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215 ohms?

Discussion in 'Electronic Components' started by A E, Oct 5, 2003.

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  1. A E

    A E Guest

    What uses 215 ohm impedance?
    Going through my new desk at work I came across an otherwise unmarked scope
    probe that reads "215 ohms 1:1".
    No one else seems to know what it's for either. It's manufactured, not home
    made.
    Any thoughts?
    Thanks,
    Alex
     
  2. mike

    mike Guest

    Take your ohm-meter and measure from tip to center pin
    of the bnc. What do you get?
    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/
     
  3. Eric

    Eric Guest

    :
    : Take your ohm-meter and measure from tip to center pin
    : of the bnc. What do you get?
    : mike
    :

    That will not work, try it with 50 & 75 Ohm coax cable.
     
  4. mike

    mike Guest

    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?
    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/
     
  5. Dave Platt

    Dave Platt Guest

    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. 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.

    The characteristic impedance can be measured only using radio
    frequency techniques, TDR or similar impulse measurements, etc. You
    simply cannot determine it using a DC ohmmeter.
     
  6. I wonder if Mike is asking for a DC measurement to see if there's some
    kind of resistive divider in the probe. You might also want to
    measure across the input and between the coax on the input and output
    sides to see if there's some kind of network in there...
     
  7. mike

    mike Guest


    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.[/QUOTE]

    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
    There you go with that braid again.
    I never mentioned characteristic impedance either...

    can be measured only using radio
    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/
     
  8. Eric

    Eric Guest

    what we are trying to tell you Mike

    it might have an impedance of 215 Ohms

    But you can NOT measure it with DC current, no were will you find it with DC
    current, full stop.










    : 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/
    :
     
  9. I think what Mike is trying to suggest is that there may be a
    resistive network inside the silly thing, and that by making a few DC
    resistance measurements you might get some hints as to how the thing
    is wired internally.

    He seems to understand that the high frequency impedance of it can't
    be measured with an ohmmeter, but many impedance matching networks are
    made from resistive components, yes?
     

  10. Hey, if we can quit wrangling over how to measure the impedance of a probe,
    maybe we can get back to the OP's actual question - I'd love to know the
    answer myself. That question was:

    "What uses 215 ohm impedance?"

    If s/he'd asked "what uses 75 ohm impedance" or "what uses 50 ohm impedance"
    or "what uses 600 ohm impedance" I'd have some answers (no doubt answers
    s/he already knows)... But I've never heard of a transmission system built
    around 215 ohms.

    Has anyone else?
     
  11. mike

    mike Guest

    I understand what you're trying to tell me. I'm denying it's
    relevance to my suggestion that the OP measure the DC resistance of
    the center conductor.
    Yes, it might. I have not done the math...
    Typical probe shield inside diameter is 0.06". Center conductor is
    ..0025, but this is past the accuracy limit of my dial caliper.
    I was too lazy to measure the dielectric constant, use 2.3 for poly.
    Do the math.
    Come back and tell us if
    you still think the Zo might be 215 ohms.
    Hint, the back of my envelope sez 126 ohms...so I guess now I HAVE done
    the math.

    And since I went to all the trouble of going up in the attic to fetch
    some probe cable, I measured it. This particular stuff is 38 ohms/foot.
    That's DC RESISTANCE OF THE CENTER CONDUCTOR.
    NOTE THAT THIS IS NOT, NOT, NOT THE Zo...IT IS DC RESISTANCE!!!!!!!
    MEASURED WITH AN OHM-METER, NOT A TDR!!!! OF THE CENTER CONDUCTOR,
    NOT THE SHIELD, BRAID, OR ANY OTHER TERMINAL.
    I DON'T KNOW HOW TO BE MORE CLEAR, SO I'LL JUST SAY IT LOUDER!!!!!
    ;-)
    Stop as full as you want. I NEVER mentioned characteristic impedance
    except to state that it was not relevant to MY suggestion that the OP
    measure the damned DC resistance of the center conductor.

    About three posts up this thread (and quoted below, so we know you read
    it) I disclosed EXACTLY why one might want to measure
    said DC resistance...yet you still tell me that the question that I
    didn't ask is the wrong one.

    So, after all this, if the OP had just answered the question I first
    asked, he would have had the answer. Maybe 20 years designing
    instrumentation for Tektronix...nah, dumb luck.

    I just love the web. It entertains me for hours (in this case days) at
    a time.
    mike

    Read
    Read it again
    Understand
    Think
    Think some more
    Post


    --
    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/
     
  12. Dave VanHorn

    Dave VanHorn Guest

    Makes perfect sense to me.
    Measurement cables don't need to be lossless, but they do need to be nicely
    damped, so they are rather resistive.

    Measure a tek scope probe lead.
     
  13. A E

    A E Guest

    That's what I was looking for, not to start a war over impedance... (cringe)
    Maybe if someone just said 'Oh that's a standard for probing klystron repeller
    plates' I'd be happy.
    But the probe just sits there, defying me all day at work. And yes, it measures
    about 200 ohms from the tip of the probe to the pin in the BNC. And there
    doesn't seem to be an attenuator in there either, just looks like a BNC plug...
     
  14. Rein Wiehler

    Rein Wiehler Guest

    gents, look at this url, www.precisionspeed.com/sender.htm ,they got
    some instrument using 215 ohms impedence. don't knows what it is good for
    rw
     
  15. what we are trying to tell you Mike
    Having read the thread to here, it is obvious to me that you did NOT read
    what Mike posted before you responded to it. Maybe you skimmed it, but
    you did NOT read it.

    Mike referred to the possibility of actual, DC measureable, resistance having
    been deliberately introduced in series along the center conductor -- which
    would, indeed, be measureable with a DC current, also along the center conductor,
    if such resistance(s) were present. He also provided sound reasoning based on
    RF propagation issues why such actual resistance might have been so introduced.

    Your objection applies to something orthogonal (in the geometric and linear
    algebra senses) to what Mike wrote. While it is a valid objection to what
    you clearly inferred Mike had written, (the inability of DC instrumentation
    to measure surge or characteristic impedances), it has nothing to do with what he
    actually wrote.

    HTH
    Kevin
     
  16. A E

    A E Guest

    Alright guys, let's all calm down here.... I think that this probe is nothing
    more than a standard 1:1 scope probe...
    I'm so occupied with exotic stuff that a simple scope probe threw me... I've
    never seen a 1:1 probe before in my life, always those 10:1s with a switch for
    the 1:1 mode, and they *never* had the series resistance printed on the body ...

    I just started this new job so I thought that those 215 ohms could be some
    uber-exotic thing, guess not!
    It's just a probe with a cable that ends in a BNC connector, that's it, that's
    all...
     
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