Lab 2 - EE 420L
Authored
by Hongzhong Li ,
Today's
date: 02/06/2015
Email: lih12@unlv.nevada.edu
NSHE ID: 2000202827
Lab
description
Operation of a compensated scope probe
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.
Ensure
that your html lab report includes your name, the date, and your email
address at the beginning of the report (the top of the webpage).
When finished backup your work.
Scope waveforms of a 10:1 probe undercompensated, overcompensated, and compensated
| 
|  |
| Undercompensated | Overcompensated | Compensated |
We
are using 10:1 probe in our experiment as you can see from the probe
setting is 10x from the images above. This means if we need the scope
input to be 0.1 of the test point, we turn on to the 10x side on the
probe.
Schematic of a 10:1 scope probe
Calculations:
Measuring Capacitance of the Cable with experiment
Difference between measuring with a cable versus a compensated probe.
We
see that probe without compensation can not obtain reasonable output
voltage. Since the capacitor is so large it will not have enough time
to charge. Without the probe to measure the output , the signal will
discharge quickly before we can get an accurate measurement. The
compensated scope probe enables to test the output voltage correctly
by reducing the capcitance and thus the circuit is able to get
charged even at high speed so we can see the change of the output with
scope.
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.
To
implement a test point on a PCB so that a known length of cable could
be connected directly to the board and load the circuitry on the board,
we need to add the probe tip circit before the test point and just add
that part to the PCB later. We can also add a resistor and capcitor in
parallel before connecting the cable to implement the test point so
that a known length of cable could be connected directly to the board.
Back up Work
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