercial: > ><http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure9.jpg> > >Is there a better way to do this measurement? > > >Needless to say I am a neophyte at RF, but do have some kit available >(thanks, ebay).We made a current transformer for measuring ~~100 amp, nanosecond pulses. It was just a small ferrite core with maybe 5 turns, a shunt resistor, and some coax to the scope. Seemed to work fine. A Rogowski coil needs no core so can be very fast. -- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Reply by ●April 10, 20132013-04-10
John Larkin <jlarkin@highlandtechnology.com> writes:> On Wed, 10 Apr 2013 21:44:52 +0100, John Devereux > <john@devereux.me.uk> wrote: > >> >>Hi, >> >>Since it seems a useful thing to have, I am making a wideband current >>probe, ~10-1000MHz, like one of these: >> >><http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure42.jpg> >> >>Every home should have one. It's from the article: >> >><http://www.interferencetechnology.com/the-hf-current-probe-theory-and-application/> >> >>1) They suggest an electrostatic shield made from copper tape, "with a >>small gap around the inside of the toroid". Well this turns out to be a >>PITA, I have shredded finger tips from copper cuts and am pausing for >>thought. >> >>What about just using very thin coax instead, and using it's shield as >>the - uh - shield? >> >> >>2) The output frequency response of a commercial probe is supposed to >>look nice and smooth like this: >> >><http://www.fischercc.com/ViewProductGroup.aspx?productgroupid=127> >><http://www.fischercc.com/productfiles/DS%20F-51%20Rev--RLSE_cd51.pdf> >> >>But he is measuring more like this for both the home-made and >>commercial: >> >><http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure9.jpg> >> >>Is there a better way to do this measurement? >> >> >>Needless to say I am a neophyte at RF, but do have some kit available >>(thanks, ebay). > > We made a current transformer for measuring ~~100 amp, nanosecond > pulses. It was just a small ferrite core with maybe 5 turns, a shunt > resistor, and some coax to the scope. Seemed to work fine.Nice.> A Rogowski coil needs no core so can be very fast.I am hoping to use this for checking cable emissions for EMC, so it needs to be reasonably sensitive. -- John Devereux
Reply by ●April 10, 20132013-04-10
Making a CT is easy beans. A hundred turns on ferrite into a suitable resistor (a terminated transmission line, in this case) will easily do MHz, maybe 20 if it's somewhat small. Amazingly, commercial CTs can suck eggs: a Triad CST306 (I think) has its lowest resonant mode at something like 300kHz! It gets better if you wrap it with aluminum or copper tape, because this reduces the leakage inductance and coupling between winding segments, suppressing the helicotoroidial resonator modes. Probably, if lossy tape were used, it would help dampen as well as shield the resonances. A high burden resistance also discourages resonances, forcing the feedpoint to not be a good current antinode. It helps a lot to use fewer turns, raising the resonant frequency. Segmented windings, arranged carefully, may help to discourage certain resonant modes. If a large ratio is required, multiple stages can be used. Past maybe 100MHz, with any approach, you're pretty much working in black magic territory. Good luck. Tim -- Deep Friar: a very philosophical monk. Website: http://seventransistorlabs.com "John Devereux" <john@devereux.me.uk> wrote in message news:87zjx6hz4f.fsf@devereux.me.uk...> John Larkin <jlarkin@highlandtechnology.com> writes: > >> On Wed, 10 Apr 2013 21:44:52 +0100, John Devereux >> <john@devereux.me.uk> wrote: >> >>> >>>Hi, >>> >>>Since it seems a useful thing to have, I am making a wideband current >>>probe, ~10-1000MHz, like one of these: >>> >>><http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure42.jpg> >>> >>>Every home should have one. It's from the article: >>> >>><http://www.interferencetechnology.com/the-hf-current-probe-theory-and-application/> >>> >>>1) They suggest an electrostatic shield made from copper tape, "with a >>>small gap around the inside of the toroid". Well this turns out to be a >>>PITA, I have shredded finger tips from copper cuts and am pausing for >>>thought. >>> >>>What about just using very thin coax instead, and using it's shield as >>>the - uh - shield? >>> >>> >>>2) The output frequency response of a commercial probe is supposed to >>>look nice and smooth like this: >>> >>><http://www.fischercc.com/ViewProductGroup.aspx?productgroupid=127> >>><http://www.fischercc.com/productfiles/DS%20F-51%20Rev--RLSE_cd51.pdf> >>> >>>But he is measuring more like this for both the home-made and >>>commercial: >>> >>><http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure9.jpg> >>> >>>Is there a better way to do this measurement? >>> >>> >>>Needless to say I am a neophyte at RF, but do have some kit available >>>(thanks, ebay). >> >> We made a current transformer for measuring ~~100 amp, nanosecond >> pulses. It was just a small ferrite core with maybe 5 turns, a shunt >> resistor, and some coax to the scope. Seemed to work fine. > > Nice. > >> A Rogowski coil needs no core so can be very fast. > > I am hoping to use this for checking cable emissions for EMC, so it > needs to be reasonably sensitive. > > > -- > > John Devereux
Reply by ●April 10, 20132013-04-10
On Apr 10, 4:44=A0pm, John Devereux <j...@devereux.me.uk> wrote:> Hi, > > Since it seems a useful thing to have, I am making a wideband current > probe, ~10-1000MHz, like one of these: > > <http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figu...= > > > Every home should have one. It's from the article: > > <http://www.interferencetechnology.com/the-hf-current-probe-theory-and...= > > > 1) They suggest an electrostatic shield made from copper tape, "with a > small gap around the inside of the toroid". Well this turns out to be a > PITA, I have shredded finger tips from copper cuts and am pausing for > thought.I think you just need to stop the current from flowing, so a layer of copper tape with some other insulating tape between the overlapping layer section should work. George H.> > What about just using very thin coax instead, and using it's shield as > the - uh - shield? > > 2) The output frequency response of a commercial probe is supposed to > look nice and smooth like this: > > <http://www.fischercc.com/ViewProductGroup.aspx?productgroupid=3D127> > <http://www.fischercc.com/productfiles/DS%20F-51%20Rev--RLSE_cd51.pdf> > > But he is measuring more like this for both the home-made and > commercial: > > <http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figu...= > > > Is there a better way to do this measurement? > > Needless to say I am a neophyte at RF, but do have some kit available > (thanks, ebay). > > -- > > John Devereux
Reply by ●April 11, 20132013-04-11
On 2013-04-10 22:44, John Devereux wrote:> > Hi, > > Since it seems a useful thing to have, I am making a wideband current > probe, ~10-1000MHz, like one of these: > > <http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure42.jpg> > > Every home should have one. It's from the article:To get a response curve that makes sense for an RF current transformer, you have to be able to excite it with a known current. The usual way to do so is by inserting the transformer into a suitably sized and carefully terminated coaxial transmission line. The line's characteristic impedance ensures that the current is flat over frequency (to within a few dB or so). The 'suitable size' bit is to make sure that the insertion of the transformer does not make a discontinuity in its impedance. I check this through reflectometry: The line with the transformer in place should look like a smooth transmission line all the way through. That's the way I test beam current transformers here. In its final application, the primary is not a wire; It's a particle beam. I get bandwidths of about 3GHz. OK, my kit is upwards of 35k$ a piece. Then again, it's all custom made, has to hold a vacuum too, be radiation resistant, etc. That has a way of driving up the cost... (Just the ferrite cores are 1k$ a piece, and I use five.) Come to think of it, the thing is warped such that many wouldn't even recognize it as a transformer. We call them "wall current monitors". Here's a picture, if anyone is interested: <http://cern.ch/psring/psring/showpicture.php?section=03> Jeroen Belleman
Reply by ●April 11, 20132013-04-11
Jeroen Belleman <jeroen@nospam.please> writes:> On 2013-04-10 22:44, John Devereux wrote: >> >> Hi, >> >> Since it seems a useful thing to have, I am making a wideband current >> probe, ~10-1000MHz, like one of these: >> >> <http://www.interferencetechnology.com/wp-content/uploads/2012/03/Figure42.jpg> >> >> Every home should have one. It's from the article: > > To get a response curve that makes sense for an RF current transformer, > you have to be able to excite it with a known current. The usual way > to do so is by inserting the transformer into a suitably sized > and carefully terminated coaxial transmission line. The line's > characteristic impedance ensures that the current is flat over > frequency (to within a few dB or so). The 'suitable size' bit is > to make sure that the insertion of the transformer does not make > a discontinuity in its impedance. I check this through reflectometry: > The line with the transformer in place should look like a smooth > transmission line all the way through.Ha! I had a thought last night it might be something like that. Be a big coax for my one though. I did wonder if a sort of giant microstrip (megastrip?) would also work :) It looks like that same company I linked to also does calibration fixtures: <http://www.fischercc.com/ViewProductGroup.aspx?productgroupid=141>> That's the way I test beam current transformers here. In its final > application, the primary is not a wire; It's a particle beam. I get > bandwidths of about 3GHz. OK, my kit is upwards of 35k$ a piece. > Then again, it's all custom made, has to hold a vacuum too, be > radiation resistant, etc. That has a way of driving up the cost... > (Just the ferrite cores are 1k$ a piece, and I use five.) > > Come to think of it, the thing is warped such that many wouldn't > even recognize it as a transformer. We call them "wall current > monitors". Here's a picture, if anyone is interested: > <http://cern.ch/psring/psring/showpicture.php?section=03>Cool, thanks! -- John Devereux
Reply by ●April 11, 20132013-04-11
Tim Williams <tmoranwms@charter.net> wrote:> Making a CT is easy beans. A hundred turns on ferrite into a suitable > resistor (a terminated transmission line, in this case) will easily do > MHz, maybe 20 if it's somewhat small. Amazingly, commercial CTs can suck > eggs: a Triad CST306 (I think) has its lowest resonant mode at something > like 300kHz! > > It gets better if you wrap it with aluminum or copper tape, because this > reduces the leakage inductance and coupling between winding segments, > suppressing the helicotoroidial resonator modes. Probably, if lossy tapeWord of the week ^^^^^^^^^^^^^^^
Reply by ●April 12, 20132013-04-12
Cydrome Leader <presence@MUNGEpanix.com> writes:> Tim Williams <tmoranwms@charter.net> wrote: >> Making a CT is easy beans. A hundred turns on ferrite into a suitable >> resistor (a terminated transmission line, in this case) will easily do >> MHz, maybe 20 if it's somewhat small. Amazingly, commercial CTs can suck >> eggs: a Triad CST306 (I think) has its lowest resonant mode at something >> like 300kHz! >> >> It gets better if you wrap it with aluminum or copper tape, because this >> reduces the leakage inductance and coupling between winding segments, >> suppressing the helicotoroidial resonator modes. Probably, if lossy tape > > Word of the week ^^^^^^^^^^^^^^^Yes I must try to use that one more often. But, Success! I built the transformer, along with the finger-shredding copper tape electrostatic shield. Seems to work well, as far as I have been able to test. Transimpedance is 20 ohms. Looks beautifully flat response when you look at the first 20MHz. Generally flat response continues to about 200MHz, with some +/- ~10dB wiggles up to 1GHz+. Don't know how much of this is the response, and how much the excitation. I might try my megastrip idea to test it further. Will try to post some pics later. -- John Devereux
Reply by ●April 12, 20132013-04-12
John Devereux <john@devereux.me.uk> writes:> Cydrome Leader <presence@MUNGEpanix.com> writes: > >> Tim Williams <tmoranwms@charter.net> wrote: >>> Making a CT is easy beans. A hundred turns on ferrite into a suitable >>> resistor (a terminated transmission line, in this case) will easily do >>> MHz, maybe 20 if it's somewhat small. Amazingly, commercial CTs can suck >>> eggs: a Triad CST306 (I think) has its lowest resonant mode at something >>> like 300kHz! >>> >>> It gets better if you wrap it with aluminum or copper tape, because this >>> reduces the leakage inductance and coupling between winding segments, >>> suppressing the helicotoroidial resonator modes. Probably, if lossy tape >> >> Word of the week ^^^^^^^^^^^^^^^ > > Yes I must try to use that one more often. > > But, Success! > > I built the transformer, along with the finger-shredding copper tape > electrostatic shield. > > Seems to work well, as far as I have been able to test. > > Transimpedance is 20 ohms. > > Looks beautifully flat response when you look at the first 20MHz. > > Generally flat response continues to about 200MHz, with some +/- ~10dB > wiggles up to 1GHz+. Don't know how much of this is the response, and > how much the excitation. I might try my megastrip idea to test it > further. > > Will try to post some pics later.OK, here it is: <http://ee.devereux.me.uk/hf-current-probe.jpg> I made a mistake, it is more like 7 or 8 ohms transimpedance. About 1 ohm impedance on the "primary",as could be expected from the 7 turns and 50 ohm secondary "burden". <http://ee.devereux.me.uk/plot0003.png> That is with 1mA from the tracking generator, so the 7.87mV display is 7.87 Ohms. -- John Devereux
