Lab 2 - EE 420L
1. Understand three different conditions of a 10:1 probe, undercompensated, overcompensated, and compensated and show three different scope waveforms.
2. How to set the type of scope probe (i.e., 1:1, 10:1, 100:1, etc.).
3. Sketch the schematic of a 10:1 scope probe.
4. Use hand calculations and circuit analysis to show that the voltage on the input of the scope is 0.1 the voltage on the probe tip.
5. Make a experiment to measure the capacitance of a length of cable by using a scope, pulse generator and a resistor. Besides, calculate the capacitance and compare to the measurement result.
6. Use two 100k resistors to make up a voltage divider. The input signal is a 0 to 1V 1MHz pulse signal. Measure the output of the divider when probing with a cable (havinig a length greater than or equal 3ft) and then a compensated scope probe. Show the result and discuss, explain the differences.
7. Briefly dicuss how to 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.
8. Don't forget to backup your report and work directory on your computer or dropbox and upload it to the CMOSedu.com for the future study and discussion.
2. The probe tip has a black switch to set the probe type as shown in the following figures. If you need the scope input to be 0.1 of the test point, turn on to the 10x side. Otherwise, the scope input will be the same as the test point signal.
3. The following figure is the draft schematic of a 10:1 scope probe.
4. Make some assumptions and use circuit analysis and alegbra to demonstrate the voltage on the input of the scope is 0.1 the voltage on the probe tip. The hand calculation process is shown below.
1. The first experiment is to measure the capacitance of a length of cable. We need to use a scope, pulse generator, a resistor and a length of cable connecting the resistor to ground. The circuit is another RC circuit like Fig.1.21.
Use the scope get the input and output transient result and measure the td (delay time) by measuring the time difference between half of output and half of input signal. Because td=0.7RC, the cable capacitance can be given by C=td/0.7R.
The following schematic is the LTspice model of the experiment. The simulation result shows delay time. Using the scope input signal to test the delay has a little difference compared to using the probe tip signal, which is acceptable.
Besides, we also need to use the capacitance meter to measure the cable capacitance for comparing with the simulation result.
2. The second one is to build up a voltage divider with two 10k resistors. The input signal is a 0 to 1V pulse at 1MHz.(If we use 100kohms, the rising time is larger than pulse width of input signal. ) Use the two methods to measure the output signal: one is probing with cable (having a length greater than or equal to 3ft, namely the capacitance is round 90pF), the other is using a compensated scope probe.
The following figures show the LTspice model. From the schematic models, we see that probe without compensation can not obtain reasonable output voltage. Nevertherless, the compensated scope probe enables to test the output voltage correctly.
Summary:
From above experiments, we can add a resistor and a capacitor in parallel before connecting the cable to implement a test point on a PCB board so that a known length of cable could be connected directly to the PCB.
