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selecting a scope, help!

Discussion in 'General Electronics Chat' started by patroclus, Jan 26, 2007.

  1. patroclus

    patroclus New Member

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    I never owned an oscilloscope, and worked with a 500MHz logic analyzer for most of my projects, as I'm 95% into digital stuff.

    Now I want to get a second hand scope...
    What would you recomend me:

    - A 20MHz dual trace with built in function generator
    - A 100MHz dual trace analog scope
    - A digital scope in the range of 100/200 MSamples/s and 40MHz bandwidth.
    - A four channel analog scope.

    What I don't like of analog scopes is that they are not able to capture single shot events, very usefull in digital stuffs.

    what would you recomend me go for?? thank you!!
     
  2. stevez

    stevez Active Member

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    It sounds like the capture of single shot events is useful or important to you. You list suggests that only one scope will do that. If you are willing to set that requirement aside then you need to judge whether the others on your list will be acceptable. Is a 20 mHz scope going to be enough? All depends on what you are doing.
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Exactly, it all depends what you want it for - a 20MHz dual channel scope is all you need for almost all purposes (probably 99+%?) - but when you need that other 1% you're totally stuffed!. A more expensive scope will also perfom better at the top end of a lower spec scope - so a 100MHz scope will be better at 10-15MHz than a 20MHz scope.

    As you're interested in digital stuff, a storage scope sounds like a good idea - but if you come across a cheap 20MHz scope, get that as well - two scopes are better than one!.
     
  4. dave

    Dave New Member

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  5. stevez

    stevez Active Member

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    I'd appreciate some help (am I correct?)from the pros on this one - if you thought you might be limited to 15 mHz waverforms where the sharpness of a corner (square wave) might be important - you might be convinced that you had a problem because the corners weren't sharp when in fact it was the limitation of the scope- the 100 mHz scope might better represent what was really there.
     
  6. patroclus

    patroclus New Member

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    It is for all purpose use, but I'm mainly into digital stuff.

    But I don't have some things clear... For example, if I want to measure a rise time, in a transistor response, for example, and this raise time is about 10ns (100MHz) I would not be able to detect it with a 20MHz scope??
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    No you wouldn't, and why would you expect to display 100MHz on a 20MHz scope? - you'd probably struggle seeing it on a 100MHz scope as well!.

    Assuming the waveform itself is low enough, and just a fast rise time, you'd see the waveform fine - which is generally what's required.
     
  8. patroclus

    patroclus New Member

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    Nigel, by the way, what use can you do of a 20MHz scope when working with PICs? I always use my logic analyzer, since I need to know what is going on in the data pins to see if it's working. What things can you do with a scope??
     
  9. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    There's very little you need a scope, or a logic analyser, for with PIC's.

    First obvious thing is to check the oscillator is running - ensure you use a x10 probe, or the capacitance will stop the oscillator anyway. Obviously you can use it to check any incoming or outgoing signals (such as RS232, IR remote etc.).

    But probably my biggest use is for debugging - allocate an output pin as a debugging pin, and insert a line of code to make it high before you call a section of code, then set it low again when the code returns. You can use the scope to monitor and measure the resulting positive going pulse.

    It's also useful technique to check interrup routines, have a line of code that toggles your debugging pin, and monitor it with the scope.

    A lot of this would probably be improved by a storage scope, but a little thought means a normal cheap scope does just as well.

    In this way you can trace problems and find what's going wrong.
     
  10. patroclus

    patroclus New Member

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    I see...
    also, is B-trigger very usefull? not all scopes have it.
    I have an oportunity to get an old 20MHz dual scope for $100 + probes. It has hold-off, several coupling modes,... but not B trigger.
     
  11. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    What is 'B trigger'?.
     
  12. patroclus

    patroclus New Member

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  13. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It's just a delayed trigger, you might use it VERY occasionally, I perhaps use it once every 2 or 3 years?, and I use a scope almost every day!.
     
  14. patroclus

    patroclus New Member

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    Ok, I'll go and get a cheap $100 scope and start playing around with it. Then, I'll be able to know exactly what I want and need.

    The scope I'm talking about is an old 20MHz dual trace, with holdoff, AC/DC/HF/LF coupling, min 100ns/div, delay up to 10ms, 1X/10X multiplier, X-Y up to 2MHz, 15cm screen,...

    I'll go and check that it is working to a tech service local, where they're selling it. Should I check something in special before assuming it works fine??

    Thank you!
     
  15. Hero999

    Hero999 Banned

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    Often a frequency counter can be useful for filling in the gaps a scope can't reach and they're pretty inexpensive even with bandwidths exceeding 1GHz .
     

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