Hi guys, As most peple here are likely aware, if you examine the coax cable for a passive oscilloscope probe you find that the inner conductor is made of a highly resistive wire -- something like nichrome, and around 50-200ohms per meter. (For some background on this, check out this article: http://www.dfad.com.au/links/THE%20SECRET%20WORLD%20OF%20PROBES%20OCt09.pdf ). What's not clear to me, though, is how one analytically determines the desired resistance of their coax in such a situation, given knowledge of the source and termination impedances of the cable. The magazine article above mentions that, at least at Tektronix, John Kobbe came up with the idea... and if you then dig into some of the Tek archives, you can find his reminisces about doing so, where he says something along the lines of, "it occurred to me that lossy coax would work well here [to greatly extend the bandwidth of a passive probe], so I just calculated what the appropriate resistance would be, ran down to the stock room, got some and tried it out... it worked great, and Howard Vollum himself took me to dinner that night as a reward!" (Just kidding on that last part...) Anyway. OK, it worked... cool! But... ummm... does anyone happen to have some pointers on how he might have gone about performing that little calculation? I'm pretty well-versed in transmission line theory and imaginary characteristic impedances don't scare me. :-) I'd prefer to read up on a little theory here than just perform the "cut and try" approach in a simulator that the article above uses. Thanks, ---Joel
Purposely lossy coax: How to determine resistance?
Started by ●February 23, 2012
Reply by ●February 23, 20122012-02-23
"Joel Koltner"> > As most peple here are likely aware, if you examine the coax cable for a > passive oscilloscope probe you find that the inner conductor is made of a > highly resistive wire -- something like nichrome, and around 50-200ohms > per meter. (For some background on this, check out this article: > http://www.dfad.com.au/links/THE%20SECRET%20WORLD%20OF%20PROBES%20OCt09.pdf ) > .** Many budget probes have a very thin, stainless steel strand with a 100 ohm resistor in series. The thinness helps to minimise capacitance while still keeping the cable diameter small. BTW Doug Ford is an old mate - formerly he designed professional audio and lighting products for local Sydney manufacturers. ..... Phil
Reply by ●February 24, 20122012-02-24
On Feb 23, 9:36=A0pm, Joel Koltner <zapwire-use...@yahoo.com> wrote:> Hi guys, > > As most peple here are likely aware, if you examine the coax cable for a > passive oscilloscope probe you find that the inner conductor is made of > a highly resistive wire -- something like nichrome, and around > 50-200ohms per meter. =A0(For some background on this, check out this > article:http://www.dfad.com.au/links/THE%20SECRET%20WORLD%20OF%20PROBES%2=0OCt...> ). > > What's not clear to me, though, is how one analytically determines the > desired resistance of their coax in such a situation, given knowledge of > the source and termination impedances of the cable. =A0The magazine > article above mentions that, at least at Tektronix, John Kobbe came up > with the idea... and if you then dig into some of the Tek archives, you > can find his reminisces about doing so, where he says something along > the lines of, "it occurred to me that lossy coax would work well here > [to greatly extend the bandwidth of a passive probe], so I just > calculated what the appropriate resistance would be, ran down to the > stock room, got some and tried it out... it worked great, and Howard > Vollum himself took me to dinner that night as a reward!" =A0(Just kiddin=g> on that last part...) > > Anyway. =A0OK, it worked... cool! =A0But... ummm... does anyone happen to > have some pointers on how he might have gone about performing that > little calculation? =A0I'm pretty well-versed in transmission line theory > and imaginary characteristic impedances don't scare me. :-) =A0I'd prefer > to read up on a little theory here than just perform the "cut and try" > approach in a simulator that the article above uses. > > Thanks, > ---JoelJoel, Thanks for the link to a nice article. I'm afraid I can't help you. I hardly do any transmission line things... And I've forgotten all that Dr. Henry Neubauer taught me as a second year EE. But the problem looks like 'ringing' in the lumped LC parts. Can a tranmission line have the equivalent of a Q? Perhaps setting that near one will give the correct lumped resistance? George H.
Reply by ●February 24, 20122012-02-24
George Herold wrote:> But the > problem looks like 'ringing' in the lumped LC parts. Can a > tranmission line have the equivalent of a Q? Perhaps setting that > near one will give the correct lumped resistance?Thanks George, I'll keep that idea in mind. I might end up having to do some simple simulations with, e.g., various loads, plotting what distributed resistance seems to work well, plot the results and see if there's any, "aha!" moments.
Reply by ●February 24, 20122012-02-24
On Fri, 24 Feb 2012 13:48:17 -0500, Joel Koltner <zapwire-usenet@yahoo.com> wrote:>George Herold wrote: >> But the >> problem looks like 'ringing' in the lumped LC parts. Can a >> tranmission line have the equivalent of a Q? Perhaps setting that >> near one will give the correct lumped resistance? > >Thanks George, I'll keep that idea in mind. I might end up having to do >some simple simulations with, e.g., various loads, plotting what >distributed resistance seems to work well, plot the results and see if >there's any, "aha!" moments.The LT Spice lossy transmission line model might help, but it doesn't include skin effect. A passive 10x probe acheives its bandwidth sort of the same way a current-mode opamp does: as the signal frequency goes up, load the hell out of the source. Do you know about this? http://www.slack.com/TE/TekConcepts/TekProbeCircuits.pdf -- John Larkin, President Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
Reply by ●February 24, 20122012-02-24
John Larkin wrote:> A passive 10x probe acheives its bandwidth sort of the same way a > current-mode opamp does: as the signal frequency goes up, load the > hell out of the source.Yeah, I'll buy that. There's some optimal point where you start giving up bandwidth with excessive loading vs. having comb filter-like effects (from the reflections bouncing around) without enough loading.> Do you know about this?I believe I've seen it before but I'd forgotten about it; thanks for the link. Tek introduced a 1GHz passive probe last year. Although it's probably mostly for bragging rights (clearly someone at Tek wanted to push the envelope a bit...), the specs are quite respectable as well. ---Joel
Reply by ●February 24, 20122012-02-24
On Feb 24, 1:48=A0pm, Joel Koltner <zapwire-use...@yahoo.com> wrote:> George Herold wrote: > > But the > > problem looks like 'ringing' in the lumped LC parts. =A0Can a > > tranmission line have the equivalent of a Q? =A0Perhaps setting that > > near one will give the correct lumped resistance? > > Thanks George, I'll keep that idea in mind. =A0I might end up having to d=o> some simple simulations with, e.g., various loads, plotting what > distributed resistance seems to work well, plot the results and see if > there's any, "aha!" moments.I googled (Transmission line +Q) and found a Ham article with equations. There was some equation that had R =3D 2*alpha*Zo.. in ohms/foot... but I didn't dig enough to figure out what alpha was... But the 2*Zo part looked promising. George H.
Reply by ●February 24, 20122012-02-24
George Herold wrote:> Transmission line +QAh, this article: http://ve2azx.net/technical/QEXaudet-Rev1.pdf ? That seems more about modeling transmission lines as lumped circuits, but it definitely looks promising as solid background information. Thanks a lot!
Reply by ●February 24, 20122012-02-24
On Feb 24, 9:53=A0pm, Joel Koltner <zapwire-use...@yahoo.com> wrote:> George Herold wrote: > > Transmission line +Q > > Ah, this article:http://ve2azx.net/technical/QEXaudet-Rev1.pdf? > > That seems more about modeling transmission lines as lumped circuits, > but it definitely looks promising as solid background information. > Thanks a lot!Yup that's it... just a google find so no idea of the quality. (NPI) For short lengths a transmission line is like a cavity... well sorta... Yeah, lumped elements is how I'm thinking about the problem. There's some critical resistive loading. It's related to the characteristic impedance. George H.
Reply by ●February 24, 20122012-02-24
On Feb 24, 3:37=A0pm, John Larkin <jlar...@highlandtechnology.com> wrote:> On Fri, 24 Feb 2012 13:48:17 -0500, Joel Koltner > > <zapwire-use...@yahoo.com> wrote: > >George Herold wrote: > >> But the > >> problem looks like 'ringing' in the lumped LC parts. =A0Can a > >> tranmission line have the equivalent of a Q? =A0Perhaps setting that > >> near one will give the correct lumped resistance? > > >Thanks George, I'll keep that idea in mind. =A0I might end up having to =do> >some simple simulations with, e.g., various loads, plotting what > >distributed resistance seems to work well, plot the results and see if > >there's any, "aha!" moments. > > The LT Spice lossy transmission line model might help, but it doesn't > include skin effect. > > A passive 10x probe acheives its bandwidth sort of the same way a > current-mode opamp does: as the signal frequency goes up, load the > hell out of the source. > > Do you know about this? > > http://www.slack.com/TE/TekConcepts/TekProbeCircuits.pdfNo! Should I print out all 116 pages? (I'm only at page 10 but it looks great.) George H.> > -- > > John Larkin, President > Highland Technology, Inc > > jlarkin at highlandtechnology dot comhttp://www.highlandtechnology.com > > Precision electronic instrumentation > Picosecond-resolution Digital Delay and Pulse generators > Custom laser controllers > Photonics and fiberoptic TTL data links > VME thermocouple, LVDT, synchro =A0 acquisition and simulation