EE 420L – Engineering Electronics II Lab
Lab 2: Operation of a Compensated Scope Probe
By: Shadden
Abdalla
Email address:
abdals1@unlv.nevada.edu
Prelab: watch scope probe video and know how
to use bode plots.
This lab contains seven parts:
1.
Scope waveforms of a 10:1 probe
undercompensated, overcompensated, and compensated correctly.
2.
A comment on where the type of scope probe is set on the scope
3.
The draft of a schematic of a 10:1 scope probe showing the 9MEG
resistor, 1 MEG scope input resistance, capacitance of the cable, scope input
capacitance, and capacitance in probe tip.
4.
Circuit analysis and algebra showing that the voltage on the input
of the scope is 0.1 the voltage on the probe tip.
5.
An experiment using a scope, pulse generator and a resistor that
measures the capacitance of a length of cable. It will also have a comparison
of the results found in the capacitance meter. This also has hand calculations.
6.
A built voltage divider using two 100k resistors with a 0 to 1V
pulse at 1MHz to the input of the divider. This will show the measurement when
probing with a cable > 3 ft and a compensated scope probe. This will also
have a discussion of the differences.
7.
A discussion about how to implement a test point on a printed
circuit board (PCB) so that a known length of the cable could be connected
directly to the board and not load the circuitry on the board.
Part 1: Scope waveforms of a 10:1 probe
undercompensated, overcompensated, and compensated correctly.
We changed the compensation using
a little screwdriver and adjusting the screw in the probe. When it is
undercompensated, the signal should look like it is still rising. When it is
overcompensated, it should look like it has risen too highly and then started
to go back down. Correct compensation should give a straight line.
This is the undercompensated scope probe. The signal is still rising because
the capacitance is too small. Correct compensation should have the correct
amount of capacitance so that the circuit rises fully and quickly.
This is the correctly compensated scope probe with the correct amount of
capacitance, ensuring that the circuit rises quickly enough to create a perfect
line.
This is the overcompensated scope probe and it
contains too much capacitance, which causes the signal to rise too quickly,
whereas the undercompensated scope probe has too little capacitance which
causes the signal to rise too slowly.
Part 2: A comment on where the type of scope probe is set on the
scope
I used a 10:1 scope probe. You
can see that value says 10X which shows that it is a 10:1 scope probe. On the
actual oscilloscope there is a setting where you can change the compensation
setting based on the type of probe you are using. It should be in the channel
setting. Here I have it set to 10X because my scope is a 10X scope.
Part 3: The draft of a
schematic of a 10:1 scope probe showing the 9MEG resistor, 1 MEG scope input resistance,
capacitance of the cable, scope input capacitance, and capacitance in probe
tip.
This LTSpice
schematic shows the capacitances and resistances that exist in the scope. The
calculations to the right support how the input is ten times the output, which
is what the 10X probe does.
Part 4: Circuit analysis and algebra showing that the voltage on
the input of the scope is 0.1 the voltage on the probe tip.
I solved the circuit by combining two impedances, solving using
the two impedances by modeling a voltage divider, and then finding the ratio
which was 10:1 just as predicted.
Part 5: An experiment using a scope, pulse generator and a
resistor that measures the capacitance of a length of cable. It will also have
a comparison of the results found in the capacitance meter. This also has hand
calculations.
We made an RC circuit using:
Resistor value: 100kohms
Capacitor: the coax cable
By simulating the RC circuit,
we can find the time delay, rise time and period of the circuit and use those
values to find the capacitance. At the end the measurement of the capacitance
will confirm the values from the simulations.
Below is a photo of the circuit on the breadboard. You can see the
black and red lead of the coax cable used as a capacitor connected to the
resistor. To the right is a wider shot showing the coax cable as a capacitor.
The input signal I used was a square signal of 15kHz and a 1V
amplitude.
To solve for the capacitance, I simulated the circuit on the oscilloscope
and measured the rise time. Using the rise time, I used the formula:
Rise time = 2.2*RC to find the value. You can see that the rise
time is 36.05ns.
Rise time = 2.2RC
36.05ns = 2.2 * 100k * C
C = 1.63* 10^-4nF (calculated value)
Actual value of C = 0.1030nF
ANALYSIS:
The value I calculated versus the value I found on the multimeter
is off about 1pF. It is very normal for it to be different because of various factors
that exist in practical simulation versus ideal calculations. The length of the
cable and the resistance, and the impedances of all the cables connected to the
practical circuit all affect the result. It is very unlikely that the result
would be exactly the same in both calculation and
simulation, however, the answer is similar.
Below is the multimeter measurement.
Part 6: A built voltage divider
using two 100k resistors with a 0 to 1V pulse at 1MHz to the input of the
divider. This will show the measurement when probing with a cable > 3 ft and
a compensated scope probe. This will also have a discussion of the differences.
NOTE:
the signals in this part are overcompensated because when I did this part, I did
not have the screwdriver needed to compensate it. I am aware of how to
compensate it and I have photos of a compensated signal above.
WITH COMPENSATED SCOPE PROBE
To the left is a photo of the simple voltage divider circuit created
on a breadboard. On the right of the breadboard you can see the compensated
scope probe. The photo on the right shows the input, a 0 to 1V pulse with a
frequency of 1MHz.
The signal measured with the
compensated scope probe charges and discharges around 0.2V. It has a smaller
capacitance which creates a smaller tau value, or a lower RC time constant.
WITH CABLE
When probing with the cable instead of a compensated scope probe,
there is more capacitance so the signal should be flatter because of a larger
tau value, or RC time constant. The larger time constant in response to a fast
frequency makes it so that the signal cannot discharge. The signal above,
measured with a compensated scope probe, charges and discharges with ease. This
signal is attempting to do so, but is struggling
because of the quick frequency. The input signal is the same as above.
Part 7: A discussion about how to implement a test point on a
printed circuit board (PCB) so that a known length of the cable could be
connected directly to the board and not load the circuitry on the board.
Printed circuit boards usually have coaxial cables connected to
them for signal inputs. In order to create a test point that allows the board
to handle the cable, one should connect a resistor and capacitor in parallel at
the test point. The capacitor value varies regarding the compensation that the
user desires, so the values are not specifically known. Adding a capacitor
footprint makes it so that different capacitor values can be tested.
Implementing a variable capacitor would be most ideal because calculations are
not always the best indication of what is needed after a board is fabricated,
so it is important for the user to be able to test the board and add
capacitance as needed.
This simulation shows how a capacitor and resistor in parallel
gives the same output that we built on the breadboard, because of the
resistance and capacitance of the scope probe. The test point would be the
output on node V(n002).
