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  1. #1
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    Seeing a crisp rectangular wave on an oscilloscope

    There do not seem to be good probes on the market. I want to see a 5 MHz rectangular wave with 500 ps switching time.
    Is anyone aware of a solution of this problem?

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    Re: Seeing a crisp rectangular wave on an oscilloscope

    There are fine probes with 1 GHz and above bandwidth, preferably resistive passive and high impedance active probes which can reproduce 500 ps signal risetime. Obviously the oscilloscope bandwidth must be sufficient as well.

    Whereever possible, we would connect the signal through 50 ohm cable directly without using a probe.



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  3. #3
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    Re: Seeing a crisp rectangular wave on an oscilloscope

    There's probe bandwidth and there's channel bandwidth.
    Probe BW is lower as far as I've seen (because what's
    the use of more BW than the 'scope front end can pass?).

    500pS risetime is (if you call it a quarter cycle) 500MHz
    (-ish) and you can find passive probes of about this range.

    The idea of using 50-ohm cable and a well constructed
    passive divider is a good approach provided that your
    source impedance (hence acceptable divider impedance)
    is low enough to make sense. 1Mohm against a 50 ohm driver
    is wasting BW. I doubt you're seeing 500pS risetimes in a
    high-Z system.

    - - - Updated - - -

    Another thing - it may not be probe BW per se, but probe C
    and source Z combining to simply degrade the edge rate.
    I doubt you'll see LVDS, PECL, CML rated for transmission
    lines with excess C - they rate, and design to, as little shunt
    C in the "load" as they can (or their customers let them) get
    away with. 10pF and 50 ohms, even, there's your 500pS
    even if probe backend and channel were infinite BW.



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