Lab 2 - EE 420L
Authored
by Shada Sharif,
sharifs@unlv.nevada.edu
6 February 2015
Pre-lab work:
- Watch the video about scope probe by Dr. Baker.
- Vary the parameters in the LTspice simulation attached.
- Know how to create bode plots and read them.
Lab Description:
- The
lab is about probing with a compensated scope probe. Many times using
an uncompensated probe can cause the circuit and elements being
measured to not act as the simulation or as desired, this is due
to the
capacitance and impedences found in the probes and uncalibration. So
this set up is bad because the output of the circuit is less than the
input voltage and there is a time constant that can affect the input,
specially if it is a square wave that would cause the output to not be
a perfect square wave.
Lab Report should include:
- Scope waveforms of 10:1 probe undercompensated, overcompensated and compensated.
- Comment on where the type of scope probe is on the scope.
- Schematic of the 10:1 scope probe.
- Hand calculation of the circuit 10:1 showing how the output is 0.1 of the input.
- Experiment done to measure the capacitance of the cable used, then proving with an actual capacitance meter measurement.
- Building a small circuit and measuring output with a compensated scope and a cable to show difference.
- Explanation of how to implement a test point on a circuit board so that no load is added.
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Experiment #1
When
connecting a probe to the scope to measure things from the circuit, we
add the capacitance of the probe through the BNC connector to the
scope. In order to eliminate this type of issue from happening there is
a screw on most probes that can be changed to adjust the capacitance of
the probe. Some probes have the screw at the tip of the probe while
others at the end where the probe is connected to the BNC connector. In
the probe we used, the screw was at the end where the scope input is.
While adjusting the screw if the capacitance is higher than what it
should be the probe would show a wave of overcompensation, and if the
capacitance is smaller we would get a wave for the undercompensated
probe.
*Notice
how all the pictures above show in the scope that the probes used are
the 10x, which means that they are the 10:1 probes.
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Experiment #2
Scopes
usually have a connector called the calibrator where the probe is
connected to in order to detect what type of probe we are using. We
used two types of scope while conducting the lab, the first one there
was a button that can be pressed after the probe is connected to the
calibrator connection, and the type of probe is measured automatically.
The other scope that was used, we had to connect the probe and through
the wave and the amplitude we knew what type of probe we are using.
This is because the wave that the scope use for the testing is a square
wave with an amplitude of 5V(as indicated by the scope) and if the wave
seen in the scope is 1/10th of 5V one can know this is a 10:1 probe and
so on. There is also other attenuations that exist like a 100:1 where
the output signal now would be lowered by a factor of 100. Also the 1:1
attenuation which is like a piece of cable.
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Experiment #3
From
the schematic shown below, the 15pF and 1 MEG Ohms are the scope
impedence and they are different from scope to the other, usually this
information is either written on the scope or in the manual of the
scope. Then there is the 90pF which is the capacitance of the coax
cable that is used, and can also vary depending on the cable and the
length of it. The parallel combination of the 11.7pF and 9 MEG
Ohms are the impedence that one fix while calibrating the probe from
the screw to compensate for the impedences mentioned above, that is the
reason why the capacitor is left variable so that it can be changed
accordingly in the probe and the circuitry holds for all output to
input ratio of 1/10 for all frequencies. Depending on the probe this
parallel combination can be at the probe tip or at the scope input. At
the probe tip as the frequency increase the capacitance of the probe
goes down and so the impedence.
As
for the waveforms shown, one is the probe tip which is the wave before
the impedences are introduced and the other one is the scope input.
Through the calibration of the probe and the C1 capacitance, the rise
time of the wave is not as slow as it would be for an under or over
compensated probe. Though the signal of the scope input has a lower
amplitude(1/10th of input) than the signal from the probe tip, the
signal has lower capacitance by 9 times and a faster rise time.
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Experiment #4
Through
this calculation one can see how the relationship between both
impedences is a 10:1 just like it was explained before. The output as a
result is 1/10=0.1 of the input but with lower overall impedence. The
DC loading on the probe tip is 9Meg +1Meg so 10Meg, and the capacitive
loading is (90p+15p)||11.7p which is 10.5p. This shows how with this
combination the overall capacitance decrease drastically. ______________________________________________________________________________________________________________________________________________
Experiment #5
The
experiment that we conducted in order to measure the capacitance of the
cable used is an RC circuit. Since the cable itself has a capacitance
because it is a coax cable, we used a known resistance of 1 MEG
ohms and a square wave input of 1V and 1 kHz. The resistor and cable
"capacitor" were connected in series and we tested with a
calibrated/compensated probe the input vs. the output of the square
wave across the capacitor. Using the measuring tool of the scope the
time delay(time takes the pulse to reach 50% of its final value) of the
signal and through that the capacitor value was calculated. To check
the answer, a capacitance meter was used to verify the calculated
result, and they both matched.
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Experiment #6
In
this experiment a voltage dividor was created using a two 100k Ohms
resistors, and we measured the output vs. the input once using a coax
cable and the other using a compensated probe. When measuring the
output with a probe either compensated or cable we are adding a
capacitor across the resistor in the output, which depending on the
capacitance added can influence the circuit. From the circuit shown in
experiment #3 due the calibration done on the probe to reduce the
overall capacitance of the probe, the RC of the circuit is small so
when measuring with a compensated probe that has a small RC, since it
is a RC circuit the capacitor takes less time to charge and as seen
below the figure on the left, the purple signal is increasing with the
impulse changing from 0V to 1V. and as the signal impulse change from1V to 0V the purple signal decrease with the same time constant.
As for measuring with a coaxial cable, the capacitance added is large
that the RC constant created is big and it take more time for the
capacitor to charge, which can be seen in the figure on the right as a
straight line. The input signal due to its high frequency it has a
small time constant, and it is smaller than the time constant of
circuit; thus, the capacitor does not even have time to charge or
discharge and the signal is seen straight and neither increasing or
decreasing.
*Notice
the output with a compensated scope is 1/2 the input, while the output
of the coaxial cable is 0. Therefore we should not probe with a cable.
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Experiment #7
To
implement a test point on a printed circuit board in order to prevent
the loading from the probe to the circuitry on the board, we can do the
same thing done before which is adding the resistor and capacitor
combination. Since the capacitance and resistance of the scope is
known, and the capacitance of the input as well, using a variable
capacitor to get a 10:1 attenuation and a resistor we can adjust the
values accordingly in order to compensate for the load, and not affect
the printed board.
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