ENGR337 Lab 2018 Spring
Lab 3 
RC circuit and the compensated probe. Due Jan 29, Monday 10 pm.

Outcome of this lab:
Understand the mechanism of the compensated scope probes.


The model of the 'probe' you used in the lab can be:

The cable has a parasitic capacitance, the value is around 100 pF, which will always be there, won't disapear. The equivalent capacitance is drawn in the circuit above.

The input resistance of the oscilloscope is 1 MEG ohm, also, the 1 MEG ohm resistor is connected to the circuit in this model.
The 100 pF capacitance slows down the probe, which means if you have a high speed input, the probe will introduce an extra delay to the circuit.

1) If we attenuate the amplitude of the input by 10 times, and then 're-plot' the curve in the oscilloscope by 10 times high, the output showing on the oscilloscope will stay the same, but the actuall input is 10 times smaller, which will relieve the driving load, and make the circuit faster.
This why the probe for the oscilloscope is different from the regular ones.

Let's attenuate the DC voltage first. Build the following circuit in LTSpice, design the unknown resistor value, to make Vo/Vi=1/10, in DC. Report the simulation result. Show why you pick up that resistor value.

2) Now, let's take care of the AC component. Build the following circuit and design the capacitance value. Report the simulation result. Show why you pick up that capacitor value.


3) Once the circuit model is done, modify the unknown the capacitance value to create waveforms of 'undercompensated', 'overcompensated', and 'compensated correctly' in your simulation. Report the results.

4) The adjustable capacitor is inside the probe head (controlled by the screw), use a screw driver to
create waveforms of 'undercompensated', 'overcompensated', and 'compensated correctly'.

5) 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).

6) Treat your scope as a capacitor, build a RC circuit, test the delay of the RC circuit, then predict the equivalent capacitance of the probe.