EE 420L Engineering Electronics II - Lab 2

Authored by David Flores

Due: February 5, 2019 (Extended Due Date: 02/09/19)

Lab Description

In this lab we will be learning what compensating a scope probe is, different kinds of scope probes (10:1,1:1) and the amount of capacitance a cable has within. This can affect output waveforms on the oscilloscope. So, we will learn exactly how these parameters are affecting them.

Pre-lab

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

Lab Instructions

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: Scope Waveforms of 10:1 Probe

Undercompensated: Overcompensated:

Compensated Correctly:

Comment: There is a variable capacitor that can be modified with a small screw. In undercompensated the waveform does not have enough capacitance in parallel with the 9MEG resistor (10:1) to match the Z2 = 9Z1 equation that capacitance should be around 11.7pF. This 11.7pF would be 1/9^th of the capacitance within the cable (~90pF, 30pF/ft) in parallel with the scope in capacitance (~15pF) If there is too much capacitance it is overcompensated, to little undercompensated, and just right would be correctly compensated.

The type of Scope probe (i.e., 1:1, 10:1, 100:1, etc.) is set on the scope by clicking on the channel number that will be used. It will multiply whatever signal is being inputted by the number so if there is a 1V pk-pk step and you have the oscilloscope set to 10 times it will show a 10V pk-pk signal.

10:1 scope probe schematic:

Comments: Here is the schematic of a 10:1 scope probe 9 MEG resistor, 1 MEG scope input resistance, capacitance of the cable, scope input capacitance, and capacitance in the probe tip. The scope in resistance and capacitances 1MEG and 15pF are part of the oscilloscope, the 90pF is the cable capacitance, and the C1 and R1 and R3 are to get this formula to work Z2 = 9Z1 for all frequencies that way we would get the correct waveform. If it is a 10:1 it divides the input by 10 if the scope is set to 1.

10:1 hand calculation:

Comments: Here are the calculations of the 10:1 scope probe schematic, using circuit analysis shows that the value of Vscope (Vout) is 1/10^th the value at Vtip (Vin). Using a Voltage divider and some algebra we can see that the value of Vout/Vin is 0.1.

Experiment 2: Measuring the Capacitance of a Wire and Probe

For this part we are building an RC circuit which allows us to solve for the capacitance if we have the values for both the resistor and the time constant knowing that the time constant is equal to 63% of the voltage input.

R = 100k, Vin = 1V, Frequency = 10kHz

Measuring the capacitance of the oscilloscope probe:

Comments: Here the yellow wire on top shows how we connected one end of that oscilloscope probe to the resistor and the other end to ground to make a RC circuit so that we could calculate the capacitance using the time constant RC.

Oscilloscope measurements: Measured Capacitance of Probe LCR: 85.69pF

Comments: From here we got a time constant of 8.4us. knowing this we have the resistance of 100k so C=time-constant/Resistance = 11.6pF which gives us a percent error of 1.97%

Experiment 3: Voltage Divider

For this experiment we built a voltage divider and we measured the output using both a cable and a compensated scope probe to see the differences. The experiment will be done at high frequencies so that we can see the effects that the uncompensated cable has compared to the Compensated scope probe. Frequency = 1MHz

Uncompensated cable Oscilloscope Measurements Compensated Oscilloscope Probe Measurements

Comment: In this experiment we noticed that there was a lot more noise in the cable measurements we used a cable about 3ft long which would have theoretically 90pF of capacitance unaccounted for. The output wave from the cable looks pretty close to the o-scope output but this capacitance could cause the amplitude of the output not to be outputted correctly because of the charge up time since it would have an RC time constant.

Implementing a test point on a PCB given length of a cable

The way to approach this would be very similar to how the Oscilloscope probe. A test point on a PCB would just need a resistor in parallel with a variable capacitor. These two components would oversee compensating the waveform correctly from the added capacitance of the wire. These values would depend on the length of the wire which gives us the added capacitance. The way to calculate these values would be Vout/Vin is equal to 1/1. We would do the exact same as the picture except we would not have the scope capacitance or resistance, the cable would have X capacitance (30pF/ft), and C1 and R1 or Z1 would need to be solved for. Having the values of vout/vin and Cable Capacitance would allow us to do so.

10:1 hand calculation:

Conclusion: This lab was very important, I feel like we should have learned this for previous labs so that we would not be so lost. I remember seeing uncompensated waveforms on the oscilloscope and I did not know what was going on. Now that we learned the ins and outs of the probe we know exactly how to compensate and even measure with any standard cable knowing the added capacitance.

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