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

Concepts

 * 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 noise that is left in with the DC coupling.

Safety Concerns

 * Electric shock to people or equipment
 * General accidents

Equipment

 * 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.

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

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%. The fall time is also known as the RC constant.

This is a square wave with DC coupling.

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 2: AC Coupling Fall Time Measurement
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).

Analysis
From the cursor measurements, the fall time, $$\tau$$, can be found using the equation: $$V(t)=V_0 e^{ \frac {-t} {\tau}} $$ From the square wave we measured the following values with the cursor: $$V_1=7.2V, V_2=4V, T_1= -35.2ms, T_2=-21.2ms $$ Using these values in this equation derived from the first: $$V_2=V_1 e^{ \frac {-|T_2-T_1|} {\tau}} $$ $$\tau$$ is determined to be 23.82ms. This is 64% of the oscilloscope's value.

Resources

 * 1) Physics 307L Oscilloscope Lab
 * 2) Wikipedia article on oscilloscopes
 * 3) National Instruments: AC and DC Coupling
 * 4) Wikipedia article on fall time

Collaboration

 * 1) Brian Josey