: Dave Platt wrote:
: >
: >
: >>ExcUUUUUUUUse me???
: >>What about hooking two ohm-meter leads to two points on a cable
: >>and reporting the result "will not work"? I'm sticking my neck out and
: >>will guarantee
: >>that if you stick one lead on the probe tip and the other on the
: >>center of the bnc, you WILL get a reading somewhere between
: >>zero and infinity ohms. What's the number?
: >
: >
: > If the coax isn't shorted somewhere inside, then a standard DC
: > ohmmeter reading of the center-conductor-to-braid resistance will be
: > as close to infinity as matters.
:
: Except that I said from probe tip to bnc center conductor. Nowhere did
: I mention braid, shield, ground or any other terminal.
:
: If the ohmmeter doesn't read nearly
: > infinite, it's due to leakage somewhere (even fingertip contact with
: > the probes or coax conductors could pull the reading down to a few
: > megohms).
: >
: > Whatever reading comes out of the ohmmeter, will have _no_
: > relationship at all to the cable's characteristic (or surge)
: > impedance. 50-ohm cable such as RG-58, 75-ohm such as RG-59, 93-ohm,
: > 150-ohm cable... they'll *all* read nearly infinite DC resistance from
: > center conductor to braid/shield.
:
: There you go with that braid again.
:
: >
: > The characteristic impedance
:
: I never mentioned characteristic impedance either...
:
: can be measured only using radio
: > frequency techniques, TDR or similar impulse measurements, etc. You
: > simply cannot determine it using a DC ohmmeter.
:
: Pet Peeve Alert:
: I love it when people chime in with authoritative statements about
: issues well beyond their area of expertise. Seems the farther it gets
: from the issues, the more people weigh in.
:
: Let's review...
: About all we actually KNOW is that the OP has some unknown 1X probe and
: a question about a number.
:
: In such instances, it's often tutorial to ask a well-formed question that
: will provide additional data and maybe even lead the OP to a direct
: answer to his question.
:
: Then we get people (who apparently didn't read the question) jumping in
: to say that the question is wrong and
: proceeding to detail why some other question would have been wrong.
:
: I don't have the Psychic Hotline on speed dial. I don't know anything
: about the OP's probe. But I can give some general information.
:
: Tutorial starts here:
: It's difficult to get a signal into a 1 Meg scope input. Most probe
: leads act like relatively low impedance transmission lines. It's not
: possible to terminate them. 'bout all you can do is stick some
: resistance in series to damp reflections. Turns out that it works a lot
: better if you distribute the resistance along the transmission line.
: Quality probes are made with special coaxial transmission line with
: resistance wire for the center conductor.
:
: So, if you were to make a resistance measurement from probe tip to the
: center pin of the BNC, you just might measure some resistance. And if
: that resistance were approximately the number in question, you might
: have an answer to your question.
:
: Flame suit on...take another shot from the hip.
:
: mike
:
: --
: Bunch of stuff For Sale and Wanted at the link below.
: laptops and parts Test Equipment
: 4in/400Wout ham linear amp.
: Honda CB-125S
: 400cc Dirt Bike 2003 miles $550
: Police Scanner, Color LCD overhead projector
: Tek 2465 $800, ham radio, 30pS pulser
: Tektronix Concept Books, spot welding head...
:
http://www.geocities.com/SiliconValley/Monitor/4710/
: