Lab 2 Operation of a compensated scope probe - EE 420L 

Authored by Jeremy Garrod  

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

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.

Using the Tektronix oscilloscope that is available in the lab, you can manually set the ratio of the probe that you are using. In the menus for each scope channel there is an option called "Attenuation" that is used to select the type of scope that will be used. The probe that is used in this lab is a Tetronix 10x probe.


               10x Scope probe


                 Menu of oscilloscope showing probe type

Experiment 3:

                             SPICE schematic of compensated probe

Using the values from the schematic above, the circuit was analyzed by hand to show that the output (Vout) was indeed 0.1 of the input (Vin). The output of the circuit is just a simple voltage divider between the two impedances Z1 and Z2. The first impedance, Z1, is the probe capacitor and resistor in parallel. The second impedance, Z2, is the the cable and scope capacitances added together in parallel with the resistance of the scope.


                                   Hand calculations for given schematic

Experiment 4:

A time delay of 924ns was measured, which gives a capacitance of c = (924ns/(0.7*100k) = 13.2pF
In order to measure the capcitance of the cable using a capacitor meter, a pair of banana probes was used. The capcitance of the banana probes was first measured. The capacitance of the banana probes and cable were then measured together and the capacitance of the banana probes were subtracted from this value. The resulting capacitance is the capacitance of the cable.

 
                    Capacitance of the banana probes along with the cable


                             Capacitance of banana probes

0.0225nF - 0.0084nF = 14.1pF

The two scope output below are of a properly compensated scope probe and just normal cable. The output of the compensated scope probe  is larger than the cable due to the fact that the probe capacitance is in series with the other capacitances, which lowers the overall capacitance since capacitors in series work like resistors in parallel. This lower capacitance creates a lower RC time constant which means that the capacitors will charge and discharge very quickly. The output of the noncompensated cable will be much lower due to the high capacitance, which creates a large RC time constant. This in turn causes the capacitor to charge and discharge slowly. This effect can be seen by the much more linear output.



                             Output of compensated probe


                        Output of noncompensated cable

Experiment 7: 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.

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