User:Ginevra Cochran/Notebook/Physics 307L

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Oscilloscope lab

Setup

Using a BNC cable, I hooked up the output from the BK Precision 4017A Function Generator to the CH1 input on the Textronix 1002 Digital Function Oscilloscope. After talking to Katie about the purpose of T connectors and terminators, I did not use any, since they're for open-ended connections. I didn't have to adjust any settings to see my sine wave on the oscilloscope, but I changed my output from square waves to curvy waves for appearance's sake. (picture of setup here)

Measure characteristics of a sine wave

I set the oscilloscope to 40 Hz, and obtained the following measurements.

By grid, the peak-to-peak voltage was about 8V twice (2.5 boxes times a scale of 2V). The period was about 21ms and 25ms (scale 5ms per box). Using the cursors, the peak-to-peak voltage was 8.08V and 7.92V and the period was 24.8ms and 24.8ms. I don't understand why both time cursors have negative positions, unless it's because they're both in the past. Using the Measure functions, the peak-to-peak voltage was 8.08V and 7.92 V. The period was about 24.76ms and 25.00ms(there was some slight positive and negative fluctuation in this value, but it remained within 0.1ms of 24.76ms and 25.00ms).

I then set the oscilloscope to 400 Hz. By grid, the peak-to-peak voltage still looked to be about 8V. The period was about 2.6ms and 2.5ms(0.5 boxes times 5ms per box). Using the cursors, the peak-to-peak voltage was 8.16 V and 7.92V and the period was 2.60ms and 2.40. Using the Measure functions, the peak-to-peak voltage was fluctuating between 8.08 V and 8.16 V (7.92 V steady)and the period was between 2.490 and 2.500ms (2.500).

I set the oscilloscope to 100 Hz and the DC offset halfway up. Using the grid, the peak-to-peak voltage was about 8.5V and the period was about 10ms. Using the cursors, the peak-to-peak voltage was 8.48V and the period was 12ms. Using the Measure tool, the peak-to-peak voltage was 8.48V and the period was 9.920ms (with fluctuation of 0.2ms). With a such a high DC offset, the wave became deformed - it was no longer sine-shaped, but had a dent in the peak. I would consider it unmeasurable, since the peak disappeared, but this may have been because the oscilloscope was set to DC coupling. (used attenuated voltage thing,

Triggering

Triggering allows the user to stop a wave at or above a certain voltage set by the trigger level. Triggering on a rising edge means the specified voltage must be reached by the positively-sloped edge of a wave. Triggering on a falling edge means that the trigger voltage must appear in the negatively-sloped edge. The difference between rising- and falling-edge triggers is only the portion of the wave that is displayed.

AC Coupling

I don't know the range on the DC offset dial, so I turned it all the way to the right. DC coupling displays the entire voltage (AC plus DC), and so when a DC offset is employed the wave shifts up or down, making it harder to observe any ripples. AC coupling removes the DC bias (showing on the alternating voltage), keeping the wave from being offset up or down and only displaying any abnormalities caused by changing the DC offset.

I set the function generator to an amplitude of 8.56 V (8.48), displayed a square wave, and switched off the DC offset. After extensive explanation and demonstration on Professor Koch's part, I finally managed to display the correct wave pattern. Using the cursors, I measured the fall time to be approximately 56ms. Using the measure function, which Christian showed me how to use to calculate fall time, I measure the fall time to be about 131.8ms. I assume that this means that the oscilloscope is calculating fall time with the 5% value instead of the 10%. I remeasured the fall time with cursors for the 5% value and got 134 ms.

According to [Wikipedia], rise time is proportional to the RC constant

[math]\displaystyle{ t_r\cong 2.197\tau\ }[/math]

So the RC constant should be [math]\displaystyle{ 131.8ms/2.197\cong 60ms }[/math] 

  5. How does this compare with the expected value for the oscilloscope? (Can you find the answer on Google?)

[math]\displaystyle{ t_r\cong 0.35/BW }[/math], where BW is the bandwidth. For the TDS1002, bandwidth is 60 MHz according to the [manual], so its rise time is [math]\displaystyle{ 0.35/60MHz = 5.8ns }[/math], and its RC constant is [math]\displaystyle{ 5.8ns/2.197 = 2.66ns }[/math]


FFT (Optional) 

  1. Find the frequency of a sine wave using FFT "Math" function
  2. Look at the harmonics in triangle and square wave
  3. Compare with what you see on this applet: Fourier series applet
  4. Be able to explain what is going on with an FFT and when it may be useful

Wikipedia

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