Lab 2 - ECE 421L 

Clayton Frister,

FristerC@unlv.nevada.edu

January 30, 2017

  

Lab 2: Operation of a Compensated Scope Probe 

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


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

Undercompensated.JPG

http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/Undercompensated.JPG      http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/Overcompensated.JPG     http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/Correctly%20Compensated.JPG  

             Undercompensated Probe                                      Overcompensated Probe                                         Correctly Compensated Probe

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).

To set the scope probe to the appropriate ratio you need to press the channel button for the channel that you are using (they are color coded). Then a new menu pops up at the bottom to which you should select probe setup, and then another menu pops up on the right and you can select 1x or 10x probe from the menu on the right. See picture below. 

http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/10x%20probe%20selection.JPG                     http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/10x%20probe.JPG

Scope Probe Selection                                                                             10x Scope Probe

 

Experiment 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.

http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/Schematic.JPG        
                                            10:1 Probe Schematic
The schematic above is the schematic shown in Dr. Bakers video. Below is a screen shot of the input wave form being attenuated to one tenth of the input, hense the 10:1 probe ratio.

 http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/Scope%20wave_in_out.JPG
                                                      10:1 Probe Input and Output Simulation

Experiment 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.  

http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/Calculation.JPG

Experiment 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 this experiment I measured an RC circuit using a resistor and a cable, with the cable being the capacitor in the circuit. The resistors actual measured value was 99.9k ohms. Using an input pulse of  1V at 100k and measuring the output against the input I got the following result.

http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/Time%20Delay%20Measurement.JPG

According to the Cmosedu book, the delay time is from the start of the rise time to 50% of  the final amplitude of the signal (which is where I have positioned my cursors in the photo above.) Using the 943.2nS measured above, and the delay time equation td = 0.7RC I can calculate the capacitance of the cable.

C = td/(0.7*R) = 943.2nS/(0.7*99.9k) = 13.3pF

First we have to measure the capacitance of the banana cables that plug into our multimeter. As seen in the picture they measured 0.0085nF. And then I connected the scope probe cable to those and measured them together. Then subtracted one from the other to get the capacitance of the cable. (See calculations below)

http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/Banana%20Cables.JPG                http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/Banana%20Cables%20and%20scope%20cable.JPG

           Banana Cables Capacitance                                                     Banana Cables and Scope Cable  Capacitance

Calculations

Scope cable capacitance = 0.0244nF - 0.0085nF = 15.9pF   compared to the calculated 13.3pF it's pretty close. 


Experiment 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.


http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/Compensated%20Probe.JPG                http://cmosedu.com/jbaker/courses/ee420L/s17/students/fristerc/Lab%202/Uncompensated%20Probe.JPG

                Compensated Probe                                                                        Uncompensated Probe

The compensated probe output compared to the input shows the 10:1 attenuation that is occuring from the corrected capacitance in the probe. The uncompensated probe on the right shows an output with large, uncorrected, capacitance that results in an almost linear output.

Experiment 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.


In order to implement a test point on a PCB so that a known length of cable could be connected directly to the board and not load the circuitry on the board you would want to place a resistor and a variable capacitor in parallel to get rid of any unwanted effects from the cable. 
 

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