Lab 2 - EE 420L: Engineering Electronics II



James Mellott

mellott@unlv.nevada.edu
02/01/2017  


Lab 2: Operation of a Compensated Scope Probe 

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. 

 

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

Below in figure 1 is the snapshots of each compensation.

 Figure 1

Experiment 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).


The type of scope probe is set in the channel menu for the Textronix oscilloscope used in the experiment. The channel menu allows the user to choose the proper attenuation for the type of probe being utilized. A probe fixed at a 10:1 attenuation was used throughout the experiment. The image on the right below in figure 2 displays the 10X attenuation printed on the BNC connector, the 10MΩ system input resistance, the typical input capacitance, 12pF, and the bandwidth, 100MHz. The image on the left in figure 2 displays the menu for the probe type set at 10X.

Figure 2

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. 

The 10:1 probe schematic below in figure 3 displays the circuit utilized to gain the attenuation necessary to account for the probe effect on the circuit. Running the same simulation as demonstrated in the pre-lab video resulted in the waveform to the right of the circuit. The pulse input from 0V to 1V results in an output of approximately 100mV. This validates the proper compensation has been achieved by the circuit.

Figure 3

Using the same techniques outlined for creating a 10:1 probe, the schematic below in figure 4 displays a 100:1 scope probe with the resulting waveform. As expected, the pulse output from 0V to 1V results in an output of approximately 10mV.  Also not the output is properly compensated as well.

Figure 4

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.  

The circuit analysis and algebra demonstrating the voltage on the input of the scope is equivalent to 0.1 the voltage on the probe tip can be seen below in figure 5. Calculations are performed assuming a scope input capacitance of 15pF and a cable capacitance of 90pF.

Figure 5

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. 

  

The simplest experiment to measure the capacitance of the cable is to create a simple RC circuit using the scope probe and cable as the capacitor in series with a resistor measured at 100kΩ. Using a voltage pulse as input and measuring the time the output of the circuit takes to reach 50% of the input, known as the delay time, allows derivation of the capacitor value through the relationships displayed in the calculations below in figure 6. The image in the middle below in figure 6 is the oscilloscope waveform for the simple RC circuit displaying a measured delay time of 13.2us for a 1V input pulse at 200Hz. Using the measured time delay resulted in a calculated value of 0.18nF. Measuring the cable capacitance with a multi-meter resulted in a value of 0.18nF.  My calculated values match that of the measured value. 

Figure 6

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.

The waveforms below in figure 7 display the and output of the voltage divider when probing the output using a compensated probe for channel 1 and an uncompensated cable for channel 4. The compensated probe matches the input pulse as can be seen below in figure 7 on channel 1.  The uncompensated probe result seen below in figure 7 is due to the uncompensated capacitance being introduced to the circuit with a resulting RC constant that is high. This is demonstrated by the nearly linear output in the output on channel 4.  The pulse is quick enough that the capacitance on the uncompensated cable never charges.

Figure 7

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. 

A test point can be implemented on a PCB such that a resistor and a variable capacitor in parallel are included in the circuit design process to prevent any effects that would occur when a known length of cable is attached to the test point. Essentially, the probe compensation is included in the circuit design and would allow probing with the uncompensated cable while minimizing the impact of the scope input and cable capacitance on the circuit operation.

 

Conclusion

Laboratory experiment two introduced the topic of scope probe compensation and an opportunity to learn about the techniques involved in compensating scope probes, as well as the effects compensated and uncompensated scope probes have on circuits. The influence uncompensated scope probes have on circuits by introducing large capacitances and altering circuit operation can be minimized by designing a circuit to compensate for the scope input capacitance and cable capacitance. This includes a resistor and capacitor in parallel at the tip of the probe calculated via a basic voltage divider to reduce the capacitance introduced to the circuit and allow for a faster signal. 

 

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