Physics307L:People/Joseph/Notebook/070829

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Oscilloscope Lab

Setup

Plugging in and turning on the oscilloscope and signal generator were the initial steps. After that was connecting the BNC cables to the correct positions. I initally plugged the channel 1 BNC cable into the SWEEP output on the function generator, which did not work at all. The correct plug was the 50 ohms port which Devon located for me. Fiddling commenced and a reasonable amount of fiddling resulted in a very fine looking sine wave.

Measuring Characteristics

see comment

Steven J. Koch 01:20, 5 September 2007 (EDT): This is most likely a problem with the scope thinking you were using a "10x" probe...I should have mentioned this before the lab started, but I think we discussed this later in the afternoon...

1. Grid Lines

Adjusting the Volts/Div knob essentially zoomed in on the sine waves, but actually made it more difficult to get a reasonable guess (because of necessary position adjustments, etc.). I stuck with 20 V as my Volts/Div. Approximating by sight and the grid lines I figured the amplitude be ~85 volts. I placed the approximate frequency at 200 Hz, based on the frequency that is displayed on screen.

2. Cursors

Hitting the CURSOR brought up some bars on the O-scope that could be moved vertically and horizontally by adjusting their respective knobs. Using the same 'resolution' as I was before, I measured the frequency to be 200 Hz(with a period of 5 ms) and the amplitude to be 87.2 V.

3. Measure

Using the MEASURE button and setting the type to Freq, the o-scope gave me exact value for the frequency: 200 Hz. I set the type to Period and it produced the value 5.004 ms, which is 4/1000ths off from the approximation that was produced from the cursors. Very high precision indeed.

We must keep in mind the O-scope is giving us a digital display, and there are inherit limitations to displaying information using a finite resolution. When I crank up the frequency to anything higher than 20KHz and the Volts/Div is set above one volt the image is nonexistent. On the other hand, if I set the frequency to below 10 Hz, the O-scope doesn't really register anything.

--- Triggering

see comment

Steven J. Koch 01:22, 5 September 2007 (EDT): yes, I believe you loved re-reading that article. It is great the way you referred to Zane's notebook and gave him credit--good science practice!

1. Re-read article; loved it. 2. Rising Edge From the article, I am led to believe the the rising edge triggering is when the sweep stops after the input signal begins rising past a predetermined voltage. Referring to Zane's Notebook led me to play with the triggering functions and I was really able to see what rising edge meant. As he states, falling edge just appears to display the opposite.

--- AC Coupling

I set the O-scope to a low frequency (~6Hz) and zooming into 5V @ 25ms did several trials at different amplitudes to get a decent spread of data.

  • Amplitude(A) @ 10 V
    10% of 10V=1 V
    time to reach=51 ms
  • A=11.5 V
    [math]\displaystyle{ \Delta\, }[/math]V = 8V
    10%=.8 V
    time=34.8 ms
  • A=4 V
    [math]\displaystyle{ \Delta\, }[/math]V= 10.6 V
    10%=1.06
    time=53 ms
  • A=2 V
    [math]\displaystyle{ \Delta\, }[/math]V = 11V
    10% = 1.1
    time=68 ms

RC Constants I read Bradley's notebook and found this: ([math]\displaystyle{ RC=\tau=\frac {t_r}{2.197} }[/math]): Using that to find the RC constants I get:

  • 23.21
  • 15.84
  • 24.12
  • 30.95

These average out to about 24 s.

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