Operation of a Compensated Scope Probe

Lab description: Copied from lab instructions

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.

Observation of Compensated, Undercompensated, and Overcompensated probe

- Show scope waveforms of a 10:1 probe undercompensated, overcompensated, and compensated correctly.
Pics/compensated_probe.PNG

Compensated probe

Pics/undercompensated_probe.PNG

Undercompensated probe

over
Pics/overcompensated_probe.PNG

Overcompensated probe

Scope Attentuation Setting

-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).

The scope probe we used was a 10:1 probe.

Pics/scope_parameters.jpg

Scope and probe combo used

LTSpice Simulation

-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.

By analyzing this simulation, it can be seen that the scope input is a 10th of the pulsed signal.

Simulated 10:1 probe schematic and waveform.

Calculations:

-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.

The circuit in question can be reduced to a simple voltage divider by calculating equivalent impedances for Z1 and Z2.

Scope circuit being analyzed. Equivalent impendances are boxed Z1 and Z2.

Calculations used to calculate ratio of voltage on probe tip.

Devised Experiment:

-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.

Our devised experiment involved integrating the cable into an RC cirucit and measurign the time delay. We pulsed a 1V square wave with a 50% duty cycle at a frequency of 30KHz. Our RC circuit utilized a 100k resistor. By analyzing the delay time, we would be able to determine the capacitance of the cable. The cable had a capacitance of 67.5pF (forgot to take a picture of the measure capacitance), which adds up since the cable was approximately larger than 2ft and cable capacitance is usually 30pF per foot.

Pics/Lab%202%20extra%20pics/TEK0002.JPG

Oscilloscope reading for our RC circuit. Measurement denotes a 4.8us delay time.

Here is the calculation that verifies our results:

Voltage divider:

-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.

Pics/compensated_voltagedivider.PNG

Compensated voltage divider output
Pics/overcompensated_voltagedivider.PNG

Over-compensated voltage divider output

The key differences between the two different outputs lies within the RC time constant differences between the two outputs. In the over-compensated circuit the RC constant is larger and provides for a longer discharge time. That is why the signal appears less stable than the compensated circuit.

Observations:

-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.

A simple test to implement on a PCB would be to have a resistor in parallel with a variable capacitor. This would allow the individual to act as a compensated probe as the user would be able to compensate for capacitance in the same manner.

Conclusion:

This lab demonstrated a valuable insight into a professional skill that often goes overlooked. Measuring waveforms is an invaluable analysis that every electrical engineer must perform. Understanding how scopes are compensated gives the analyzer another degree of accuracy for their observations and also gives methods as to how a scope can be compensated manually. Overall, this was a very beneficial lab.