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Basic oscilloscope recommendations

Discussion in 'Electronic Equipment' started by spasmous2, Jun 12, 2007.

  1. spasmous2

    spasmous2 Guest

    I need to buy a 4-channel oscilloscope and am hoping someone can
    suggestion what make/model might be suitable. I only require about
    10MHz bandwidth for the transient signal, and it repeats every 0.1 to
    10 seconds.
    out really quickly whereas I'd like it to stay on screen until the
    next trigger. I was thinking an LCD screen might be the solution to
    that problem? Does that necessitate a digitial scope?
    really I don't need anything fancy, like FFT or USB storage. I just
    want 4-channels and a persistent trace on the screen. And 50 ohm input
    impedence.

    Can anyone point me in the right direction? Thanks.
     
  2. KC

    KC Guest

    Take a look at the Hameg scopes. They sell excellent product without
    the fancy gadgets for a fair price.
     
  3. mike

    mike Guest

    Yes, you need digital storage.
    Never, ever buy a scope with a bandwidth less than 100MHz.
    Requirements change. You ALWAYS need more bandwidth than
    you anticipated.
    For slow signals, memory depth is an important parameter.
    Draw the waveform you want to look at.
    Put dots on it representing samples close enough together
    so you can see what you want to see. Figure out the time
    between samples. Then count how many of those samples
    will be needed to cover the area of interest. If you expect
    to see 10 MHz. bandwidth stuff over a whole 10 second interval,
    you're gonna need a LOT of memory depth...times 4. If you can't afford the
    memory, you're gonna need delayed triggering or some other
    means to see what you want to see.

    You haven't disclosed the nature of the waveforms you want to see,
    but what you have said indicates that your situation may require
    far more than "basic oscilloscope" features.

    Don't skimp on a scope. It's your best tool for a lot of
    design/debug situations. Requirements change, and waveforms
    are NOT slowing down.
    mike
     
  4. Hammy

    Hammy Guest

    Do you really think memory depth is all that important? When you can
    just hook up a DSO to your PC via USB or RS232 and record the waveform
    for as long as you need, save it in a wmv file and review at your
    leisure. Or am I missing something?
    I agree.
     
  5. mike

    mike Guest

    Only if you want high resolution for a long time.
    Do the math. Acquisition time divided by the time between samples
    equals the memory depth requirement.
    Yes, there are different techniques, but not typically found on
    entry-level scopes.

    When you can
    YOUbetcha! IFF your DSO has the capability to spit out real-time data.
    AND your PC has the port and the capture software and the I/O bandwidth
    to acquire the data. And if you have the software to transform it into
    wmv, although I don't know why you'd want that.
    Remember, we're talking 10MHz. analog scope bandwidth here.
    Or am I missing something?
    yes
     
  6. spasmous2

    spasmous2 Guest

    Thanks mike for your thoughts. Digital scopes seem to start around
    $2000, which is steeper than I was hoping. On ebay there are many
    analog scopes that are high quality for much less than this.

    I would get an analog but I just don't know if the screen persistence
    is adequate. The signal I am looking at is around 20ms duration and
    repeats every second or so, so the update of the screen would be
    very slow on an analog scope. I really want the trace to stay on the
    screen until the next trigger. Can analog scopes accomodate this?
     
  7. Jean-Yves

    Jean-Yves Guest

    sure it will not...
    you can get quite cheaps digital scopes on ebay too...
    but for that job you sure need a digital scope.
     
  8. spasmous2

    spasmous2 Guest



    I am a little confused about digital storage oscilloscopes. I have
    seen some DSO's that appear to have an analog screen (eg.
    http://www.harlanlabs.com/2430a.jpg) whereas others are obviously
    digital because of the LCD color display (eg.
    http://www.testequity.com/Images/new/tektronix/tds2024.jpg).

    I may be just mixed up, but can the first model (Tektronix 2430A)
    acquire a 20ms transient, then hold it on the screen until the next
    trigger, which may be a second later - or even hold it on the screen
    indefinitely?
     
  9. DaveM

    DaveM Guest

    I haven't been following the entire thread, but hopefully I won't get flamed for
    breaking in here.
    I understand that you're looking for a 4-channel scope. Don't know if this is
    an absolute requirement, but if you can get by with a 2-channel scope, an analog
    storage scope is certainly usable at the time intervals you describe.
    They have several storage modes; one is a timed storage, where the trace is
    reset after a preset elapsed time, or triggered reset, where the trace stays on
    the screen until the next sweep trigger. The brightness is quite good,
    especially on slow sweep speeds.
    If you want to try a cheapie to begin, I recommend that you look at Ebay for a
    Tektronix storage scope. The model 265 scopes were a staple in their day, and
    there are lots of them on Ebay, usually in the $200-$300 range. If you can find
    one that the seller will verify that it is a working scope, they can be a
    bargain. On the other hand, they are obsolete, no longer supported by
    Tektronix, and some parts are hard to find.

    There are other Tektronix models of analog storage scopes on the market, usually
    for reasonable prices.

    --
    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
    address)

    Life is like a roll of toilet paper; the closer to the end, the faster it goes.
     
  10. mike

    mike Guest

    The absolutely positively most important thing is to define with
    precision what you want your scope to do. Your current definition
    I interpret to take about 3 orders of magnitude less memory depth
    than my interpretation of your original requirement.

    How much of the signal do you need to see on the screen at one time from
    one acquisition? And how fine a detail do you need to see when you
    magnify it? That determines what you need from a digital scope.

    Second thing to worry about is how much your time is worth.
    If you make money with your time, and your time gets $100 an hour,
    your break-even point is 20 hours. That's a no-brainer.
    If lack of measurement delays a project, it's mismanagement.
    A system that's difficult to use on an occasional problem is
    frustrating. A system that won't easily do your primary requirement
    is EXPENSIVE. Stated another way, look at the life-cycle costs
    of the project, not just the $$$ spent on a scope.

    If it's a hobby, that's another story. Hobbies are DESIGNED to waste
    your time. It's their job...
    With a hobby, you might have more fun trying to squint at a
    flashing display. Heck, I once built a spot welder. Was trying
    to look at the current waveform on an analog scope. I went as far
    as hooking a usb webcam up to the trigger-out on my scope and
    captured the trace afterglow on the computer. I had great fun
    for several months messing around with it. Then a digital scope
    fell into my lap and I had the welder problem solved in a matter
    of a few minutes. Bummer...I had to find something else to waste
    my time.

    My favorite scope is a TEK TDS500 series. Mostly because people
    give them away dead. All you gotta do for most of them is
    wash the boards and replace 140 electrolytic caps.

    Analog storage scopes can be interesting if you get them VERY
    cheap. I'd avoid 'em.
    mike
     
  11. spasmous2

    spasmous2 Guest

    I apologize for being unclear, I am trying to be as precise as I can!
    The signal I expect is around 10-20 ms duration. It repeats every 1
    second (approximately) but I only care about the first 10-20 ms. I
    want around 2us resolution but probably 10us is more than adequate.
    The scope will sit in a corner and display these types of waveforms
    for the rest of its life and nothing else.

    I will look at the waveforms - but hopefully not for the rest of my
    life ;) My viewing pleasure is paramount in this purchase - I would
    like the 10-20ms of interest to be displayed brightly for 1 second, or
    longer if necessary, until the next 10-20 ms of interest comes along.

    My current (analog) scope triggers and shows the 10-20 ms but then
    goes dark for the remaining 980-990 ms.
     
  12. Hammy

    Hammy Guest

    Then maybe you should be looking at transient recorders.
    Heres just one combo dso Transient recorder.

    http://www.maplin.co.uk/module.aspx?ITAG=SPEC&ModuleNo=30367&doy=15m6#spec
     
  13. Jean-Yves

    Jean-Yves Guest

    the 2430 will do that.
     
  14. peter

    peter Guest

    50 ohm impedence?! Is this a typo? Oscilloscopes are designed to have high
    input impedence on the order of mega ohms so as not to disturb the circuit
    being monitored.
     
  15. Jean-Yves

    Jean-Yves Guest

    my high end fluke combiscope has a 1Mohm input impedance
    ... and a special button for a 50 ohm impedance...
    I dont know what this is for but as it is a special feature of a high
    end scope it must be usefull for something...
     
  16. mike

    mike Guest

    It's done for signal quality. Active probes are designed to work into
    50 ohms.
     
  17. Jim Yanik

    Jim Yanik Guest

    above 250Mhz,scopes often have 50 ohm terminations.
    TEK 485 and 2445/65 series have switchable 1M/50ohm input Z.
    TEK 7A24,7A19,and 7A29 7K plug-ins all have only 50 ohm input Z.

    Some active FET probes require a 50 ohm termination,and the TEK AM503/A6302
    current probe system needs one,too.
     
  18. Jamie

    Jamie Guest

    my scope has a 50 ohm input option on it.. other than that, it's a 1 meg
    input.
     
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