Lab 2 - ECE 420L Engineering Electronics II Lab  

Authored by: Frank Sanchez,

sanchezf@unlv.nevada.edu

2/10/2017 



Experiment 1:Show scope waveforms of a 10:1 probe undercompensated, overcompensated, and compensated correctly.

Compensated
                                                                                                                            OverCompensated                                                                                 UnderCompensated
file:///C:/Users/frank/SkyDrive/EE%20420/420L/lab%202/compensated.jpegfile:///C:/Users/frank/SkyDrive/EE%20420/420L/lab%202/overcompensated.jpegfile:///C:/Users/frank/SkyDrive/EE%20420/420L/lab%202/undercompensated.jpeg



Experiment 2: Comment on where the type of scope probe (i.e., 1:1, 10:1, 100:1, etc.) is set on your scope (some scopes detect the type of probe used automatically).
 

                   1:1 Scope Probe                                                      10:1 Scope Probe                                              100:1 Scope Probe                                          1000:1 Scope Probe
file:///C:/Users/frank/SkyDrive/EE%20420/420L/lab%202/1%20to%201_scope_probe.jpegfile:///C:/Users/frank/SkyDrive/EE%20420/420L/lab%202/10%20to%201%20scope_probe.jpegfile:///C:/Users/frank/SkyDrive/EE%20420/420L/lab%202/100%20to%201%20scope_probe.jpegfile:///C:/Users/frank/SkyDrive/EE%20420/420L/lab%202/1000%20to%201%20scope_probe.jpeg

The pictures above show the of probe used for the experiment. The channel menu on the oscilloscope allows the user to properly attenuate a probe from 1x to 1000x. Other parameters in regards to the BNC connector inclued its input resistance of 10M-ohms, 12pF input capacitance, and a bandwidth of 100MHz.
bnc%20connector.jpg

10%20to%201%20scope_probe_schematic.PNG


The circuit above is a representation of a circuit taught to the student during the pre-lab. This circuit represents a probe attenuation of 10x to 1x ratio.

Running a simulation for this circuits proves that its a clear representation of  a probe and its parameter effects and how its can output 1V from a 100mV input.

10%20to%201%20scope_probe_waveform.PNG
10%20to%201%20scope_probe_calculation.PNG 

cable%20capacitance.PNG
time%20delay%20measurement.jpg
time%20delay%20hand%20calculation.PNG
In order to measure the capacitance of the cable; you can create an RC circuit using the scope probe and resistor of 108.5k.  By the use of  a voltage pulse as an input; one can measure the time  the circuit takes to reach around 50% of voltage pulse, know as time delay. My experimental result ended up being a time delay of  960ns. for a 1 volt input pulse. This in hand gave me a calculated value of close to 13pF.
Measuring the cable capacitance with a multimeter, gave a result of about 18.1pF. But by subtracting the capacitance of the multimeter, you'd get around 11.1pF which is close to the calculated capacitance value.
compensated%20probe.jpguncompensated%20probe.jpg

The result on the left shows a compensated output to the input which has a  10:1 attenuation which has a capacitance that is being introduced to it. The result on the left shows an uncompensated output, that helms a big capacitor that is taking a while to charge.

In order to implement a test point on a pcb so that a known length of cable can be connected directly to the board, is by placing acapacitor and a resistor in parallel. This is done in order to prevent any effects from the cable connected to the pcb to occur.


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Conclusion:

Lab 2 gave us an idea on how a compensated or uncompensated scope probe can affect a circuit. If it being an uncompensated scope probe which introduces a big capacitance or having a compensated circuit which compensates for the input capacitance and cable cap. This lab gives us an understanding on what might affect a circuit while using a measuring device.


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