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작성자 키트 작성일2017-08-31 13:05 조회473회 댓글0건

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EE 420L Engineering Electronics II - Lab 2

 

 

Eric Monahan

 

 

 

monahan@unv.nevada.edu

 

 

2/10/16

 

 

 

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.

 

 

 

Undercompensated Probe                                                           Overcompensated Probe                                                                  Compensated Probe

 

3660040649_uFO6ERH7_image002.jpg           3660040649_DjdJEbQ1_image004.jpg             3660040649_9ztlPExd_image006.jpg  

 

 

 

 

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 left below displays the 10X attenuation printed on the BNC connector, the 10MΩ system input resistance, the typical input capacitance, 12pF, and the bandwidth, 200MHz. The image on the right displays the menu for channel 2 with the probe type set at 10X. 

 

 

 

 

 

 BNC Connector                                                                             10X Attenuation

 

 3660040649_jX52EZri_image008.jpg            3660040649_DBOqGwhJ_image010.jpg 

 

 

 

 

 

The table below is directly from the Tektronix website and offers a perspective on the range of different values for the parameters mentioned above. Based on the table, the model is possibly a P6117 or an earlier model of the same type.

 

 

 

 

 

3660040649_joxJU4kV_image012.jpg

 

 

Source:  Tektronix

 

 

 

 

 

 

 

 

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 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.

 

 

 

 

 

Schematic  10:1 Probe                                                                                                                             10:1 Probe Waveform

 

3660040649_SpYx9Ey1_image014.jpg            3660040649_QBswdf2n_image016.jpg 

 

 

 

 

 

Using the same techniques outlined for creating a 10:1 probe, the schematic below 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. 

 

 

 

 

 

Schematic  100:1 Probe                                                                                                                                    100:1 Probe Waveform

 

 

3660040649_5zFCgsqO_image018.jpg         3660040649_Ve0SCZnP_image020.jpg 

 

 

 

 

 

 

 

 

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. Calculations are performed assuming a scope input capacitance of 15pF and a cable capacitance of 90pF. The image on the left displays the circuit with impedances in parallel circled and labeled to correlate with the calculations displayed in the image on the right.

 

 

 

 

 

Circuit with Labeled Impedances                                                                                                    Circuit Analysis

 

 

3660040649_VDHAjrC7_image022.jpg                3660040649_QqVdEuWo_image024.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. 

 

 

  

 

 

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 108kΩ. Using a voltage pulse, 3660040649_WbrFEjxH_image026.png as input and measuring the time the output of the circuit takes to reach 50% of 3660040649_VPNpH1vG_image028.png , known as the delay time, allows derivation of the capacitor value through the relationships displayed in the calculations in the center image. The image to the left below is the oscilloscope waveform for the simple RC circuit displaying a measured delay time of 980.0ns for a 1V input pulse at 100kHz. Using the measured time delay resulted in a calculated value of 13pF. Measuring the cable capacitance with a multi-meter resulted in a value of 28pF. This is higher than the calculated value obtained, but may be due to variations in the different methods of measuring. For example, the probes used to measure the cable capacitance on the meter are long cables with their own capacitance that varies depending on how far apart or close they are held to one another. 

 

 

 

 

Delay Time of RC Circuit                                                                                            Cable Capacitance Derivation

 

3660040649_nO4q0z2Q_image030.jpg 3660040649_qbHX5hWw_image032.jpg  3660040649_LNmfkO1R_image034.jpg  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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 waveform on the left below displays the input and output of the voltage divider when probing the output using a compensated probe. The center image displays the output measured with an uncompensated cable, pictured in the image on the right. The compensated probe has a 10:1 attenuation and results in approximately 100mV output for a 1V input. This is due to the compensation that reduces the capacitance introduced to the circuit and the small RC resulting in the capacitor taking less time to charge and thus allowing for a measurable signal.  The uncompensated probe results in a large capacitance being introduced to the circuit with a resulting RC constant that is high and difficult to measure due to the extended time necessary for the introduced capacitance to fully charge. This is demonstrated by the nearly linear output in the center image. The cable is effectively acting as a wire.  

 

 

 

 

 

Compensated Probe                                                                    Uncompensated Cable                                                                   Actual Cable used in Experiment 6

 

3660040649_DZMQnuYf_image036.jpg          3660040649_vTPaA5eg_image038.jpg            3660040649_lqu3Xy5S_image040.jpg

 

 

 

 

 

 

 

 

 

 

 

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. The result of laboratory experiment two is a more insightful comprehension of the impact a measuring device can have on a circuit.

 

 

 

 

Return to Monahan Lab Report Directory

 

 

 

 

 

Return to EE420L Spring 2016 Student Directory

 

 

 

Return to EE 420L Labs

 



3660040649_rCbfSHD1_25EC25A0259C25EB25AA25A9_25EC2597258625EC259D258C.png3Ftype3Dw740

100대1 프로브 회로를 찾다가 발견한 글입니다.^^

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