Lab 2, More Spice and the Compensated Probe
Outcome of this lab:
Be more familiar with Spice. Understand the mechanism of the compensated scope probes.

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Instructions:

1. Time delay of pulses.

1.1 Build the following circuit in LTSpice using symbols (not Spice Code). Run the simulation and explain the results. (10 points)



1.2 Make the following changes to the circuit and tell if you can charge up the 100 p capacitor to the full voltage range, explain why and why not?

a. Reduce the voltage supply's Von to 2 V, keep all other parameters unchanged. (10 points)



b. Change Von back to 5V, Change the Ton and Tperiod to 1u and 2.1u separately. (you may need to increase your simulation time to show more pulse periods). (10 points)



c. Change all the parameters back to the original ones in the figure in Section 1.1. Replace the 100p with a 10p capacitor. Simulate it and explain the results. (10 points)



d. Change all the parameters back to the original ones in the figure in Section 1.1. Replace the 1k resistor with a 100 ohm resistor. Simulate it and explain the results. (10 points)



1.3 Build the following circuit on your breadboard, use the function generator to generate the square wave, use the oscilloscope to probe the signal. Adjust the frequency of the square wave on your function generator to keep your capacitor fully charged to the full voltage range. Meaure the time delay use the oscilloscope (Cursors). Use your thumb drive to save the input/output waves on the oscilloscope. (if you do not have a thumb drive, take a picture use your phone). (10 points)




2. Time delay and amplitude attenuation of sinewaves.

2.1 Build the following circuit in LTSpice AND on the breadboard. Calculate, simulate, and measure the time delay and Vo/Vi of this circuit. Compare the results in a table. (10 points)



3. The Compensated Probe

The circuit model of the oscilloscope probe you used in the lab:



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.

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. This will relieve the driving load and make the circuit faster.

3.1 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. (10 points)



3.2 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. (10 points)
(The large resistors perform as AC blockers in the circuit).



Hint:



3.3 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). Change it to 1:1, show/discuss what will happen on the scope (Use the same signal input in section 3.2). (10 points)

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The end of the lab