EE 420L – Engineering Electronics II Lab – Lab 2
Email:
skellj1@unlv.nevada.edu
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Simulation and experimentation to test the operation of a
compensated scope probe.
Pre-Lab
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Watch the scope probe discussion video.
Lab Tasks
·
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 algebra (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.
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Part 1: Compensated Scope Probe Waveforms
The 10:1 compensated scope
probe used to probe in this lab is shown below.
The scope probe used for this lab has a small screw on the BNC
connector that can
be adjusted to change the shunt capacitance of the probe.
The waveforms below show undercompensated, overcompensated, and
correctly
compensated scope probe examples. Further explanation and LTspice simulations
of these waveforms follow.
Undercompensated Scope Probe
Overcompensated Scope Probe
Correctly Compensated Scope Probe
Part 2: Drafting 10:1 Scope Probe Schematics
For the following schematics
and simulations, 20pF was used for the oscilloscope’s internal capacitance.
This value is given in the TDS2000C Series Digital Storage
Oscilloscopes data sheet.
The capacitance of the cable we
used was measured using the capacitance measuring system in the Keithley
2110 Multimeter, shown below.
We see from the picture below that Ccable was
measured to be 0.0639nF
or roughly 64pF.
In order to adjust the probe
compensation on the oscilloscope, the user must:
1.)
Press the channel menu button for the channel they have their
scope probe connected to.
2.)
Click the second button from the bottom (in correspondence with
the on-screen menu).
3.)
Adjust the probe compensation. Potential compensation values are
1X, 10X, 100X, and 1000X.
Undercompensated LTspice
Schematic and Waveform
·
Note that for the
undercompensated case, the shunt capacitor C1 is too small.
Correctly Compensated LTspice
Schematic and Waveform
·
The 9.33 pF was calculated using circuit analysis and algebra
(see hand calcs below).
Using the previously calculated C1 = 9.33pF, the
below results were obtained.
Overcompensated LTspice Schematic and Waveform
·
Note that for the overcompensated case, the shunt capacitor C1 is
too big.
Part 3: Devising an Experiment
to Measure Cable Capacitance
·
The idea for our experiment is quite simple, and
can be entirely explained by the list that follows.
1.)
Using a length of cable as a
parallel plate capacitor, and a 100k resistor, implement a simple RC circuit.
2.)
Using the pulse generator,
input a pulse to our RC circuit.
3.)
Observe the output waveform of
the RC circuit, and measure 5*Τ (T=RC) experimentally.
4.)
Using the experimental value of
5*R*C, solve for C.
5.)
Use the multimeter to measure the
actual cable capacitance for comparison to hand calculations.
Shown below is our breadboard
implementation of the RC circuit using a roughly 2 foot
cable for
a capacitor.
Here, we see our pulse input in blue, and our RC
circuit output in yellow. At 5.00 us per division, we
chose our experimental 5*T value to be roughly 34
us, based on the plot.
Below are the hand calculations
for the experimental cable capacitance.
Finally, we tested our cable
capacitance using the multimeter. Our result was nearly spot on.
The measured capacitance of the
cable was 63.9 pF, while the calculated value of the cable capacitance
was 68 pF.
This experiment was successful in estimating the cable capacitance.
Part 4: 10:1 and 1:1 Probing of
a 1 MHz Voltage Divider
Below is the test setup for the
100k in series with 100k voltage divider.
The waveforms below show the input (blue) and the
output (yellow) of the voltage divider pictured above.
|
Probed with a 10:1 compensated scope probe.
|
Probed with a 4-foot long
coaxial cable.
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In order to understand what is going on in the above oscilloscope
waveforms, we will look at the spice results below.
10:1 Compensated Scope Probe
This schematic
shows the voltage divider set up with a 1 MHz frequency square wave with VL =
-1V and VH = 1V.
The voltage
source models our function generator input signal. For analysis, we can look at
the equation for the
current through
a capacitor.
In the circuit
above with the voltage divider and the compensated scope probe circuitry, the
input voltage is first
cut in half by
the resistive voltage divider, and is then divided by ten by the second voltage
divider within the
scope probe circuitry.
This means the voltage in the oscilloscope after the divisions is only 50mV.
The output
capacitance,
in this case, only has to charge to 50mV (dV in the circuit with the scope probe is 10 times smaller
than
dV in the circuit with the cable). So, regardless of the large RC
time constant of the attached circuitry, we can see
the output of
the divider beginning to charge and discharge the output capacitance, but never
fully charging or discharging.
1:1 Coaxial Cable
This schematic
(above) shows the same voltage divider circuit with the same 1 MHz input
signal, put instead of
probing with a
10:1 probe, we simulate probing using a 4-foot long coaxial cable, loaded with
roughly 30fF of
capacitance
per foot of cable.
The actual
capacitance of our cable was measured by the multimeter to be 111.3pF, as is
shown above.
In this
circuit, following our previous explanation of what is going on with the scope
probe circuit, dV is much
larger. In the
previous case, the voltage required to charge the output capacitance was only
50mV thanks to
the 9 MEG
internal resistance of the probe. For the circuit probed with cable, the output
capacitance needs
500mV to fully
charge. This takes a much longer time, and thus, the waveform below is showing
the capacitance
very slowly charging
and very slowly discharging.
Part 5: PCB Test Point for 10:1 Compensated
Probing
The diagram I
created below shows how a PCB test point could be implemented so that a known
length of cable
could be
connected directly to the board and not load the capacitance and internal scope
RC on the board.
Essentially,
we create an on-board scope probe from a parallel resistor and variable capacitor.
The oscilloscope
used for this
lab has an internal 1MEG resistor and 20pF capacitor. In order to create a
compensated scope probe
on the board,
we simply need to connect a 9MEG resistor in parallel with a variable capacitor
that can range between
around 5.55pF
(for one foot of cable) to around 19pF (for five feet of cable). These values
for the variable capacitor
assume that
coaxial cable is loaded with roughly 30pF/foot. We then route the signal
through a header into the
oscilloscope,
and the overall Vout/Vin for the system will be 1/10.
