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oscilloscope probes VS BNC cable

Discussion in 'Electronic Design' started by john, Sep 14, 2007.

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  1. john

    john Guest

    Hi,

    Is BNC cable is appropiate cable to make oscilloscope readings or
    oscilloscope probes? My co-worker is insisting that BNC Cable is more
    shielded than the probes? I suspect that its not all about shielding
    only. There is a reason that probes exsists.

    We are trying to measure from an amplifer ( multiclamp700B). Please
    advice

    Thanks
    John
     
  2. Phil Allison

    Phil Allison Guest

    "john"
    ** Yes - it is done all the time where a piece of equipment provides the
    signal for the scope.


    ** Might be - bit it hardly ever matters.


    ** Yes - they exist to allow you to " probe " inside equipment & and
    pick up signals from wherever you like.

    Most scope probes are made with thin, low capacitance per foot cable ( using
    a very thin inner conductor) and are fitted with an adjustable head
    incorporating a 1:1 or 10:1 switch. The 10:1 position giving a 10 megohm
    load in parallel with a few pF of capacitance which is more suitable for
    probing sensitive circuit locations.

    Also scope probes are *designed* to work with standard 1 megohm in parallel
    with 20 pF input circuits of typical scopes.


    ** If it has a BNC output - then use a BNC to BNC lead.





    ....... Phil
     
  3. rebel

    rebel Guest

    First up, "BNC " is a family of connectors, not a cable type.

    But assuming you mean coaxial cable such as 50ohm transmission line, the use of
    a proper CRO probe system provides a lower capacitance cable. Part of the
    objective is to not load the device under test.

    The shielding on the proper probe cable is indeed typically not as complete as
    even something as mundane as RG-58C/U, but that really IS secondary.
     
  4. D from BC

    D from BC Guest

    Scope probes have a capacitance adjustment.
    This is to compensate for cable capacitance and scope input
    capacitance.
    It can make square waves look more square.

    But I guess the same circuit can be added to some qualified coax + a
    BNC connector. (Example: Maybe cable tv coax could be ok?)


    D from BC
     
  5. Rich Grise

    Rich Grise Guest

    It's not necessarily "better shielded". Most scope probes have a
    very tiny center conductor in their cable, yielding a high-impedance,
    low-capacitance cable. As D said, the cable comprises part of a
    capacitive divider.

    I have a salvaged piece of RG-58/U with a BNC on one end and bare
    wire at the other. I've soldered a 1K resistor in series with the
    center conductor, and it's "good enough", at DC and audio frequencies.

    But I was thinking of making a capacitive divider with it - I've got
    a 3' piece of coax, which is listed at, I think, 27 pF per foot. So,
    with 3 ft, that'd be 81 pF. Which, if I remember my old formulas, I
    would need to stick an 8.1 pF cap in series with my center conductor
    to yield a 10:1 probe.

    I haven't checked my numbers, or tried anything like this, primarily
    because I don't have an 8.1 pF cap lying around. And, of course, a
    trimmer would be better, to trim out the tolerances and square up those
    edges! :)

    But, the coax should be good enough for the audio range, but I
    recommend an isolation resistor between it and the DUT.

    Hope This Helps!
    Rich
     
  6. Phil Allison

    Phil Allison Guest

    "D from BC"


    ** ONLY divider probes have that.


    ** ONLY against the effect it has on the divided down signal.


    ** ONLY divided down ones.


    ** NO.

    Cos it ain't a divider.



    ........ Phil
     
  7. You didn't really mean that, did you?

    Cheers
     
  8. D from BC

    D from BC Guest

    Oops... That's right.. No comp on 1x probes (direct to scope).

    I think I just assumed everybody uses 1x10x divider probes..

    Divider probes and other reactances (without googling) is:

    ------probe---+---\/\R1\/\---+
    | |
    |------| |---- +------+---------+
    Ccomp | | |
    \ = Ccable = CinScope
    / | |
    R2 | |
    \ | |
    / | |
    Gnd Gnd Gnd

    What I meant was R1,R2 and Ccomp could be put on a qualified coax to
    make a divider probe.


    D from BC
     
  9. D from BC

    D from BC Guest

    Noooo... I don't mean it's going to make 45% duty look like 50% duty.
    :p

    1x probe cable capacitance and scope input capacitance can have an RC
    effect on some circuits with fast edges. The scope can present a
    slower than actual edge times.

    With a 10x probe the circuit sees less applied capacitance. The
    circuit sees a capacitor divider composed of Ccomp in series with
    Ccable + Cscope.
    Therefore a closer to actual edge time can be viewed.

    Hope I got that right :|...
    D from BC
     
  10. It can be if the device you are "probing" provides a low impedance
    signal output designed for driving a coax cable. For example, if you
    are measuring the output from a signal generator it's common to use a
    standard BNC cable rather than a probe because it's a more convenient
    and provides a better quality connection. Also, a standard BNC cable
    can give you more bandwidth for your $$, as quality high speed probes
    are expensive.
    Yes, probes exist to allow you to more conveniently "probe" point of
    interest in a circuit with minimum loading (they have lower
    capacitance). Also, a 10:1 probe allows you to measure much higher
    bandwidth (faster) signals than just a standard coax cable can.
    In that case you would use a BNC cable, with a BNC T piece if needed.

    Dave.
     
  11. Rich Grise

    Rich Grise Guest

    What if my scope has 1M, 25 pF at its input, such that R2, 1M, is to the
    right of Ccable; Ccable is approx, 27 pF per foot, and it's just under
    3' long - would R1 be 10M and Ccomp about 9.1 to 10.0 pF?

    Is there a formula for gimmicks somewhere? :) Do I have to buy a 10M
    resistor to make a proper probe?

    Thanks,
    Rich
     
  12. The resistor divider has to drop the voltage to 1/10th, so
    the resistors have to have a ratio of 9 to 1, since the
    voltage is proportional to R2/(R1+R2). So a 9.1 Meg
    resistor is pretty good. Likewise, the cap that parallels
    the 9.1 meg resistor has to be about 1/9th of the total
    input capacitance (cable capacitance plus scope input
    capacitance. Now you see why they go to so much trouble to
    build special low capacitance cable for scope probes. I
    wonder if you can pull out the center conductor and pull in
    a very slinky high E guitar string (tinned with silver
    solder at each end for electrical connections).
     
  13. D from BC

    D from BC Guest

    E 0.20mm
    http://www.mohomods.com/strings.htm

    RG59 cable core wire AWG 22 = 0.64516mm
    http://rocky.digikey.com/WebLib/Alpha Wire/Web Info/9055,58,59 9803,30,48,58 p279.pdf


    Diameter reduction figure: 0.65/0.2 = 3.25


    100Mhz Scope probe core wire diameter ???

    Not here http://en.wikipedia.org/wiki/Test_probe
    Not here
    http://www.tek.com/products/accessories/oscilloscope_probes/passive.html

    Nope... but it's a page on making a DIY scope probe...
    http://www.cromwell-intl.com/radio/probes.html

    Fk it... I'll guess 33 AWG which ~ the E string

    Anyways...
    Getting an E string preferrably centered in 3ft of RG59 (or other)
    might be tough.
    Pulling out the coax core wire might be difficult.
    Some oven heating might help. :p

    RG6 replaces RG59.
    http://en.wikipedia.org/wiki/RG-6
    It has a fatter 18 AWG wire.
    The guitar string idea gets better stuck in RG6.

    RG6 5.00 dB attenuation per 100' at 750 MHz
    http://www.atechfabrication.com/products/coaxial_cable.htm
    But I think that's with video source and load impedances.


    D from BC
     
  14. I would look for low capacitance (high impedance) teflon
    insulated coax. Besides, it is slippery when you try to
    push the E string through it. E.g.:
    http://glacier.lbl.gov/~gtp/DOM/delay/Belden_RG-180.html
     

  15. Exactly. It is all about impedance and impedance matching.

    one must be sure to make the proper settings choices on the scope as
    well.
     
  16. D from BC

    D from BC Guest

    I'm getting a bad vibe about that RG-180...I think it's going to be
    pricey, min quality'd and difficult to find a source..
    RG6 can probably be found a Radio Shack, London Drugs, Home Depot or a
    surplus electronics store.
    Or perhaps stop the local cable guy and give'm 5 bucks...


    D from BC
     
  17. Does D from BC mean you are in British Columbia? I have
    some RG-180 I got on eBay. It is sort of stiff, but not
    much worse than most coax that is not made to be an o-scope
    probe. If governments don't get in the way too badly,
    perhaps I could send you 10 feet, or so, in an envelope, to
    play with.
     
  18. D from BC

    D from BC Guest

    Yup..I'm in British Columbia.
    Land of really bad traffic, great skiing, marijuana and Chinese food.
    :p
    Surrey BC might still be car theft capital of North America.
    Weather is like 50% continuous sun and 50% continuous rain.
    And a V6 is must due to some roller coaster type roads.
    Also, the women are in great shape because they don't make enough
    money to eat alot :p

    About the wire..
    Thanks for the offer..but I have cheapo 100Mhz probes I got off ebay.
    I have an interest in DIY probes but stay away from making one :)



    D from BC
     
  19. There are some situations where a piece of cable can give much better
    results than a probe, where the circuit impedance is low (like your
    amplifier output perhaps).

    A common situation for me is measuring mV ripple on the output of a
    switch mode power suppy. A normal x10 probe would attenuate the signal
    unnecessarily, and the ground lead would pick up a *lot* of magnetic
    noise from the switching inductor. Simply connecting a "BNC" (RG58)
    cable gives much cleaner results.

    Also see e.g. Jim Williams' classic AN47.

    <http://www.google.com/search?q=ltc+an47>
     
  20. Rich Grise

    Rich Grise Guest

    Well, I _could_, but it's the only BNC I have and it came already
    attached to the coax. I guess I'll just save up my pennies and buy
    a proper probe when I can afford it. :)

    Thanks!
    Rich
     
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