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First shot with a scope

Discussion in 'Electronic Basics' started by Rikard Bosnjakovic, Nov 30, 2005.

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  1. I consider myself a pretty much novice in the world of electronics, and
    I've never used a scope before so I bought a used one and got it
    yesterday. It's in very good condition, but I have yet not learned how to
    use it for best results.

    Reading the manual, I set all buttons and knobs to "initial state" to be
    able to put up zero volts as a reference in the crt. The probes looks like
    a hook with an additional small crocodile clamp. At first I thought i
    could use either the hook or the clamp, but by reading the manual it looks
    like the cramp should be put to ground somewhere near I am about to measure.

    However. The scope has a separate ground-input, so I took one of the
    ground-cables and attached it, and connected it to GND (Vcc- on my board I
    was about to measure) instead.

    So far so good. Putting the hook on +5 gives me a straight line. Trying
    some more, I put the hook on the output pin of an oscillating 555-timer on
    the board and the crt gives me a "square" wave, except for the vertical
    lines between the edge rise/fall but I take it that it's the natural
    behaviour of a scope.

    Adding another hook in channel B, I thought that I'll measure +5V
    (regulated) and +9V (from a wallwort) at the same time to see what
    happens. The +5V is a straight thin line, while the +9V is a very thick
    line. I changed the time-knob (volts/time i think it was) to a faster
    value, and the "line" looks like a sawtooth line and I guess that this is
    the behaviour of any wallwort.

    So, what I want to do now are the following steps:

    1. I want to learn how to calculate/find out what the lowest and highest
    value of the "sawtooth-line" is for the wallwort, using my scope.

    2. When I push a mechanical button on my PCB, there will be lots of noise.
    How can I "see" this noise on my scope? The dot is way to rapid for me to
    see just about anything, and even if I slow it down, anything to the left
    of the dot (the scope sweeps left to right) is gone. Is there a way to
    "freeze" the measuring? Just like an EKG.

    In case you want the manual for the scope, it's available here (about 8MB):

    http://bos.hack.org/tmp/Hitachi V-1050-Service Manual.pdf
     
  2. Noway2

    Noway2 Guest

    Congratulations on getting a scope, they are a valuable tool that will
    go along way in your understanding of electronics.

    In answer to your questions, first regarding grounding. The scope
    requires a reference (ground) to which the signal (probe) is measured
    against. The most common method of connecting this ground is to use
    the "small crocodile clamp". This method should be fine for basic
    measurements of slow signals. If you compare grounding this way to the
    one you used where you connected the ground to the input plug, you
    will notice that the loop of wire formed by the signal to ground
    connection is much smaller with the "small crocodile clamp". In a
    nutshell, smaller is better as the larger this loop area, the more
    noise you have contaminating your measurement.

    Second, the sawtooth waveform you are seeing on the 9V supply is
    probaly the ripple voltage that "rides" on the DC output. Most
    supplies have a specification for the ripple, typically measured in
    milivolts (mV). To measure this with the scope, you need to determine
    what you have set as the "volts per division" on your scope. Then
    using the grid lines on the scope display, observe the signal. One
    block or division on the grid lines represents the X volts of your
    volts per division setting. For example, is you are measuring at 1
    volt per division and the sawtooth is 1/2 of a block tall, then you
    have a 1/2 volt peak - peak sawtooth.

    Third, regarding seeing the noise from a switch, this is where the time
    base (or seconds per division) comes into play. The time base settings
    determines how much time each grid block on the display represents
    (measured from left to right). The switch bounce is probably on the
    order of a few miliseconds. What you can experiment with is setting a
    trigger point, which is a voltage at which the scope will "fire" and
    start to display. You can set a trigger for a voltage about halfway
    between the switch being open and closed, to catch the transition.
    Once you can see the switch transition, you can play around with the
    time base, volts per division, and trigger settings to better observe
    the noise.

    Lastly as far as 'holding' the image on the display, your scope may or
    may not be able to do this. This type of functionality falls under the
    realm of storage scopes. Most new scopes purchased today are "Digital
    Storage Scopes" where the data is sampled, processed by a computer, and
    then displayed on a screen. Since the data is aquired and processed by
    a computer it is much easier to have a persistant display. This would
    be in comarison to most analog scopes, which may or may not have some
    form of storage functionality.
     
  3. ehsjr

    ehsjr Guest

    Congratulations! A 'scope is a great piece of equipment
    to use. I recommend that you google on something like
    how to use an oscilloscope
    or similar.

    Better would be having someone show you how. The previous
    reply covered things - I'll just add that6 when you want to
    use you scope to look at switch noise, you will need to
    press the switch many times. Each instance of switch closure
    can produce a little different pattern on the scope, so it
    takes more than one look to get an appreciation of what
    switch noise looks like. A good place to start is with the
    timebase set to 2 mS per division, and the vertical input
    set to 5 volts per division.

    Ed
     
  4. I noticed that most of your questions were answered, but didn't see this
    one. A rectangular waveform (such as a square wave) with a fast risetime
    and fast falltime moves the beam on the scope face too quickly for the
    phosphors to become charged so those parts of the waveform will be very dim.
     
  5. quietguy

    quietguy Guest

    Hi There

    If you go to the Tektronix site, they have downloadable pdf docs on
    understanding and using oscilloscopes (XYZ of oscilloscopes at
    www.tektronix.com) which are great reading for the learner CRO user. They are
    free to download, and are relevant to all brands of CRO, not just Tektronix

    David
     
  6. Jasen Betts

    Jasen Betts Guest

    ["Followup-To:" header set to sci.electronics.basics.]
    ther's a grid over the front of the scope display the lines divide it into a
    number of divisions, somewhere on the scope near the input connector will be
    a control marked "volts per division" (or similar)

    so counting divisions from the zero point will tell you how many volts the
    signal is, as you're probably using a "X10" probe it may be out by a factor
    of 10. use the 5V supply (or the reference connector on the face of the
    scope) to guide you in interpreting the volts per division control.
    Unless that scope has a storage function (it probably doesn't)
    you'd need to arrange for it to be pushed many times in quick succession...

    contact bounce is somewhat unpredictable, I've not had a lot of success
    attempting to quantify it.

    Bye.
    Jasen
     
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