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Oscilloscope calibration

Discussion in 'General Electronics Discussion' started by Supercap2F, Mar 27, 2014.

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  1. Supercap2F

    Supercap2F

    550
    150
    Mar 22, 2014
    Hi everyone!

    There is a oscilloscope on ebay right now at a good price and the seller says that it needs calibrating. Is that hard? Can someone tell me how to do it?

    Thanks :)

    Dan
     
  2. shrtrnd

    shrtrnd

    3,802
    507
    Jan 15, 2010
    Calibration, real calibration, is expensive.
    Who knows if this guy knows if the o-scope works or not. Maybe it's not giving
    him correct readings and so he thinks it need calibration. Maybe the readings are
    off if it's broken.
    Just my 2 cents worth.
    You don't just figure-out how to calibrate something, you need the correct calibration
    standards to ensure it's calibrated.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,480
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    Jan 21, 2010
    Calibration can be as simple as having a signal source with a known (and accurate) frequency source and amplitude, and using that to confirm that the horizontal timebase and vertical amplifiers are accurate.

    *real* calibration is a little more complex and a lot more expensive.

    "needs calibration" can mean anything from

    "This is a perfectly good scope that is fully functional and accurate, but the calibration certificate has expired a week ago"

    to

    "Following an incident, the timebases are all completely wrong and the vertical accuracy is shot, I haven't looked to see if there is any damage, so I'll just say it needs calibration"

    As a beginner, it will be hard for you to know the difference, and may (in the worst case) result in you effectively wasting your money.

    Look at eBay this way: Sure you can get stuff cheap, and that is money saved. Every time you save some money, you can think of that as capital to emotionally offset a purchase where you do your dough.

    I've bought lots on eBay and I am so far ahead on buying stuff that I can justify spending a several hundred dollars on something described as "for parts or not working", and another hundred on freight, knowing that if it didn't work I couldn't return it. If it didn't work, I'd be able to shrug and console myself that I was still ahead. (But I still wouldn't risk it for something anywhere near $1000).

    You probably can't do that. The money you're planning on spending is probably a lot for you, and you're probably not in a comfortable position of having already got so many bargains that this one matters less.

    1) Look for auctions where they offer some form of return guarantee
    2) Understand how to test the equipment before you buy it
    3) be prepared to test it when it arrives.

    And also:

    4) Ask the buyer how well it works if there are not pictures of the actual item in use.
    5) Be patient.
    6) look at the sellers feedback profile and the nature of the other things they sell.
    7) If it's an auction (as opposed to "buy it now") decide how much you're prepared to spend and bit that amount as close as possible to the end of the auction (within the last 5 seconds is ideal). Don't chase it (and bidding at the last second pretty much means you can't).
    8) look for people selling lots of scopes. Chances are they know how to test them (if in doubt, ask)

    You could also sign up to a local freecycle group and ask if anyone has a scope they're throwing out.

    If there's a hacker-space nearby you could join that. Apart from perhaps getting some more hands on help and experience (my local one runs things like "arduino nights" teaching people how to use microcontrollers for example) you might find someone there who can help locate an oscilloscope or help you test it. They may even have oscilloscopes for communal use; if they are within easy travelling distance, maybe you don't need to buy your own just yet.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    Great advice Steve!
     
  5. Supercap2F

    Supercap2F

    550
    150
    Mar 22, 2014
    Hey Guys

    Are the calibration adjustments internal or is it on the face of the oscilloscope? Do digital oscilloscopes need to be calibrated to? So far on ebay I have saved about $1700 but I make about $500 a year so if I got a dud oscilloscope it would hurt pretty bad. I have looked on the internet but as far as I can tell there are no electronics clubs close to me.

    Thanks

    Dan
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,480
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    Jan 21, 2010
    Let me preface this by saying I've never tried to calibrate a scope.

    Calibration is internal. There are likely a myriad of adjustments and if you're lucky they'll be labelled.

    However you need access to equipment that is more accurate than you want the scope to be. And you need to know exactly how to do it -- which means access to a service manual.

    You can do a quick check using the calibration output on most scopes. It will have a defined frequency and amplitude. This is not calibration, this is a quick check that things are not grossly in error. If it fails, you also need to know if it's the signal source of the scope.

    Practically speaking, most people use equipment that has never been calibrated. For most of us, within 1% may be perfectly fine.

    Don't get hung up on calibration. Get something that works.

    And even then, ask yourself if you want to spend the equivalent of 6 months of your earnings on a single piece of equipment. For far more years than I care to remember, all I had was an analogue meter and a soldering iron. Even with the best scope in the world, you'll still use your multimeter for 99% of your measurements.
     
  7. Supercap2F

    Supercap2F

    550
    150
    Mar 22, 2014
    Well I was not thinking of calibrating it myself I just wondering if there was a chance of me bumping the knobs or doing some thing that would mess up the calibration. Would it
    just be easier for me to buy a cheep usb oscilloscope or some thing like that?

    Thanks
    Dan
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,480
    2,826
    Jan 21, 2010
    No, there's no real chance of just bumping something to change the calibration. Certainly nothing as easy as a knob on the front panel!

    The problem with cheap USB oscilloscopes is that they have a very low bandwidth and are certainly not safe to use at voltages higher than maybe 50V.

    I have one. It samples at 1,000,000 samples per second. Whilst that might sound a lot, it really limits you to frequencies under 100kHz (if you want a reasonable indication of the shape of the waveform) but in practice 40kHz is the limit I would not exceed.

    This limits their usefulness to audio frequency, low voltage projects.

    Having said that, I have used mine for exactly those sort of things. They are also small, you can carry them in your pocket (I've actually misplaced mine at present), and operating from batteries means you don't have to be so careful where you place the ground lead.

    A lot of people dismiss these as toys. I'm not so quick to do that, but it is important to realise that they have major limitations.

    If you're going to consider one of these, you need to carefully think about what you intend using it for. Perhaps you should tell us.
     
  9. Supercap2F

    Supercap2F

    550
    150
    Mar 22, 2014
    Probably what I would use a oscilloscope for is to fine tune circuits. And I'm reading a book on op-amps that needs a oscilloscope (I mean what's the point of making a op-amp triangle wave generator if you can't see if the frequency etc. you calculated is correct?). Ebay has a handheld oscilloscope on it right now item number:221344056699 what do you think of it?

    Thanks

    Dan
     
    Last edited: Mar 28, 2014
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