Purposely lossy coax: How to determine resistance?

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

"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

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,
> ---Joel


Joel,  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.

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.

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

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

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.

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!

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.

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.pdf

No!  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