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Homebrew HV hiZ scope probe

Discussion in 'Electronic Design' started by TheGlimmerMan, Oct 20, 2012.

  1. It comes on spools- manganin is better than nichrome, but nichrome
    isn't all that bad. In a pinch, brass or stainless are pretty awful
    tempco-wise, but relatively high resistance.

    These guys have 25m lengths of it with Kapton insulation:-

    http://www.iceoxford.com/Cryogenic-spares/Wiring.htm

    If you need many pounds/kg of it, it's easy to buy.
    Except for cable TV coax where it seems pretty easy to pull out (esp
    accidentally).. the dielectric is slippery PE, I think, and the center
    wire a thick smooth solid wire. But it's awfully stiff for test prod
    wire.


    Best regards,
    Spehro Pefhany
     
  2. Good suggestiin the P6106 manualon, but no luck. Tektronix lists
    itself as manufacturer of most parts, cable included. Interestingly,
    it lists the cable as '39 ohm' cable. The resistance of the central
    conductor of a 1m P6106 cable is about 130 ohms. The geometry of the
    cable suggests a Z0 much higher than 39 ohms, so I don't know what
    this is supposed to mean.

    Jeroen Belleman
     
  3. Frank Miles

    Frank Miles Guest

    Hmmn. Maybe 39ohms/ft (series resistance)?
     
  4. Fred Abse

    Fred Abse Guest

    P6015 manual describes the cable as "50 ohms per foot".
     
  5. Fred Abse

    Fred Abse Guest

    Not in the P6015 manual.
     
  6. Fred Abse

    Fred Abse Guest

    The Tek probes used 100meg resistors in the probe head, with a 100K
    effective load in the termination unit. Much more tractable to compensate.
    Any reason for going to 1000meg in your design?
     
  7. tm

    tm Guest

    1/10 th the circuit loading.
     
  8. josephkk

    josephkk Guest

    Gloves? Like hell. Use a mandrel (or a dowel rod).

    ?-)
     
  9. josephkk

    josephkk Guest

    Copper clad steel rather frequently, no good for broadband resistivety.
    But that is what you pull out.

    I would likely as not try to pull in constantan (perhaps nicked from
    thermocouple wire).

    ?-)
     
  10. Ralph Barone

    Ralph Barone Guest

    What was the code name for the coax with the helically wound insulation,
    leaving mostly air as the dielectric? You might stand a chance replacing
    the center conductor on that stuff.
     
  11. Guest

    I've known it as "heliax".
     
  12. Ah yes, exactly. I still get caught sometimes when some
    engineering outfit doesn't use metric throughout.

    Jeroen Belleman
     
  13. Yes, and it would be even harder to do that job as the elements that
    suspend the center conductor would get trashed on the removal, and then
    the replacement would no longer be "at the center", especially when you
    bend, form, or use the cable.


    The 'heliax' is an integrated structure, and breaking the bonds holding
    the center will mean the replacement can move about however it gets
    pushed. Not good.
     
  14. Jasen Betts

    Jasen Betts Guest

    Use coax with a high tensile steel core weld a piece of
    nichrome wire to the other end first
    you may have to ripple it off, like how a caterpillar walks.
    the outer is elastic, but the core is not,
    probably a few hour's work plus gettign the welds made (perhaps by a
    jeweler?)
     
  15. Fred Abse

    Fred Abse Guest

    There's a problem with that. The proposed 1:1000 single-stage probe
    requires a parallel compensating capacitance 1/1000 of the total
    scope-plus-cable capacitance, which is likely to be in the order of 50pF,
    which implies a compensating capacitor of 50 fF, or 0.05 pF. across the
    probe series resistor. This is probably impossible to realize.

    The Tektronix 1:100 HV probe used a special "leaf-and-collar" capacitor of
    a few pF, across the 100M probe resistor, which is taking things about as
    far as they can go, and still withstand the voltage gradient across the
    resistor. The capacitor dielectric was Freon, rather than air, giving a
    higher dielectric strength. Freons have a permittivity of around 2, which
    helps as well. The probe actually had a load of 100K at the scope end of
    the cable, not just the 1meg/22pF scope input, giving 1:1000 ratio.
    Compensation adjustment was done at the scope end of the cable, as is
    ubiquitous in higher-end probes today.
     
  16. Fred Abse

    Fred Abse Guest

    Says so on the schematic in the probe manual.

    Probe cables, at least modern ones, use foam dielectric, hence lower
    capacitance per foot.

    I've got some probes with removable cables, I'll TDR one when I get the
    time. I'll have a go at open and shorted measurements, too, and calculate
    the complex Zo. With a resistive center, Zo will be significantly complex
    at higher frequencies than "regular" coax.
     
  17. Fred Abse

    Fred Abse Guest

    He's just taking the pith.
     
  18. tm

    tm Guest

    Certainly not disagreeing with you Fred. In fact I don't know of any 1000:1
    probes that are good at high frequency measurements. But for DC
    measurements, you would want a 1Gohm probe for best accuracy measurements.

    Also, I think I read somewhere that the Freon 114 used in the 6015 probe had
    a permittivity near to 1. There was some discussion of using F-11 as a
    replacement but its permittivity is up there and the probe does not work
    well with it.

    There is some good information on the yahoo tekscopes group archives
    concerning this probe.

    Regards
     
  19. tm

    tm Guest

    I meant to say any 1 G ohm, 1k:1 probes.
     
  20. josephkk

    josephkk Guest

    Nicrhome is ok for heater uses, but i prefer constantan for electronic
    purposes. Check out the properties differences, it will be worth it.

    ?-)
     
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