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suggest basic func/signal generator for basic scope test/setup ?

Discussion in 'Electronic Equipment' started by robb, Oct 5, 2007.

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  1. robb

    robb Guest

    there is quite a variety of sig/func generators on the bay ...
    just wondering if there is a basic series or brand to looking for
    to test/setup a tek 2236 scope ?
    thanks
    rob
     
  2. Tony Hwang

    Tony Hwang Guest

    Hi,
    You have a built-in calibrator in the 'scope. One I'd like will be
    multi wave form capable with variable frequency and out put level.
    If your set up means calibration, then the generator should be
    calibrated against known reference as well. I send in my 'scope
    to a lab to maintain it's calibration official.
     
  3. JeffM

    JeffM Guest

    http://www.google.com/images?q=Fluke-5820A

    How are you planning to verify that the crap you buy on ebay
    is giving a valid output?
    Anything without a recent NIST-traceable certificate
    is only adding more variables to your situation.

    If you have doubts about the accuracy of your scope,
    take it to a metrology lab
    for calibration with NIST-traceable standards.
     
  4. mike

    mike Guest

    You need to be a lot more descriptive about what you're trying to
    accomplish.
    If you have other uses for the generator, use those criteria for your
    decision. If ALL you want is to calibrate the scope, forget it.

    For calibration purposes, a generator won't do you any good unless it
    has the waveform precision you need and also has traceable
    calibration. You can't afford that.

    It's a binary situation. If you use your scope in business, and
    maintain any certifications, you must send it in to have it done right.

    For hobby use, my experience says that a TEK scope either works "close
    enough" or it needs fixed. A scope is a poor tool for making
    measurements. It's great for "looking at the waveform", but unless
    you have a scope with built-in counter and DVM, you won't get accurate
    measurements.

    Here's how you check a scope.
    If it's a CRT scope, there are some things that mess up everything.
    Wrong high voltage can shrink or expand the display. A sure sign
    is if the display changes size when you change intensity. Make sure
    that's not a problem before you go tweeking stuff. Ditto for
    other power supplies, but the effects can be much less dramatic.

    Do all the functions work? check.
    Does it trigger? check.
    Use a T-connector to put your DVM and a variable power supply into the
    vertical input. Compare deflection against your DVM. Check
    Stick a transformer from the AC line into the input. Wall wart etc.
    Does it look like 60 Hz. or 50Hz. depeding on where you live.
    You can check several sweep ranges.
    Find a known signal source. Crystal oscillator in your computer,
    27MHz CB walkie talkie etc. Use that to check the faster sweeps.

    Now comes the hard part.
    In a perfect world, frequency response and transient response are
    equivalent. In the real world, where you're looking at tranisents,
    I prefer to optimize transient response.
    Stick the calibrator output into your vertical input.
    (I'm assuming the scope has one. If not, skip the step.
    Most generators are worse than the scope.)
    If the corners are mostly square, you're done.

    Don't open the case if you don't have the manual with calibration
    procedures and the equipment specified. You can substitute
    equipment, but your question suggests you don't have the knowledge
    to do that. Virtually any <non calibration> generator you pick
    will be worse than your scope. Just getting the waveform into
    the scope without degrading it is an art.

    RESIST TEMPTATION TO MAKE IT PERFECT.
    YOU RISK MAKING IT MUCH WORSE.
    RESIST TEMPTATION TO LET YOUR BUDDIES TALK YOU
    INTO MAKING IT PERFECT.
    IF IT AIN'T BROKE, DON'T FIX IT.
    IT WILL NEVER BE PERFECT, DON'T EVEN TRY.
    If it's far enough out of calibration to need significant tweeking
    it's probably broke. Fix it before you try to calibrate it.
    Leave the covers on and enjoy your scope.

    The above assumes the scope works and nobody's been inside
    messing with adjustments.

    Scopes are great for looking at waveforms. If you need to
    know the frequency of something, use a counter. If you need
    to know the voltage of something, use a DVM.

    mike
    designed scope calibration generators in a former life.
     
  5. robb

    robb Guest

    I know just enough to realize that best option for calibration
    is... send it to a lab.

    however, i want to be able to test my scope's range of
    functioning initially and (on occasion) then use the generator
    to inject signals into circuits that i might be hobbying with
    later (or any other usefullness)
    just **hobby** use....
    it would be a short lived, fault ridden business buying a abused
    scopes from the bay and trying to start some electronics related
    business with that equip.

    these were purchased on hobby budget and will be used as hobby
    tools.

    i would just like the peace of mind (on hobbyist budget) that
    the scope is not dead or way off on certain settings....
    so , my thinking is to run scope through a range and
    combination of measurements to know if it gives the expected
    behavior (read, some close or near expected measurement/wave/func
    form )
    The tek 2236 does have a built in autoranging DMM (w/frequency,
    period, width, 0-2G-Ohm meas, etc ) that can be used through a
    "separate" leads port or will display the measurements from
    channel 1 concurrently with CRT plus more stuff
    my scope passes this test but i read "online researching" that
    the tek 2236 has virtually NO power supply isolation and the
    measurements get munged by it's own noisey power supply ???
    anyone know about this

    i have seen where there appears to be slight noise (small fuzzy
    waves and pulsing) in the sweeps and was wondering if this was
    the isolated power supply issue or another fault in the scope ?
    great, thanks for that, i will try those reccomendations ....
    the built-in scope probe calibrator is what i used for first
    trace test and probe calibration ( 500 mv P-P 1 kHz )

    FWIW the probe calibration gave picture perfect CRT image of over
    and under shoot wave form when turning the calibration screw on
    probe. that is leading edge of square wave turned up or down
    while rest of wave remained flat and steady ???

    when adjusted the form was square, even width and flat across the
    CRT.
    learned that lesson a while ago

    thanks mike for taking time to help an amateur hobbyist, thanks
    for info and ideas

    robb
     
  6. robb

    robb Guest

    Hey Jeff, can always count on you shooting straight, narrow and
    succinct.

    i do not need traceable calibration

    as an amateur hobbyist, i just want to put the new scope through
    wide range of measurements to see if there are any gross problems
    .... if scope measures close on varied ranges then i have peace of
    mind that scope is in fair working order.

    so if i test an unknown scope with unknown signal generator and
    they both come close in agreement i think i can be
    **statistically** comfortable that both are in close enough for
    hobby work condition.
    if they do not agree well then i can use some other means to test
    signal from generator then go from there.

    this is part of the hobby :) i have 2 x 2236 for scopes from the
    bay.... a scope and parts spare for under $90 (*delivered*) one
    appears to be functioning well, the other maybe working well and
    that suits my hobby budget

    calibration is what... $150 ? thats more than the two and
    shipping combined

    thanks again for the response, thoughts and straight shooting
    robb
     
  7. robb

    robb Guest

    yes, calibration test, but just for peace of mind and something
    to test the range of the scope
    i would not change anything or tweak if the scope was close,
    maybe attempt a repair if grossly out of spec

    calibration would likely cost much more than i paid for the
    scopes including delivery.

    thanks for reply ,
    rob
     
  8. Bob Masta

    Bob Masta Guest

    If you want to test the time base at the low end,
    you can use the signal generator from my Daqarta
    software with your sound card. Sound cards use
    crystal time bases, so you get pretty decent accuracy.
    At a sample rate of 48000 Hz the max sound card
    output will be under 24000 Hz, so you won't be
    able to calibrate below 50 usec per division. At the
    low end there is no particular limit, since the generator
    maintains the timebase accuracy for as long as you
    want to wait. (Of course, you need to use a pulse or
    square wave to pass the sound card's AC coupling cap.)

    Daqarta also has a wide variety of available waveforms,
    including different kinds of noise, plus you can provide
    your own Arbitrary waveform or even play a WAV file.

    Sound cards have no intrinsic amplitude calibration...
    they are perfectly repeatable, you just don't know what
    the attenuator steps are, or the full-scale range. Daqarta
    has an Auto-Calibration feature to figure out the attenuator
    steps, but you need something external to calibrate full scale.
    But once you do that, you're all set. The attenuation range
    is usually quite wide (more than 60 dB) and you can also
    change the size of the digital waveform to take that even
    farther... 90 dB is fairly easy to get.

    By the way, the Daqarta signal generator is FREE.
    At the end of 30 sessions or 30 days, the input capabilities
    will stop (unless you purchase), but the output and analysis
    capabilities keep on working. You are welcome to use it forever like
    that.

    Best regards,


    Bob Masta

    DAQARTA v3.50
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
     
  9. redbelly

    redbelly Guest

    Or: holding a bare wire or unbent paper clip, touch it to the input.
    Look for 50 or 60 Hz as appropriate.

    Mark
     
  10. robb

    robb Guest

     
  11. Krohn-Hite 1400A...very cheap good quality. does sine square triangle. It
    can also sweep frequency and amplitude.

    Krohn-Hite makes very good oscillators and have been in business for many
    years.
     
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