# Purposely lossy coax: How to determine resistance?

Started by February 23, 2012
```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:
).

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:
> .

** 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
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.

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

I believe I've seen it before but I'd forgotten about it; thanks for the

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

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
>
>
> 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
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.
>
>
> 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

```