Feb 6: Intro to Using Oscilloscopes, AC Coupling, and DC Coupling

Oscilloscope Basics
-We used the oscilloscope to visualize voltage as a function of time

-VOLTS/DIV = used to control number of volts per displayed vertical division
-SEC/DIV = used to control time per displayed horizontal division
-TRIGGER = was difficult to use but was used to align a repeating signals to make it easier for us to see the graph of voltage vs. time (capture and display by differentiating rise and fall of a wave)

Practice
-Used the function generator to square waves and used the scope to measure the rise time of the square wave (i.e. time to pass from 10% to 90% of its full amplitude).

Approach: we measure the rise time by using the cursors, which we used to first determine the full amplitude (of 10.2 V), which we then used to calculate 10% of the full value to find the lower and upper 10%. We then moved the lower and upper cursors to the bottom and top 10% and looked for delta t on the oscilloscope, which was 20.4 ns.

Though the square wave appears to rise instantaneously, after adjusting (zooming in and out using the SEC/DIV knob), which saw that the square wave was not perfectly square (with its vertical sides shooting up instantaneously). By careful adjustment, we were able to see that there was still a slanted slope (though steep). 

-SYNC or TTL Connector

SYNC or TTL can be useful because it makes triggering easier. We watched the signal from the SYNC connector as a square wave and noted that it had a sharp transition (because it's a square wave and has sharp rising and falling edges), and while the amplitude of the square wave remained constant (ranging from 0 to voltage we input using the function generator), the frequency matched that of the sine wave from Channel 1 (main signal connector we used). To summarize, it served as a reliable time marker.

 

Notice that the the sine wave can change in amplitude but the square wave signal does not. Because of this consistency(?) or constant nature, we can trigger the scope more easily. A sine wave rises and falls more slowly and has noise that may get in the way of clearly distinguishing between a rise and a fall, whereas a square wave rises and falls much more quickly. Because of the rapid rise and fall, even with noise, we can better distinguish rise and fall.

Bridging to AC/DC Coupling
-Using the oscilloscope, we looked the same sine wave (with the square wave connected to SYNC) and saw the difference between the two. 

DC Coupling (left) vs. AC Coupling (right) 

Offset voltage = 5.00 V (set using the function generator)

AC coupling places a capacitor in series with the scope input, which "washes away" DC information or the offset voltage (as seen in the photos above). In other words, it keeps the time varying signals (or at least a fraction) and lets them through but blocks DC information. DC coupling does not have the capacitor.

Why would anyone want that (wash away information)?

-AC coupling can be used to filter out the offset voltage (filter out a signal that has both AC and DC components or information). This filtering could be useful in getting rid of unwanted signals or noise.

Lab 1-6: AC Voltage Divider

How would the analysis of the analysis of the voltage divider be affected by an input voltage that changes with time?

Observation:

The output voltage (in blue) got divided by 2 and looks like what would happen if we had used a DC voltage divider. 

Explanation (looked this up online to better understand but may be wrong): In AC circuits, we deal with impedance, which unlike resistance (in DC circuits) has magnitude AND phase. However, looking at the AC voltage vs. time curve, we can see that local regions (zoomed in) of AC circuits are essentially like DC circuits (in which the impedance is close to resistance with zero phase angle).

With an offset onto the signal (we used 3.00 V), the AC signal "washed away" the offset (so three 1.00 V spacings lower than the DC signal), which was expected.

Transistors ('Electronically Controlled Switch')

-three terminals on a transistor: gate, drain, source

-A popular transistor: MOSFET

-In a MOSFET, an applied voltage to the gate controls the resistance between the source and drain (with enough positive charge collected, a negatively charged channel, or a conducting channel, is created, connecting the two N regions). Thus, the applied voltage is like a switch for the resistance with two options: R = 0 (ON) or R= inf (OFF). 

-This applied voltage that controls the resistance is V-GS (potential difference between the gate and source).

-For an n-channel MOSFET, if V-GS < or = to 0 V, then the channel is non-conducting. If V-GS > 1.5 V, the channel is "open" and conducting.

A Better Way to Control a Motor

-Last time, we saw that DC output is not very effective in running a LEGO motor (not enough voltage). However, DC output is useful in that it has enough voltage to collect positive charges on the gate.

We wired a MOSFET to enable an Arduino to control a LEGO motor (input voltage = 5V)

Setup

Using the oscilloscope, we checked that we were able to turn the motor on and off using the code above.

(on then off)

Now, we considered the fact that LEGO motors are designed to run at 9V. Therefore, instead of using Arduino, we used a 9 V battery to provide the input voltage. 

Observations:

-Compared to using the Arduino, a lot more torque was required to stall the motor. 

-On the oscilloscope, the signal looked similar as above, with the applied voltage at 9 V instead of 5 V.

-Note that in the code (when using the battery) that we had to wait for twice as long before looping again because it took a longer amount of time for the motor to slow down and eventually stop before running again.