Lab 2 - EE 420L
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(Undercompensated Probe) (Overcompensated Probe)
(Properly compensated probe)
Part 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).
On the oscilloscopes being used in this lab, we can find the Scope Settings under the "Channel Menu" for whichever channel you are currently using. In our case, the channel we used was "Channel 1" (yellow), thus the "Probe Setup" option can be found on the bottom right of this menu. From there, the user can select which level of compensation he/she would prefer to use in their readings. These copes can accommodate a range of x1 to x1000, however, the scopes we are using in lab are only 10:1 Scopes.
(<<< various settings found here)
(Scope menu with option found here^^)
Part 3: Draft the schematic of a 10:1 scope probe showing: the 9 MEG resistor, 1 MEG scope input resistance, capacitance of the cable, scope input capacitance, and capacitance in the probe tip.
(Schematic of 10:1 Scope Probe with listed capacitances and resistanceses)
In the above schematic, we can see the various capacitances and resistances at play when using a scope. We would expect to see the wave read, at the "tip" point, displayed on the scope.
Part 4: Using circuit analysis, and reasonable/correct values for the capacitances, show using circuit analysis and alegbra (no approximations), that the voltage on the input of the scope is 0.1 the voltage on the probe tip.
(Calculated value for C1 using schematic drafted in previous part)
Below we can see examples with the calculated values, as well as with an over/under compensated probe. For the properly compensated probe schematic, we can see that the output of 100mV is what we would expect from a 1:10 attenuation with an input of 1V. In the next two examples, we can see how changing the capacitance value effects the output reading. With values greater than 11.6p, we get an overcompensated reading, and with values smaller than 11.6p, we get an undercompensated reading.
(Properly compensated)
(Over compensated)
(Under compensated)
Part 5: Devise an experiment, using a scope, pulse generator, and a resistor, to measure the capacitance of a length of cable. Compare your measurement results to the value you obtain with a capacitance meter. Make sure you show your hand calculations.
For our experiment, we used a probe cable to act as the capacitor for an RC circuit, along with a 100k resistor. By doing so, we can read the delay time of this "circuit" to calculate the capacitance of the cable with the equation for time delay. Td = 0.7RC
(Time delay measurement for "RC" circuit)
(calculated time delay)
(measured capacitance of cable used)
We measured a capacitance of about 42.1pF, however, the cable we used was closer to 2 ft. in length so this value doesn't really meet the expected capacitance of 30pF per foot.
Part 6: Build a voltage divider using two 100k resistors. Apply a 0 to 1 V pulse at 1 MHz to the divider's input. Measure, and show in your report, the output of the divider when probing with a cable (having a length greater than or equal to 3 ft) and then a compensated scope probe. Discuss and explain the differences.
(measurement with uncompensated cable)
(measurement with compensated probe)
In the above oscilloscope results, we can see that when measuring with the uncompensated cable, the RC time constant is too high for the output pulse to reach its full value before the signal drops low. When using the compensated probe, we can see a clear time constant with much nicer output readings. Seeing both of these results gives a clear illustration into how the large capacitance of uncompensated cable can effect your readings.
Part 7: Finally, briefly discuss how you would implement a test point on a printed circuit board so that a known length of cable could be connected directly to the board and not load the circuitry on the board.
We can implement a test point for a PCB can be created using a resistor and capacitor in parallel between the probing point on the board and where the cable is actually connected to compensate for the cables capacitance. Knowing the length of the cable will allow you to calculate the needed parallel capacitance needed for compensation, however, using a variable capacitor in parallel with the resistor could allow some more flexibility when having difficulties finding a good capacitor size.
End of Lab 2!
_________________________________________________________________________________________________________
Ensure
that your html lab report includes your name, the date, and your email
address at the beginning of the report (the top of the webpage).
When finished backup your work.
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