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How to calibrate TEK 465B 'scope?

Discussion in 'Electronic Basics' started by Eric R Snow, Jul 19, 2007.

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  1. Eric R Snow

    Eric R Snow Guest

    Greetings All,
    A couple years ago I installed the meter kit in my TEK 465B
    oscilloscope. I have all the manuals for the 'scope but not anything
    against which to test it. It seems to be OK but calibration would be
    great so can anybody point me to a good place to buy the proper signal
    generator? I'm just a rank amateur but the thing has beenh helpful. A
    good example is how I tuned up my neighbor's generator after some hack
    had messed up all the adjustments. I looked at the output from a 9
    volt AC wall wart that I first plugged into the mains. I noted the
    wave form (not a perfect sine wave, but bent over) and the frequency.
    The frequency was correct for the scale I used so at least I know
    that's working correctly. I then plugged the wall wart into his
    generator and used both an optical tach and the 'scope to measure
    frequency. Along with a DVM I adjusted the carb and governor so that
    the generator put out 60 cycles and 120 volts with at least a 300 watt
    load. Unloaded the frequency went up to about 66 cycles and the
    voltage went up to about 130 volts. With a full load the frequency is
    pretty steady and the voltage drops to 115 volts. He now loads it up
    with a little space heater before he plugs in his fridge and some
    lights. There was no way to keep the voltage above 105 volts and the
    frequency above 52 Hz when the thing was loaded if the unloaded
    frequency was 60 and the voltage 120.
    ERS
     
  2. Bob Masta

    Bob Masta Guest

    You can download my Daqarta software and use its free sound card
    signal generator. (You don't have to buy the software: When the
    30-day/30-session trial period expires, only the signal *input* stops
    working. The signal generator keeps working, along with all the
    spectrum and waveform analysis displays.)

    Since sound cards have crystal-controlled sample rate clocks,
    the signal generator accuracy and stability will be excellent.
    However, note that the output is limited to frequencies (typically
    20 kHz or less). That's fine for calibrating the slower sweep
    ranges, but not much use for the fast end. With (say) a 10 kHz
    square wave output, you'd have 100 usec per cycle, or 50 usec
    per phase. With the scope on 5 usec/div, this will just fill a 10-div
    trace. Still, it's a good start, and it's free.

    Note that sound cards don't come with any sort of level
    calibration. Daqarta can be calibrated for your card, and
    has some features to help automate this. But somewhere
    along the line you will need to provide a reference level to
    tie into the calibration. This gets into a chicken-and-egg
    problem unless you can borrow a known-good generator
    to get you going.

    Another sound card issue is that they don't pass DC,
    so you can't (for example) just measure a battery with
    your DVM and then adjust Daqarta to read the same.
    And most inexpensive DVMs lack a sensitive AC Volts
    range, or you could just read the sound card output
    and adjust the calibration to match the meter. One
    workaround I have come up with is to measure a
    battery with the DVM (since they always have sensitive
    DC ranges), then use the sound card to read the peak
    voltage that the AC coupling passes. You have to keep
    shorting the input and repeating until you get a good
    bounce-free spike capture, then assume the peak is
    the same as the battery voltage.

    If you are more adventuresome, you can use the
    DVM to measure a high AC voltage, say from an
    isolation transformer on the 120V mains, and divide it down
    to a volt or so for the sound card to read. It's easy to
    get a known divider ratio, so you should be able to get
    a fairly accurate reference.

    Best regards,



    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
     
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