Lab 2 - EE 420L 

Author:  Nicholas Mingura

Email:    mingura@unlv.nevada.edu   

 2/9/19

Lab Description:

Perform, and document in your html lab report, the following:

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

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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Lab:

Part 1:

For the first part of this lab students were to measure a waveform with a scope being undercomensated, overcompensated, and compensated correctly. This is achieve by turning a small screw on the BNC connector that adjusts the shunt capacitance on the probe. 

Image 1: Overcompensated 10:1 probe measuring a 5V amplitude with 1k Hz frequency

Image 2: Undercompensated 10:1 probe measuring a 5V amplitude with 1k Hz frequency

Image 3: Correctly compensated 10:1 probe measuring a 5V amplitude with 1k Hz frequency

Part2:

In the lab the oscilloscopes can adjust the reading that they get according to what ratio you like to use on your probe, whether that be 1:1, 10:1, 100:1 etc. To adjust the oscilloscope you use the channel menu for the chanel you are mesuring click probe setup and use the set to 1x or 10x, after geting to 10:1 and pressing 10x it will take you to 100:1, however in the lab our scope probes only have a switch for 1:1 and 10:1.

Image 4: Scope probe menu displaying the probe set up and set to 1x or 10x.

Part 3:

Image 5: Schematic of a 10:1 scope probe in Ltspice  

Part 4:

In Image 5 the values used were shown because they make te tranfer funcion .1 as seen by the hand calculations below in Image 6.

Image 6: Hand calculations showing the scope probe in Image 5 has a transfer function of .1

As seen in part 1 of the lab we can recreate the overcompensated, undercompensated and correctly compensated values by changing the capactiance of the tip seen in the table below.

Capacitance	Circuit	Simulation
8p
11.6p
15p

Table 1: Simulated values of a wave form with varying tip capacitance.

Part 5:

For the experiment to measure the capacitance of the cable, a probe cable was used as a capacitor in series with a resistor to measure the time delay, then the time delay was used to find the capacitance in the cable.

Image 7: Delay time measured from the experiment mentioned above.

Image 8: Calculations determining capacitance of the cable

Image 9: Measured capacitance of the cable.

Part 6:

Below you can see that when using the uncompensated cable the output doesnt have enough time to reach the full value before droppoing low, however on the compensated cable it gets a clear high before going low.

Image 10: Uncompensated cable measuring a voltage divider.

Image 11: Compensated cable measuring a voltage divider.

Part 7:

A test point on a PCB can be implemented with a resistor and capacitor in parallel between the measured point and the actual cable. If the cable length is know than the capacitance value can be chosen to compensate for the cable's capacitance.

Image 12: Circuit diagram of how the PCB test point can be implemented.

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