User:Kirstin Grace Harriger/Notebook/Physics 307L/Oscilloscope Lab

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Lab 00: Oscilloscope Exploration

SJK 14:13, 24 September 2010 (EDT)
14:13, 24 September 2010 (EDT)
Overall, looks like a very good primary notebook!


  • Triggering: Triggering is how the oscilloscope detects the wave being generated by the function generator. The triggering mechanism detects the wave by looking for a specific voltage value on either the rising or falling part of the wave. This value is the starting point of the wave that gets displayed.
  • Coupling: There is an AC and a DC signal from the function generator. The oscilloscope offers the option to choose between them when displaying the wave function. AC coupling generates a wave function while DC coupling generates the same function but with a constant voltage added on. AC coupling makes the signal go through a capacitor in an RC circuit, which acts as a high pass filter and smooths away noiseSJK 22:06, 21 September 2010 (EDT)
    22:06, 21 September 2010 (EDT)
    The DC component is not necessarily noise. In fact, I'd usually think of noise as higher frequency component of a signal. Lower-frequency "noise" may be more often referred to as "drift" or "offset."
    that is left in with the DC coupling.

Safety Concerns

  • Electric shock to people or equipment
  • General accidents


SJK 22:08, 21 September 2010 (EDT)
22:08, 21 September 2010 (EDT)
This looks like a very good recording of the setup and procedure in your primary lab notebook here. Great that you include the make and model numbers of the equipment. The photos are a very good way of recording and presenting information. It looks to me like you've provided the necessary information to replicate these experiments at a later date.
  • Oscilloscope: Tektronix TDS 1002
  • Wave Generator: BK Precision 4017A 10MHZ Sweep/Function Generator
  • Wires and Connectors: BNC Cable


Part 1: Basic Waveform Measurement

We used a BNC cable to connect the function generator to Channel 1 on the oscilloscope and turned both machines on. We set the triggering to CH 1 on the oscilloscope. We changed settings on the function generator to make sine, triangle, and square waves at 3 different voltages, 3V, 8V, 13.4V, one consistent frequency, 120Hz, and 0 DC offset. We took measurements for the period and voltage of each wave type at each voltage from the oscilloscope. We then measured the period and voltage of each wave type set to 8V, 120Hz, and a DC offset of a 1/2 turn of the knob. To take our measurements, we used 3 methods: counting grid squares by eye, aligning the cursors to the edges of the wave to let the oscilloscope count the changes in axis that define the voltage and period, and using the oscilloscope's measure function.

Oscilloscope and function.jpg
This is our set up. The oscilloscope is on the top, and the function generator is on the bottom.

Oscilloscope sinewave.jpg
This is a sine wave that was created by the function generator and is being measured by the oscilloscope.

Part 2: AC Coupling Fall Time Measurement
To compare DC and AC coupling and measure the AC fall time, we used a square wave with a voltage of 8.6V, a frequency of 11Hz, and a DC offset of 0. To measure the fall time, we used the oscilloscope's measure function and then used the cursors to find points on the wave we could use in an equation to find the fall time. At low frequencies the inherent properties of the AC circuit allowed us to measure the fall time. The fall time represents how long it takes the voltage to drop by 10%. SJK 22:14, 21 September 2010 (EDT)
22:14, 21 September 2010 (EDT)
This is a typo. You probably meant fall time from 100% to 10% of the value, not "by 10%." Also, tau is the RC constant, and tau is different from the time from 100 to 10%.
The fall time is also known as the RC constant.

Square wave.jpg
This is a square wave with DC coupling.

AC coupling.jpg
This is the same wave with AC coupling. The fall time is the change in the time axis spanned by concave-up curve that makes up the peak of the square wave.


Part 1: Basic Waveform Measurement

{{#widget:Google Spreadsheet |key=0AoEBBJie7L2cdEI4N3pPdUtrUndCZzNvdWV5M0JCalE |width=550 |height=300 }}

Part 2: AC Coupling Fall Time Measurement
SJK 22:22, 21 September 2010 (EDT)
22:22, 21 September 2010 (EDT)
Good use of the spreadsheet above for your data. The most quantitative part of this lab was the fall time estimation. If this were a "real" lab, you would for sure have made more measurements of the fall time, and then figured out a method for estimating the uncertainty in your fall time estimation. Likely, this would be mean +/- standard error of the mean.

We measured a fall time of 37.1 ms with the measure function, and with the cursors we measured the end points of the concave-up curve that makes up the peak of the square wave to be (-35.2ms, 7.2V) and (- 21ms, 4V).


From the cursor measurements, the fall time, , can be found using the equation:

From the square wave we measured the following values with the cursor:

Using these values in this equation derived from the first:

is determined to be 23.82ms. This is 64% of the oscilloscope's value.

SJK 22:19, 21 September 2010 (EDT)
22:19, 21 September 2010 (EDT)
I am not checking your math, but assuming you're doing it correctly. In contrast to your lab summary, I think your calculation using the cursor are _more_ accurate that the "measure" function. I think the measure function does not work properly when the wave is truncated like you have, and further more, "fall time" as defined in the oscilloscope is not tau, but is ln(10) * tau I think.


SJK 22:20, 21 September 2010 (EDT)
22:20, 21 September 2010 (EDT)
Good links for resources and good link to Brian
  1. Physics 307L Oscilloscope Lab
  2. Wikipedia article on oscilloscopes
  3. National Instruments: AC and DC Coupling
  4. Wikipedia article on fall time


  1. Brian Josey