Lab 2 - EE 420L 

Authored by Jacob Reed

reedj35@unlv.nevada.edu

Due: 2/4/2019

Pre-lab work

• Watch the video scope_probe and review scope_probe.pdf (associated notes).
• Vary the parameters in the simulations found in probe.zip to ensure you understand the operation of a compensated 10:1 scope probe.
• From lab 1 ensure that you understand the operation and analysis of simple RC circuits (likely a quiz on this). 
• Ensure that you can read/create Bode plots and plot the corresponding signals in the time-domain at a particular frequency.
• Read the write-up seen below before coming to lab

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.

Lab Work

Experiment 1: Scope Waveforms

For this experiment, I will show the waveforms of a 10:1 that is undercompensated, overcompensated, and compensated correctly.

The figure to the left shows the screen on the oscilloscope where the user can

choose the type (attenuation) of probe. One just simply has to press the button

of the channel on the scope being used by the probe and it can be set to either

1X or 10X. I used a 10X probe for this lab.

Experiment 2: Schematic of 10:1 scope probe

For this experiment, I will be showing a schematic and simulation of a 10:1 scope probe.

Schematic of a 10:1 scope probe                                                                                  Waveform of a 10:1 scope probe

As can be seen above, an input pulse from 0 to 1V shows a proper output voltage

of 0.1V or 100mV which is 1/10th of 1V.

Experiment 3: Circuit Analysis
 
For this experiment, I will be showing, using circuit analysis, that the voltage on the input of the scope is 0.1 the voltage on the probe tip. We can see that the circuit analysis will just be a simple voltage divider using Z₁ and Z₂. We can see that the cable capacitance and scope input capacitance are in parallel so I will just combine them for 105pF.

                       
 
Experiment 4: Measuring capacitance of a length of cable
 
For this experiment, I will be using a function generator, scope & probe, 100k resistor, and a cable that we will be measuring the capacitance of. We made a simple RC circuit with the 100k resistor and the cable is the capacitor in this situation. In order to measure the capacitance of this, we can make use of the time constant of the circuit to approximate the value of the cable capacitance. Unfortunately, I did not get a photo of the capacitance of the cable using a meter, but it was 67.5pF. When the input is a voltage pulse, we can measure the time it takes for the output to reach 50% of the input. This is going to be the delay time which is 0.7RC. Once we measure the time, we can just solve for C in the equation and find an experimental value for the capacitance of the cable. We ended up measuring a delay time of 4.8µs. With t_d = 0.7RC, or C_cabled =

µs/0.7(100k) = 68.6pF. This is very close to our measured value using a capacitance meter.


Scope waveform showing a 4.8µs delay time

 
Experiment 5: Measuring output of a voltage divider with a cable and compensated probe

The waveforms below show the difference between probing the output of a voltage divider with a properly compensated probe and one that is not properly compensated. Checking the compensation of the properly compensated probe beforehand shows what the output looks like when a probe is overcompensated.


Compensated probe                                                                                      Overcompensated probe
 
Experiment 6: Implementing a test point on a PCB

 An appropriate test point on a PCB would have a resistor and variable capacitor in parallel. This would allow the user testing the PCB to be able to change the capacitance as to compensate for the non-compensated probe/cable being used on the test point. 

Conclusion:

    This was a rewarding lab for me because I now have a deeper understanding about probe compensation. I also learned about the attenuation of a probe and how it works. When working on problematic circuits in the future, it will be easy to rule out probe compensation being the root of the problem. An improperly compensated probe may have adverse effects on the circuit operation due to the introduction of a large capacitance. In hindsight, I may have built circuits and measured incorrect waveforms due to an uncompensated probe. Having the knowledge on probe compensation will help the studentsobtain cleaner waveforms for the circuits they are testing.

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