Download LAB 6 - Chabot College

ENGR 43
Lab Activity
Student Guide
LAB 6 – Advanced Oscilloscope Operation
Student Name: ___________________________________________________
Overview
Getting Started
In this activity you will learn some
additional procedures for making effective
use of the oscilloscope, plus observe some
of the limitations of oscilloscope
measurements.
Lab Activity and Deliverables:
It should take students approximately 2
hours to complete the lab activity, and 1
hour of homework time to complete the lab
report.
Before Starting This Activity
Equipment & Supplies
Item
Tek MSO2014 oscilloscope
Fluke 271 function generator
Tek TP2221 probes
BNC-clip lead
Probe compensation tool
100 kΩ resistors
1 kΩ resistors
Proto board or NI-ELVIS
USB flash drive (provided by
student) for waveform storage
Refer to the following documents for scope
probe operating instructions and background
reference information:
Tektronix TP2221 200 MHz 1x/10x Passive
Probe Instructions.
Tektronix ABCs of Probes Primer
Links to these documents are on the
GoogleDocs listing
(http://tinyurl.com/engr43-lablinks)
Learning Outcomes For Activity
Relevant knowledge (K), skill (S), or
attitude (A) student learning outcomes
Quantity
1
1
2
1
1
2
2
1
1
Special Safety Requirements
If you use the NI-ELVIS proto board section
for your test connections, do not make any
connections to the right or left strips on the
prototype area. These connect to the NIELVIS circuitry, which is not used in this
activity.
K1. Describe the proper use of 1x/10x scope
probes and the procedures for optimal
measurement integrity
S1. Compensate a 10x scope probe
S2. Measure low amplitude or noisy
signals with acquisition averaging
Lab Preparation
S3. Set triggering controls for appropriate
channel, slope, and level
No special setup requirements
A1. Appreciate the need for proper
oscilloscope measurement techniques
to capture accurate measurements
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ENGR 43
Lab Activity
Student Guide
white background. Do not select “Save
Waveform” for this activity, as this will
save the numeric data values of the
waveform, not the screen display.)
Task #1 – 10x Probe
Compensation
The standard resistance for an oscilloscope
input is 1 MΩ, but the parallel input
capacitance can have significant variation
between models, and even some variation
between input channels on the same scope.
Proper balancing of the probe capacitance
with the scope input capacitance is
necessary to make accurate measurements
with a 10x probe.
1. Refer to the above illustration. Connect
the probe to the calibration signal on the
oscilloscope front panel.
2. Press AUTOSET or otherwise adjust
your oscilloscope to display a stable
waveform.
3. Adjust the trimmer in the probe until you
see a flat-top square wave on the display.
(See above illustration for location of
adjustment port.)
4. Refer to the example traces for over-,
under-, and properly compensated
probes.
5. Connect a USB flash drive to the front
USB port on the scope. Press the Save
button to save screen images of over-,
under-, and properly compensated
probes. (Note: when saving screen
displays from the Tek scopes, using the
Ink Saver option saves the screen with a
Lab 6 – Advanced O-scope Operation
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Lab Activity
Student Guide
Task #2 –Low-Frequency and
High-Frequency Loading Effect of
1x and 10x probes.
8. Repeat the measurements of steps 5
(10x) and 6 (1x) for the 100 kΩ voltage
divider.
9. Change the frequency of the function
generator to 100 kHz. Repeat steps 5
through 8 for the higher frequency.
This task demonstrates the potentially
adverse effects a scope probe can have on
the operation of a circuit due to the resistive
loading effect.
Hz
Probes
VA
VB
10x
1
1x
kHz
100 kΩ 10x
1x
10x
100 1 kΩ
1x
kHz
100 kΩ 10x
1x
10. Which test condition(s) appear
abnormal?
_________________________________
1. Refer to the schematic shown above.
Connect two 1 kΩ resistors (brown,
black, red, gold) in series with the
function generator as shown. The red
lead of the BNC-clip lead connects to
node A and the black clip connects to the
circuit ground at node C.
2. Set the function generator to output a 1
kHz sine wave, 1 Vpp.
3. Connect P2221 probes to channels 1 and
2 of the MSO2014 scope. Set the probes
and the scope input channels for 10x
probes.
4. Connect the channel 1 probe to node A,
and channel 2 probe to node B. The
ground clips for both probes connect to
node C.
5. Measure the peak-to-peak voltages and
nodes A and B. Since R1= R2 in this
voltage divider, we expect to see 50% of
the node A voltage at node B. Enter your
measurements in the table below.
6. Set the probes and the scope inputs for
1x probes. Repeat the measurements and
enter the values in the table.
7. Build a second voltage divider circuit,
this time with 100 kΩ resistors (brown,
black, yellow, gold).
Lab 6 – Advanced O-scope Operation
ENGR 43
R1 & R2
1 kΩ
_________________________________
_________________________________
11. For a visual indication of probe loading
effect, remove the channel 1 probe from
node A (leave the probe ground clip
connected). The output frequency should
still be at 100 kHz. Select the Trigger
menu and set the trigger input to channel
2. Set the channel 1 and 2 probes and
inputs for 10x. While leaving the
channel 1 probe disconnected, observe
the channel two trace at node B. Connect
the channel 1 probe to node B and note
the effect on the channel 2 trace. Switch
the channel 1 probe to 1x and note the
effect on the channel 2 trace. What do
you observe?
________________________________
________________________________
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© 2012
ENGR 43
Lab Activity
Student Guide
Task #3 – Filtering noise with
sample averaging.
5. Connect the channel 2 probe, set to 10x,
to node A. Set the trigger input to
channel 2. What improvement do you
see?
________________________________
This task demonstrates how to measure lowamplitude or noisy signals by using sample
averaging to reduce asynchronous
interference.
________________________________
________________________________
6. Set the channel 1 probe to 1x (always
change the input channel probe selection
when you change the probe multiplier
switch). What improvement do you see?
________________________________
________________________________
1. Refer to the schematic shown above.
Connect a 100 kΩ resistor (brown,
black, yellow, gold) in the R1 position,
and a 1 kΩ resistor (brown, black, red,
gold) in the R2 position. Connect the
function generator and set the output for
a 10 kHz sine wave with a peak-to-peak
amplitude of 1 volt. Connect the channel
1 probe to node A, set the probe to 10x,
and verify the amplitude and frequency
of the signal.
2. Calculate the expected voltage across R2
(node B to ground). Round your answer
to the nearest millivolt.
________________________________
7. Low-voltage signals are difficult to
measure because of outside interference,
or noise. Sometimes we want to measure
the sum of the signal and the noise, but
sometimes we are only interested in the
signal. We can take steps to minimize
the noise, such as minimizing wiring
length and shielding our circuit from
electrostatic interference, but we can
never eliminate all noise. However, we
can take advantage of the fact that
outside interference is asynchronous (not
synchronized) to our signal.
8. Select the Acquire menu. The Average
mode should be set to Off. Change the
Average mode to 4 samples. What
improvement do you see?
________________________________
VR2 = __________ mV
3. Remove the channel 1 probe from node
A and connect to node B. Select the
Measure menu and add channel 1 pk-pk
as a new measurement. Save this screen
image on your flash drive.
4. Are you able to measure the expected
voltage with 1 mV precision? Why not?
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________________________________
_______________________________
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Lab Activity
Student Guide
9. Increase the number of samples until the
channel 1 measurement is stable within 1
mV. How many samples are needed, and
what is the measured voltage?
4. Connect the channel 2 ground clip to the
circuit ground (node C). Save this
waveform on your flash drive.
5. Connect the channel 1 ground clip to the
circuit ground. Save this screen image
on your flash drive.
6. Experiment with connecting the channel
1 probe ground clip through various
lengths of 22 ga. wire. How long of a
wire can you use before seeing the
effects (probe ringing)?
_________________________________
_________________________________
10. Save this screen image on your flash
drive.
Task #4 – Probe grounding.
_________________________________
This task demonstrates the importance of
proper probe grounding for signals with fast
rise/fall times.
_________________________________
Deliverable(s)
Paste your saved waveform displays into the
appropriate places in the Performance
Report (which starts on the next page of this
document). Print your Performance Report,
and save your completed Lab 6 Activity
Guide and Performance Report in your Lab
Activity Binder.
1. Refer to the schematic shown above.
Connect two 1 kΩ resistors (brown,
black, red, gold) as shown.
2. Set the function generator for a 100 kHz
square wave, amplitude 1 Vpp.
3. Set the channel 1 probe multiplier (and
scope input) to 10x. Connect the probe
to node A, but leave the probe ground
clip disconnected. In this configuration
the circuit ground for the probe is made
through the AC power plugs on the
function generator and the o-scope. Set
the scope to display two complete
periods of the square wave. Save this
screen image on your flash drive. What
anomaly do you see?
_____________________________
_____________________________
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© 2012
ENGR 43
Lab Activity
Student Guide
LAB 6 – Advanced Oscilloscope Operation
Student Name: ___________________________________________________
Note: Print and turn in the performance report pages along with the lab activity procedure pages.
Scope Probe Compensation
Paste your scope screen images from Task #1, step 5 in the spaces below.
Over-compensation
Under-compensation
Proper compensation
Scope Probe Loading Effect
Based on your data from Task #2, what general rules can you make for using 1x and 10x probes?
Sample Averaging Mode
Paste your screen images showing the waveform with and without sample averaging.
No
Avg
Avg
Samples
=______
Probe Grounding
Paste your screen images showing the waveform with and without proper grounding.
Bad
Proper
Ground
Ground
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© 2012