Lab 2 - EE 420L 

Andrew Buchanan

Febuary 13 2019

Buchaa2@unlv.nevada.edu

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.

Experiment 1 Scope probe compensation

This first experiment is to show how an over compensated and under compensated scope probe looks.

The Oscope probe has  a little screw on the BNC connection that allows you to adjust the compensation of the probe by adjusting the capacitance in the tip of the probe. The probe we used was a 200MHz 10x probe.

This is my scope probe under compensated

This is my scope probe over compensated

This is the scope probe compensated correctly

Experiment 2 setting up the scope probe

To set the Oscope probe you would push the channel button, then select the type of probe from the menu on the right and the second selection from the bottom.

This is my Drafted schematic of a 10:1 scope probe showing the 9MEG resistor, 1MEG scope input resistance, capacitance of the cable, scope inpute capacitance, and capacitance in the probe tip. We measured the cable's capacitance with the Multimeter with a value of 105.7pF and from the data sheet for the Tektronix TDS2014 Oscope you can find the value of 20pF for the input impedance.

This is our multimeter Measurment of the wire capacitance

This is the input capacitance from the Oscope data sheet.

For a 10:1 probe C1 is equal to (1/9)*C2, and R1 is equal to to 9*R2. Hand calculations for the probe scope are below.

This is the probe correctly compensated

This is the probe over compensated

This is the probe under compensated

Experiment 5 Measure the capacitance of a length of cable

For this part we came up with a way to measure the length of a cable with an experiment.

We connected the cable  to the bread board in series with a 100K resistor to form an RC circuit.

We then pulsed the cable with a 10KHz pulse and measured the charge time.

Knowing that 5*Tau is the charge time we used the equation Tau=RC to find what C is Equal to, and we got 100pF

This is very close to our measured 105.7pF from the multimeter

50us/5=10us; 10us/100k=100pF

Experiment 6 Voltage divider

We measured the voltage divider using a scope probe then we connected the cable across the probe and took another measurment

The probe by itself (left) has a lower capacitance than the probe with the cable atached (right). We can see that the probe by itself charges a little bit, but when the capacitance is increased with the cable you can see that the wave flattens out.

Experiment 7 Test point

In the design of a PCB you need to be able to vary the capacitance with the length of cable. You can solve this problem by considering a variating capacitor, like the one on the scope probe. You can change the capcitance to compensate for the capacitance of the cable.

Return to Labs