EE 420L Engineering Electronics II Lab

Lab 2- Operation of a Compensated Scope Probe

email: matacarl@unlv.nevada.edu

2/06/19

Pre-lab Requirements:

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 frequency.
Read the write-up seen below before coming to lab

Lab description:

This lab’s main purpose is to understand the circuitry and application of compensated scope probes in comparison to a regular coaxial cable.

Requisites:

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

b) 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).

c) 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.

d) Using circuit analysis, and reasonable/correct values for the capacitances, show using circuit analysis and algebra (no approximations), that the voltage on the input of the scope is 0.1 the voltage on the probe tip.

e) 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.

f) 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.

g) 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.

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

Below is a photo of the oscilloscope screen showing 3 scope probes; compensated (yellow trace), undercompensated (blue trace), and overcompensated (pink trace).

Location showing the type of scope probe (i.e., 1:1, 10:1, 100:1, etc.) setting on the Oscilloscope

The photo below shows the steps to follow to find the type of scope that is needed. Next to the oscilloscope are photos for the scope probe used (10X1).

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.

The values for the cable and the C_scope cable used below were values calculated in the lab.

Using circuit analysis, and reasonable/correct values for the capacitances, show using circuit analysis and algebra (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

For this experiment a 100k resistor and a 6ft coaxial cable was used. The value given by the meter was closed to 173pF. Thus, this is the value used for LTspice simulations and experimental part.

Hand Calculations:

Simulation Results:

Below shows the circuit used to test the capacitance of the 6ft coaxial cable for this experiment.

Experimental Results:

Photo on the left shows the 173pF Coaxial Cable (6ft) in series with a 100k resistor to make an RC circuit. Photo on the right shows the results from the oscilloscope. The measure parameters in the red box show the fall, rise, and Delay time.

Using the values from the screen of the oscilloscope we can calculate the experimental capacitance.

The difference in capacitance value from the meter and the experimental test result is about 23pF.

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.

Circuit and plots with compensated probe	Circuit and plots without compensated probe



Here by using a compensating scobe probe the capacitance of the circuit has been decreased, causing the Time Constant to decrased as well, which is the reason why we are able to see a small increase in the output voltage; if no probe was used then the capacitance would be 9 times greater, increasing the Time Constant significantly. Even though the capacitance has been decreased, the Time Constant of the circuit is still greater than the period for which is being set, 1𝞵s, for this reason circuit does not have time to charge to its final Voltage value. The photo below show the ouput (blue) as almost a triangular wave, due to the period time, which does not allow the RC circuit to fully charge.	For the case when there is no compensating probe the capacitance is much greater becaue the cable capacitance and scope input capacitance add up to over 100pF, making an RC circuit with a Time Constant much much grater than the 1𝞵s period signal from the input. Thus the output (blue) does not have any time to charge, causing the output signal to look like a line.

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.

The design below would be the approach I would take to implement test points. The input BNC would be used to connect the coaxial cable and test the capacitance with a meter. The meter would connect to the headers, which would be connected to the signal pin and frame of the input BNC. The value from the meter would be a good estimate, however if we want to test the cable under a load and compensate experimentally then the selector switch comes into play. The selector switch would be used to select the test mode without affecting the rest of the circuit. By using this method we have the same circuit as the circuit simulated above near top of the page, which we know creates good results. In this set up the variable capacitor could be a through hole component.

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