Lab 2 - EE 420L: Engineering Electronics II
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: Show
scope waveforms of a 10:1 probe undercompensated, overcompensated, and
compensated correctly.
Below in figure 1 is the snapshots of each compensation.
Figure 1
Experiment 2: 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).
The type of scope probe is set in the channel menu for the Textronix oscilloscope
used in the experiment. The channel menu allows the user to choose the proper attenuation
for the type of probe being utilized. A probe fixed at a 10:1 attenuation was
used throughout the experiment. The image on the right below in figure 2 displays
the 10X attenuation printed on the BNC connector, the 10MΩ system input
resistance, the typical input capacitance, 12pF, and the bandwidth, 100MHz. The
image on the left in figure 2 displays the menu for the probe type set at 10X.
Figure 2
Experiment 3: 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.
The 10:1 probe schematic below in figure 3 displays the
circuit utilized to gain the attenuation necessary to account for the probe
effect on the circuit. Running the same simulation as demonstrated in the
pre-lab video resulted in the waveform to the right of the circuit. The pulse
input from 0V to 1V results in an output of approximately 100mV. This validates
the proper compensation has been achieved by the circuit.
Figure 3
Using the same techniques outlined for creating a 10:1 probe,
the schematic below in figure 4 displays a 100:1 scope probe with the resulting
waveform. As expected, the pulse output from 0V to 1V results in an output of
approximately 10mV. Also not the output
is properly compensated as well.
Figure 4
Experiment 4: 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.
The circuit analysis and algebra demonstrating the voltage on
the input of the scope is equivalent to 0.1 the voltage on the probe tip can be
seen below in figure 5. Calculations are performed assuming a scope input
capacitance of 15pF and a cable capacitance of 90pF.
Figure 5
Experiment 5: 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.
The simplest experiment to measure the capacitance of the
cable is to create a simple RC circuit using the scope probe and cable as the
capacitor in series with a resistor measured at 100kΩ. Using a voltage
pulse as input and measuring the time the output of the circuit takes to reach
50% of the input, known as the delay time, allows derivation of the
capacitor value through the relationships displayed in the calculations below
in figure 6. The image in the middle below in figure 6 is the oscilloscope
waveform for the simple RC circuit displaying a measured delay time of 13.2us
for a 1V input pulse at 200Hz. Using the measured time delay resulted in a
calculated value of 0.18nF. Measuring the cable capacitance with a multi-meter
resulted in a value of 0.18nF. My
calculated values match that of the measured value.
Figure 6
Experiment 6: 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.
The waveforms below in figure 7 display the and output of the
voltage divider when probing the output using a compensated probe for channel 1
and an uncompensated cable for channel 4. The compensated probe matches the
input pulse as can be seen below in figure 7 on channel 1. The uncompensated probe result seen below in
figure 7 is due to the uncompensated capacitance being introduced to the
circuit with a resulting RC constant that is high. This is demonstrated by the
nearly linear output in the output on channel 4. The pulse is quick enough that the
capacitance on the uncompensated cable never charges.
Figure 7
Experiment 7: 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.
A test point can be implemented on a PCB such that a resistor
and a variable capacitor in parallel are included in the circuit design process
to prevent any effects that would occur when a known length of cable is
attached to the test point. Essentially, the probe compensation is included in
the circuit design and would allow probing with the uncompensated cable while
minimizing the impact of the scope input and cable capacitance on the circuit
operation.
Conclusion
Laboratory experiment two introduced the topic of scope probe
compensation and an opportunity to learn about the techniques involved in
compensating scope probes, as well as the effects compensated and uncompensated
scope probes have on circuits. The influence uncompensated scope probes have on
circuits by introducing large capacitances and altering circuit operation can
be minimized by designing a circuit to compensate for the scope input
capacitance and cable capacitance. This includes a resistor and capacitor in
parallel at the tip of the probe calculated via a basic voltage divider to
reduce the capacitance introduced to the circuit and allow for a faster signal.
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