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trouble viewing waveforms on scope

Discussion in 'Electronic Basics' started by tempus fugit, Oct 30, 2003.

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  1. tempus fugit

    tempus fugit Guest

    Hey all;

    I recently got a scope (my 1st!), and am having some trouble getting some
    waveforms to sit still. I can get nice clear displays of higher frequency
    stuff (several 100 kHz), but at lower frequencies (say a 1000 Hz sine wave),
    I can't get a decent display. I can see the wave, but it doesn't stop - it
    shoots across the screen, then it's blank, then another shoots across the
    screen. I've fiddled around with the trigger level, as well as the vernier,
    but I can't get it to stop. Is this normal for lower frequency stuff, or am
    I doing something wrong?

    The scope is an HP 1707Bm btw.

  2. (I am not familiar with this model, so this is general advice.)
    There is a whole section of controls related to the trigger. The most
    important is the source selector. Make sure it is set to the vertical
    channel you are using.

    Then there is filter selection for the trigger operation (DC, AC, high
    frequency reject, low frequency reject, noise filter, etc.

    Next is slope selection (positive going or negative going. Then
    trigger level.

    You also should consider the position of the normal/auto/single
    switch. Normal only triggers a trace when the trigger conditions are
    met. Auto include a timer that will trigger a trace if the trigger
    conditions are not met for some small amount of time. Single arms the
    trigger mechanism for one trace, and then must be reset before another
    is possible.
  3. Baphomet

    Baphomet Guest

    In addition to your suggestions John, I would add check the "Holdoff"
    control. While I am not particularly familiar with H.P. scopes, I think they
    were one of the few who used this schema. The Holdoff control provided a
    lockout period after the initial trigger, presumably to prevent false or
    multiple triggering on complex waveforms. Good luck with your new scope
  4. tempus fugit

    tempus fugit Guest

    Thanks John. I'll try messing around with these other controls too. So I
    should definitely be able to get the waveform stopped on the screen?

  5. If the waveform is repetitive, you should be able to trigger at the
    same point on the wave, each cycle, and have all the copies on top of
    each other.

    If the waveform does not repeat itself, exactly, then you will see the
    different waves on top of each other, but all starting at the same
    condition of voltage and slope.
  6. tempus fugit

    tempus fugit Guest

    Thanks Baphomet. I checked the Holdoff control - it is off. To more
    specifically describe my problem, if the sweep time control is set to
    anything slower than 2mS, the waveform starts flashing (I can see it, but it
    flashes on and off the screen). This happens when a sine wave is connected,
    or when nothing is connected. At faster sweep times, there is a straight
    line with nothing connected to the probe. If I go below the 2mS setting, the
    line "draws" itself across the screen (same for if a 1kHz sine wave is
    input). I have scoped waveforms at higher frequencies successfully and
    cleanly - they look like I would expect them to - and I can "stop" a lower
    frequency (e.g. 1kHz) sine wave at the higher speed, but of course I can
    only see part of the sine wave, because the sweep time is too fast to
    display it all.

    Anything else you can suggest? Is my scope malfunctioning, or am I?

  7. Baphomet

    Baphomet Guest

    I wonder if you have hum (50 Hz.) in the power supply feeding your sweep
    circuit. It's really starting to sound like that although long distance
    diagnostics like this are akward. Since you probably don't have another
    scope to diagnose your scope, you might try bridging your filter
    capacitor(s) with a known good one. Just a long shot I guess.
  8. tempus fugit

    tempus fugit Guest

    lol - if only I could just test it with my scope.

    On the sweep dial, there are 2 controls - the center one is the main sweep,
    and the outside one says "delayed sweep". If I switch the delayed sweep all
    the way clockwise (to "off") I can't even get a trace on the screen, so I
    usually have to keep it pretty much in the same position as the main sweep
    control. Is that normal?
  9. It might be. Your scope has two sweep generators. Usually they are
    referred to on the controls as A and B. A (main) is the sweep
    generator triggered by all those settings we have been discussing. So
    you need to find the sweep selector control and set it to A or Main.
    The B or delayed sweep is triggered after an adjustable time delay
    (set with a ten turns dial) after the trigger that starts the A sweep.
    If you set the sweep selector to intensify, the timed part of the A
    sweep will show dimly, while the B or delayed sweep will be brighter,
    so you can see where it begins, and how long it lasts.

    Ordinarily, you use delayed sweep to view a small fraction of a long,
    repeating sequence (like a single line of a video frame). You sync
    the scope so the whole frame shows with the main (A) sweep, then
    switch the sweep selector to intensify, and turn the delayed (B) sweep
    up in speed to light about the right fraction of the whole frame, and
    slide that bright section horizontally to what you want to see with
    the ten turn time delay pot. Then you switch the sweep selector to B
    or delayed sweep, to just display that bright section stretched out to
    fill the screen.

    But what you are trying to do right now is just view the main or A
    sweep. If the sweep selector is set ot A, it shouldn't matter where
    the delayed sweep speed is set, or where the delay pot is set.
  10. tempus fugit

    tempus fugit Guest

    Thanks John. OK, so the sweep controls are not the same as the channel
    (which are A and B) controls/inputs? There are a lot of buttons on this baby
    that I'm unsure of. You now your stuff though. The delayed sweep is indeed a
    10 turns button. I'll try to hunt down the main sweep selector that you
    refer to, and let you know how I make out. I think the problem is one of
    operator error, though, because I'm sure I was able to properly display the
    slower waveforms shortly after I got it.

    I didn't think this thing would be so tricky to manoever.
  11. Baphomet

    Baphomet Guest

    Keep the faith baby. I got my first exposure to delayed sweep scopes when I
    was about 13. I had a friend who was an engineer at Times Facimile (later
    absorbed by Litton Industries). They manufactured huge high speed rotating
    drum fax machines that the newspaper industry used to typeset AP/UPI
    feeds.They had all of these cool Tektronix 545B's on carts all over the
    place. The staff there graciously allowed me to play with the scopes and I
    became most fascinated with the delayed sweep portion. After perhaps half a
    day experimenting, I got the delay and intensification portions down
    reasonably well. The staff was both amazed and delighted because none of
    them had ever used these features so could offer little guidance. I
    attribute my continuing on in electronics to their encouraging attitude and
    allowing my experimentation. There is this expression "Tall Oaks From Acorns
    Grow" which I think applicable.

  12. tempus fugit

    tempus fugit Guest

    I haven;t been able to find any button or switch to select the main vs.
    delayed sweep. I could post a photo of the front panel on
    alt.binaries.schematic.electronic if you guys will be able to check it out
    over there.

  13. Fred Abse

    Fred Abse Guest

    My experience is similar. Many people using 'scopes on a daily basis have
    no idea what the delayed sweep is for. They can have been using the same
    instrument for years, and never found out. Quite a few times, I've been
    into somewhere or other to look at a problem "they" can't sort out, and
    have asked whether they have a 'scope I can borrow. Ten minutes later, the
    conversation goes something like:

    "Come take a look, there's the sucker!"

    "Geez, how did you do that, we've been looking there for a week, and not
    seen it."

    "Just use the delayed sweep, like this" (turning multiturn pot a bit one
    way, then the other)

    "Delayed sweep, what the hell's that?"

    I see them in about equal measure with the "take the ground off and use
    rubber gloves" idiots.
  14. Charlie+

    Charlie+ Guest

    On Thu, 30 Oct 2003 12:40:51 -0500, "tempus fugit"

    If you can get from your library (or buy) a book called
    Hands-On Guide to Oscilloscopes
    by Barry Ross - Pub. McGraw-Hill ISBN: 0-07-707818-7
    It contains all the info you want on scope controls, how they work and
    how to take measurements with each - exactly what you want and
    written at a very understandable level! Just happen to have a copy
    on my desk and I learned from it how to use delayed sweeps etc.
    properly having got only halfway there fiddling on my own !
    Yes - try setting trigger source as Internal, Mode as DC, and then
    setting trigger A Level control to get exactly the picture you want -
    assuming your particular scope has those controls!
  15. Baphomet

    Baphomet Guest

    Please do so. Do you have schematics of both the trigger and sweep circuity
    if that should prove necessary?
  16. tempus fugit

    tempus fugit Guest

    If only I did. I'll post over there, though and hopefully it's just
    something I'm doing wrong so i don't have to open her up.

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