Lab 2: Operation of a Compensated Scope Probe - ECE 420L 

Authored By: Joey Yurgelon

Email: yurgelon@unlv.nevada.edu

January 30, 2015

Pre-lab Work:

• 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

• Students will learn the inner workings of the common scope probe, and understand the conversion an input signal takes from probe to scope.

• 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.

Experimental Results: 

    Exercise #1: Show Scope waveforms of a 10:1 probe undercompensated, overcompensated, and compensated correctly.

• Top is the LTSpice Simulations and Bottom is the Experimental Results

Undercompensated Probe Scope Waveform	Compensated Probe Scope Waveform	Overcompensated Probe Scope Waveform

   Exercise #2: Comment where the type of scope probe (1:1, 10:1, 100:1, etc. ) is set on your scope.

• The scope probes inside of the lab generally have a way to toggle between 1x and 10x attenuation. It is located on the handle of the probe. Not all probes in the lab have this feature, however, some of them are a constant attenuation. If this is the case, they are commonly 10x probes, and are usually printed on the BNC connector. The scopes also have a probe compensation portion on their screen that allows the student to connect to and quickly compensate their probes correctly. 

Probe Compensation on Scope

Attenuation switch is located on the body of the probe

    Exercise #3 & #4: Draft the schematic of a 10:1 scope probe, perform the circuit analysis to show that the voltage on the input of the scope is 0.1 the voltage on the probe tip.

• There are two capacitors that need to be compensated for to offer a correct reading on the oscilloscope. There is an input capacitance on the scope, and a capacitance in the cable that may load down the circuit if a 1:1 probe is used. To compensate for this, a 0Meg resistor is set in parallel with a variable capacitor inside the probe end. What this allows the probe to do is act as sort of a voltage divider, and cancel out the effects of the unwanted capacitance in the scope and line. Unfortunately, the trade off is the change in signal amplitude as noted in the calculations. The calculations themselves are pretty straight forward, however. One can make use of equivalence to simplfy the circuit into a common voltage divider relation. This relation will be noted as Vout = Vin* (Z2)/(Z1 + Z2) where Z1 is (R1)/(1+jwR1C1) and Z2 is (R2)/(1+jwR2C2) by parrall impedences. This relationship, once numbers, are inputted will clearly show that the scope input signal is 1/10 the probe signal. 

Circuit Probe Draft Schematic

Probe Circuit Analysis

    Exercise #5: Devise an experiment to measure the capacitance of a length of cable. Compare the result with a capacitance meter. 

•  In this exercise, we needed to devise an experiment to determine the unknown capacitance of a coaxial cable. We built a simple RC circuit that we could relate the rise time to the capacitance in the cable. By setting up the experiment with a 1k resistor and a 1k Hz pulse signal, we could determine the rise time of the output. This rise time landed right around 365ns. From our calculations we could determine the capacitance by the equation C = Trise/(2.2R). This capacitance came out to be 165.9pF. To compare our result, we connected the cable to the multimeter and received a result of 191 pF. 

  Experimental Setup and Theory	Experimental Rise Time Measurement
  Multimeter Capacitance Measurement

  
  

• One can see that the difference between the two is the ability to really measure the amplitude of the signal. The 10:1 probe at 1 MHz, adds around 10 pF into the circuit. The output seen below shows that the capacitance nearly forms a perfect integrator of the square wave input. If the frequency was adjusted, however, the capacitor would have time to discharge before the next pulse happened which would reduce the DC offset while preserve more of the square wave input. The 1:1 probe has the complete opposite effect, the capacitance loading effect just prevents any sort of reasonable measurement from taking place. When unsure of our results in the lab, Professor Baker had us sim the 10:1 probe to confirm that at 50% duty cycle with a 1MHz input signal the offset would be about a 1/3 the input. This can be verified below.

• 10:1 Probe Measurement	1:1 Probe Measurement
  10:1 Probe Schematic	10:1 Probe Simulation

  
  

• Just like the probe set up, one would have connect a resistor and a capacitor in parallel so that the effects of the scope and attached cable do not alter the signal trying to be measured.

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