Lab 2 -
EE 420L
Authored
by: Roman Gabriele Ocampo
Email: ocampor5@unlv.nevada.edu
Date: February 10, 2014
Operation of a Compensated Scope Probe
Prelab:
- Watch the scope probe video available on the
EE420L Lab 2 page
- Review operation and analysis of simple RC
circuits
- Read the lab write-up provided on the lab website
Lab
Description and Goals:
The goal of this lab is to understand the function and operation of the
compensated scope probe when it comes to measuring waveforms on the
oscilloscope .
- Show the waveforms of a 10:1 probe that is
undercompensated, overcompensated, and correctly compensated.
- Show where the scope probe type is set on the probe as well as the oscilloscope.
- Sketch the schematic of a 10:1 scope probe
- Using hand
calculations and the sketch from above, show that the voltage on the
input of the scope is 1/10 of 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.
- Build
a voltage divider using two 100k resistors and show the difference
between measure the output of the divider when probing with a cable
versusprobing with a compensated scope probe.
Compensation of a scope probe:
The following are the waveforms of a 10:1 probe that is first undercompensated and then overcompensated.
Below is the waveform for a probe that is compensated correctly.
Probe Type Setting
Some
of the probes in the lab have a switch to change the type from 1:1 to
10:1. This switch is found directly on the probe, as illustrated below.
On
the oscilloscopes used in this lab, the attenuation settings are found
within the "Probe Setup" tab in the channel menu, as shown below.
Schematic of a 10:1 Probe
Hand Calculations on the 10:1 Probe Schematic
The following hand calculations demonstrate how the voltage on the input of the scope is 1/10 of the voltage on the probe tip.
Experiment 1: Finding Cable Capacitance
The goal of the first experiment is to utilize a scope, pulse generator, and a resistor, to measure the capacitance of a length of cable. The following circuit was built:
The time
required for an RC circuit to charge up to half the input signal is
defined as t=0.7RC. Therefore, capacitance can be found if t and R are
known: C=t/(0.7R).
The following waveform was captured from the oscilloscope:
Half
the input signal is obtained at around 6.8us. Therefore, C=97.1pF.
Measuring the cable using a capacitance meter gives a value of 117pF.
Experiment 2: Voltage Divider Measurements
For
experiment 2, a voltage divider was created, and then probed in two
ways. The voltage divider was probed using a compensated 10:1 scope
probe as well as just a length of cable. The following waveforms were
found:
Measurement using the scope probe:
Measurement using the cable:
This
experiment illustrates that use of a compensated scope probe is
necessary to obtain reasonable output voltage measurements. Without the
scope probe, the measured output voltage signal suffers from its
capacitors charging and discharging too slowly, which causes the
displayed waveform to be horribly incorrect.
Summary:
In
order to implement a test point on a printed circuit board, that point
must first be attached to a capacitor and resistor in parallel.
Otherwise, the capacitance associated with a length of cable would
cause loading effects onto the PCB when attached.
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