Scope probe with low resistance cable

Discussion in 'The Projects Forum' started by Oldi, Nov 1, 2013.

  1. Oldi

    Thread Starter New Member

    Nov 1, 2013
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    I'm working on serious project in which I need probe like scope probe, but with low resistance (ohmic not impedance) cable with extreme stable capacitance. Working frequency is in audio range.
    Where to get such probe or manufacturer?
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Define "low resistance" and "extremely stable capacitance"
     
  3. Papabravo

    Expert

    Feb 24, 2006
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    Why do you think this extreme low resistance is a requirement for your application?
    Why do you think the capacitance of a given cable is "unstable"?
    What parameters do you imagine control the capacitance of a cable?
     
  4. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    How much money do you have to spend ???
     
  5. MrChips

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    Oct 2, 2009
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    You can purchase high quality low loss audio cable for higher prices. Personally I really don't know if it makes that much of a difference.
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    I claim it doesn't, but the Golden Ears claim otherwise.
     
  7. Oldi

    Thread Starter New Member

    Nov 1, 2013
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    Resistance of 1 ohm is OK (scope probe have few 100). Probe will be for low capacitance measurement with resolution 0.01pF, so must be stable, let say max +- 0.05pF change because of manipulation and heating because of hand. Antenna and audio cable have more then +- 0.1pF because of movement douring measirement different objects.
     
  8. MrChips

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    Oct 2, 2009
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    The capacitance of RG-58 is about 100pF/m.

    I don't know how you are going to achieve 0.01pF resolution.
     
  9. bance

    Member

    Aug 11, 2012
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    Very short leads?
     
  10. #12

    Expert

    Nov 30, 2010
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    Is my math right when it says the answer is about 20 thousandths of an inch?
     
  11. MrChips

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    No, I think it's more like 3 thou.

    It's about 3pF per inch.
     
  12. nsaspook

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    Last edited: Nov 1, 2013
  13. The Electrician

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    Oct 9, 2007
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    Flexible coax isn't going to be as stable as you require. Try semi-rigid coax:

    http://www.micro-coax.com/products/cable/semi-rigid/
     
  14. Oldi

    Thread Starter New Member

    Nov 1, 2013
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    I can measure with 10PPM precision, so 100pF cable capacitance is not the problem. If is stable ofcourse.
     
  15. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    "require" or "want"?


    OP, what is the goal? It seems that nearly everybody in the history of electronics has done this without the capabilities you are requesting. The surprising thing is, you are asking for these capabilities for the audio range.

    Could you help us understand WHY you need the performance standards you specified?
     
  16. Oldi

    Thread Starter New Member

    Nov 1, 2013
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    Because of measurement capacitance from 2 to 100pF with at least 0,1pF stability. Audio frequency because of nature of measurement object. I can't tell more.

    Semi-rigid is not possible because of life time, cable must be flexible almost like scope probe, RG58 style will be OK.

    Maybe cable with aluminum foil? I check one with 3mm diameter for TV aerial, but it's not OK.
     
  17. DickCappels

    Moderator

    Aug 21, 2008
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    Take apart a good quality scope probe (get an old style Tektronix probe), the cable is low capacitance and is has a fairly stable capacitance, much lower and better than coax cable for that purpose. If you don't need a shielded cable, you can merely use a pair of wires (insulated or not) held apart by spacers every couple of cm. Then you can choose your resistance when you select the wire size.
     
  18. Oldi

    Thread Starter New Member

    Nov 1, 2013
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    Scope probe cable have resistance 100 and more ohm and cable must be coaxial.
     
  19. Papabravo

    Expert

    Feb 24, 2006
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    No scope cable of mine has such a resistance. Regardless of what the OP wants it may not be available to his specifications. I really don't understand anything he has said because it looks and sounds like complete nonsense. Maybe he has some technical insight that the rest of us lack but who can tell? We just can't get a clear idea of what he is trying to accomplish.
     
  20. Oldi

    Thread Starter New Member

    Nov 1, 2013
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    I have 5 probes with X1/X10 switch (Pomona, Hitachi and noname). Resistance in X1 position is from 220 to 540ohm. I opened few broken probes (1 from HP) and at all cable have few 100ohm resistance of inner conductor.
    That's facts.
     
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