Lab 2 - EE 420L 

Authored by Shada Sharif,

sharifs@unlv.nevada.edu

6 February 2015
 
Pre-lab work:
Lab Description:
Lab Report should include:
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Experiment #1


When connecting a probe to the scope to measure things from the circuit, we add the capacitance of the probe through the BNC connector to the scope. In order to eliminate this type of issue from happening there is a screw on most probes that can be changed to adjust the capacitance of the probe. Some probes have the screw at the tip of the probe while others at the end where the probe is connected to the BNC connector. In the probe we used, the screw was at the end where the scope input is. While adjusting the screw if the capacitance is higher than what it should be the probe would show a wave of overcompensation, and if the capacitance is smaller we would get a wave for the undercompensated probe.




*Notice how all the pictures above show in the scope that the probes used are the 10x, which means that they are the 10:1 probes.

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Experiment #2

Scopes usually have a connector called the calibrator where the probe is connected to in order to detect what type of probe we are using. We used two types of scope while conducting the lab, the first one there was a button that can be pressed after the probe is connected to the calibrator connection, and the type of probe is measured automatically. The other scope that was used, we had to connect the probe and through the wave and the amplitude we knew what type of probe we are using. This is because the wave that the scope use for the testing is a square wave with an amplitude of 5V(as indicated by the scope) and if the wave seen in the scope is 1/10th of 5V one can know this is a 10:1 probe and so on. There is also other attenuations that exist like a 100:1 where the output signal now would be lowered by a factor of 100. Also the 1:1 attenuation which is like a piece of cable.

                Probe with an option of 10:1 or 1:1                                               Calibration connector and BNC                                                          10:1 / 10x probe

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Experiment #3

From the schematic shown below, the 15pF and 1 MEG Ohms are the scope impedence and they are different from scope to the other, usually this information is either written on the scope or in the manual of the scope. Then there is the 90pF which is the capacitance of the coax cable that is used, and can also vary depending on the cable and the length of it. The parallel combination of  the 11.7pF and 9 MEG Ohms are the impedence that one fix while calibrating the probe from the screw to compensate for the impedences mentioned above, that is the reason why the capacitor is left variable so that it can be changed accordingly in the probe and the circuitry holds for all output to input ratio of 1/10 for all frequencies. Depending on the probe this parallel combination can be at the probe tip or at the scope input. At the probe tip as the frequency increase the capacitance of the probe goes down and so the impedence.

As for the waveforms shown, one is the probe tip which is the wave before the impedences are introduced and the other one is the scope input. Through the calibration of the probe and the C1 capacitance, the rise time of the wave is not as slow as it would be for an under or over compensated probe. Though the signal of the scope input has a lower amplitude(1/10th of input) than the signal from the probe tip, the signal has lower capacitance by 9 times and a faster rise time.

  

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Experiment #4


Through this calculation one can see how the relationship between both impedences is a 10:1 just like it was explained before. The output as a result is 1/10=0.1 of the input but with lower overall impedence. The DC loading on the probe tip is 9Meg +1Meg so 10Meg, and the capacitive loading is (90p+15p)||11.7p which is 10.5p. This shows how with this combination the overall capacitance decrease drastically.

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Experiment #5


The experiment that we conducted in order to measure the capacitance of the cable used is an RC circuit. Since the cable itself has a capacitance because it is a coax cable, we used a known resistance of  1 MEG ohms and a square wave input of 1V and 1 kHz. The resistor and cable "capacitor" were connected in series and we tested with a calibrated/compensated probe the input vs. the output of the square wave across the capacitor. Using the measuring tool of the scope the time delay(time takes the pulse to reach 50% of its final value) of the signal and through that the capacitor value was calculated. To check the answer, a capacitance meter was used to verify the calculated result, and they both matched.
 



*The capacitor measured is smaller than the hand calculation, but still very close enough to say that this type of experiment works for measuring capacitance.

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Experiment #6

In this experiment a voltage dividor was created using a two 100k Ohms resistors, and we measured the output vs. the input once using a coax cable and the other using a compensated probe. When measuring the output with a probe either compensated or cable we are adding a capacitor across the resistor in the output, which depending on the capacitance added can influence the circuit. From the circuit shown in experiment #3 due the calibration done on the probe to reduce the overall capacitance of the probe, the RC of the circuit is small so when measuring with a compensated probe that has a small RC, since it is a RC circuit the capacitor takes less time to charge and as seen below the figure on the left, the purple signal is increasing with the impulse changing from 0V to 1V. and as the signal impulse change from1V to 0V the purple signal decrease with the same time constant. As for measuring with a coaxial cable, the capacitance added is large that the RC constant created is big and it take more time for the capacitor to charge, which can be seen in the figure on the right as a straight line. The input signal due to its high frequency it has a small time constant, and it is smaller than the time constant of circuit; thus, the capacitor does not even have time to charge or discharge and the signal is seen straight and neither increasing or decreasing.

 

*Notice the output with a compensated scope is 1/2 the input, while the output of the coaxial cable is 0. Therefore we should not probe with a cable.

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Experiment #7

To implement a test point on a printed circuit board in order to prevent the loading from the probe to the circuitry on the board, we can do the same thing done before which is adding the resistor and capacitor combination. Since the capacitance and resistance of the scope is known, and the capacitance of the input as well, using a variable capacitor to get a 10:1 attenuation and a resistor we can adjust the values accordingly in order to compensate for the load, and not affect the printed board.

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