Buying Oscilloscope Probes

Discussion in 'General Electronics Chat' started by Sparky49, Nov 9, 2014.

  1. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    Hi everyone.

    I have just bought a new (old) digital oscilloscope from Ebay. Should be arriving in a few days and I'm really excited, it's my first digital oscilloscope and it has a pretty decent bandwidth (400MHz).

    Anyhow, until now, I have used the 'cheap' 15MHz probes from Ebay to do measurements, but with the higher frequencies I am planning on measuring I presume these will not be so suitable.

    So, what do I need to look for in an oscilloscope probe? Googling hasn't provided many clues, reviews tend to be for the cheapest, low bandwidth probes on the market. Bandwidth is presumably a key characteristic, but surely there must be differences between the £50 and £500 versions? (I'm not buying a £500 probe lol).

    Many thanks for your time.

    Sparky
     
  2. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    I use Uni-T probes from ebay and they never gave me any problems at all.. I would find a set of probes that have the needed comp range for your scope and you should be good .. Unless you want to find some used probes and try them ....
     
  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    1,215
    I have a 500 MHz LeCroy scope and I use the inexpensive Tektronix copies for normal use (<100Mhz) and one or two high end probe for >100 Mhz. EBay EBay.
     
  4. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    Yeah, that's the plan I was aiming for.

    But what sets the higher end probes apart from one another? I can get a new 450MHz probe from one place for £50, and a 300MHz probe from another for £100. What else is there to consider?
     
  5. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
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    Well the age old question ... Do you really have a use for them at the high of a frequency?¿? Are you going to build something that really need all that bandwith or can you just use 100 MHz or slightly higher and be fine ???
     
  6. Sparky49

    Thread Starter Active Member

    Jul 16, 2011
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    417
    "Do you really have a use for them at the high of a frequency"

    Yes, hence why I am asking what other characteristics of probes set them apart from one another.
     
  7. tindel

    Active Member

    Sep 16, 2012
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    I've always been curious if you can really get 400MHz bandwidth with passive probes... I just haven't had time to test it. I have some passive probes at work that claim 500MHz... which I think is phooey. I'd recommend active probes, with the circuitry near the tip to obtain the lowest parasitic capacitance possible, but active probes are expensive. and as always... keep your probe leads as short as possible to minimize inductance, as well.
     
  8. crutschow

    Expert

    Mar 14, 2008
    12,973
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    Cost is usually (but no always) related to the quality and durability of the probes. For high frequency you want one with the lowest 10X probe capacitance since even a small capacitance provides a significant load at 500MHz. For example 10pF has an impedance of about 40 ohms at 400MHz.
     
  9. WBahn

    Moderator

    Mar 31, 2012
    17,715
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    Is it possible that the first are passive and the second are active?
     
  10. tggzzz

    New Member

    Mar 14, 2015
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    Well, £500 for a scope probe is pretty cheap - some are >£10000!

    Fortunately there is one type of very high performance probe that is relatively easy to make at home - a "low impedance Z0" probe. The name is a misnomer, since it can have an impedance higher than a standard 10:1 probe.

    See the references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/ for when to use different types of probe, and how to make your own probe.
     
  11. Esposito

    New Member

    Apr 21, 2015
    4
    0
    The original probes supplied with the 2235 were Tek P6109's. However a P6121 or P6122 will also match up. And I've used the older modular Tek P6105 probes also. The input is 1 megohm w/20pF capacitance. A 1 megohm input is very standard and most aftermarket probes will work with this input.

    These probes are all 10X type. Meaning they reduce the signal by a factor of 10 to provide low circuit loading and maintain good bandwidth up to 100mhz, which is the 3dB point of the scope. (50% down) For all but the most critical pulse analysis, low cost aftermarket probes will be ok. They will work but may lack the durability of Tek probes.

    All 10X probes must be compensated (adjusted) to the scope's input by looking at the scope's square wave calibrator output and adjusting for a sharp corner without overshoot or undershoot. This is usually done with a small screwdriver adjustment at the probe's input connector, or sometimes by turning the probe's body.
     
  12. tggzzz

    New Member

    Mar 14, 2015
    4
    3
    Not really. At 100MHz a 15pF tip capacitance has a load of only 107 ohms.

    A "low impedance Z0 probe" has a load of 500ohms or 1000ohms!

    3dB = 50% power reduction = 30% voltage reduction (remember dB = 20log10(v1/v2) )
     
  13. RichardO

    Well-Known Member

    May 4, 2013
    1,228
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    The simple answer is to buy the best probe you can afford -- your measurement can _never_ be better than the probe. I once paid more for probes to go with a used scope than I paid for the scope itself. I have never regretted it.

    Also remember that the system risetime is equal to the square root of the sum of the squares of the risetime of the scope and the probe:
    System_risetime = sqrt (scope_risetime ^2 + probe_risetime ^2).
     
  14. Himanshoo

    Member

    Apr 3, 2015
    228
    6
    does normal bnc connector work at high frequencies?
     
  15. tggzzz

    New Member

    Mar 14, 2015
    4
    3
    Yes; the Keysight N2874 is a 1.5GHz probe with BNC connectors: http://www.keysight.com/en/pd-1661742-pn-N2874A/passive-probe-101-15-ghz-13-m?cc=GB&lc=eng At these frequencies, the BNC connector is not the major problem.

    For more information on probes, see https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
    For a homebrew accessory that improves the signal integrity visible with standard 10:1 probes in the hundreds of MHz range, see https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/
     
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