EE 420L
Analog
Integrated Circuit Design Laboratory
Laboratory
Report 2: Operation of a Compensated Scope Probe
AUTHOR:
Bryan Kerstetter
EMAIL:
kerstett@unlv.nevada.edu
FEBRUARY
6, 2019
General
Overview
Probing can either make or break an electronics
project. Poor probing can lead one to believe that their project is not
working. This laboratory educates students about good probing habits while
introducing the compensated scope probe.
Prelab
I watched Dr. Baker’s
video about the compensated scope probe while reviewing the concepts
covered on my Laboratory
Report 1.
Description
of Laboratory Procedures
Waveforms of a 10:1 Compensated Scope:
Undercompensated, Compensated, and Overcompensated
For the entirety of this laboratory, the Tektronix
TPP0200 Voltage Probe (Figure 3) was used. This particular probe is classified
as a 10:1 compensated scope probe. A screw driver can be used to adjust the
scope capacitance. I assume, this is done by adjusting the distance of charged
plates. Or possibly, adjusting the overlap area of the two plates. These concept can be explained in Equation 1.
[1]
|
Variable |
Meaning |
Units |
|
A |
The Overlap Area of the Two Plates |
|
|
|
Separation Distance |
|
|
|
Permittivity of Free Space
|
|
|
C |
Capacitance |
Farads |
Figure 1
Figure 2
Figure 3
LTspice
Model
Here we can see that the capacitance of the scope
probe (C1), determines whether the scope probe is compensated properly. If C1
is to large then the scope probe becomes overcompensated. If C1 is too small,
then the scope probe is undercompensated. Figure 4 shows three cases of scope
probes where the respective probes are compensated, undercompensated, and
overcompensated. Figure 5 shows the inspected traces, where we can see the
actual input voltage vs the inspected voltage traces. Note that the inspected
voltages are 1/10 the input voltage. Hence, as to why the scope probe in theses
simulations are classified as a 10:1 compensated scope probe. Figure 6 zooms on
the compensated, undercompensated, and overcompensated voltage traces.
Experimentally, we attached three separate compensated scope probes to the
oscilloscope test signal. Then we adjusted the compensation of each probe (see
Figure 2) such that there were compensated, undercompensated, and
overcompensated scope probes all to be observed. Our experimental results can
be seen in Figure 7.
Figure 4
Figure 5
Figure 6
Experimental
Results
Figure 7
Attenuation Adjustment on
the Tektronix TDS 2014 Oscilloscope
The probe type can be adjusted on the
Tektronix TDS 2014 Oscillscope by the following procedure demonstrated in the
list below and in Figure 8.
1.
Push the CH 1 MENU button
2.
Push the buttoned
demarked by BOX 2 in Figure 8 to toggle probe adjustments
3.
Look at the probe
adjustment (BOX 3 in Figure 8)
Figure 8
Draft of 10:1 Scope Probe Schematic
The Tektronix TPP0200 Voltage Probe datasheet, the
capacitance ranges of the scope probe tip can be seen in Figure 9.
Figure 9
The oscilloscope used in
this lab (from this point onwards) is the Tektronix
TDS 2014C. According to oscilloscope
datasheet (figure 10), the input impedance of the oscilloscope is a 1 MΩ
resistor in parallel with a 20 pF capacitor.
Figure 10
Additionally, we measured the capacitance of the cable
to be 82 pF. We then compensated the scope probe properly. To determine
reasonable/correct values, I used the equation derived in my first laboratory
report (Equation 15).
[2]
[3]
Therefore, we can say that a reasonable scope probe
capacitance is 11.33 pF for our 10:1 scope probe. Figure 11 shows our drafted
schematic with reasonable/correct values.
Figure 11
10:1 Scope Probe Attenuation Factor Proof
We may then simplify Figure 11 down to what is seen in
Figure 12.
Figure 12
[4]
Based upon Figure 12, we can see that 
.
[5]
[6]
[7]
Also, based upon Figure 12, we can see that 
and 
.
[8]
Therefore, the attenuation factor of our scope probe has been
proved.
Device an Experiment to Measure the
Capacitance of a Coaxial Cable Without a Farad Meter
Theory
By this point, it has been demonstrated that a coaxial cable has
capacitance. This capacitance can be measured with a farad meter (multimeter,
LCR meter, etc.). However, how might one measure capacitance without a farad
meter? Previously, in Laboratory 1, we covered a basic review of RC circuits.
This knowledge will allow us to measure capacitance with just an oscilloscope.
Equations 9-12 and Figure 13 demonstrate the knowledge required for this
experiment.
[9]
[10]
[11]
[12]
Figure 13
Calculating
Expected Values
With a farad meter (LCR Meter), we measured the capacitance of
the cable to be 173 pF.
R1
= 100 kΩ [13]
Ccable
= 173 pF [14]
[15]
[16]
[17]
[18]
In preparation for our experiment, we may determine what the frequency
of our Vpulse input square wave.
[19]
We also decide that pulse should be from 0 V to 5 V.
Figure 14
In the experiment we connected the circuit as seen in Figure 14.
The oscilloscope output can be seen in Figure 17. A 1.8-meter cable was used.
Figure 15
Figure 16
Figure 17
Comparing
Hand Calculations and Experimental Results
|
|
Hand
Calculations |
Experimental
Results |
|
|
|
|
|
|
|
|
|
|
|
|
Figure 18
Experimental
Calculation of Cable Capacitance
To calculate the cable capacitance without the use of a farad
meter we may use any one of Equations 15-18. In this laboratory we decided to
use Equation 17.
[18]
[19]
Comparing
Measured and Calculated Cable Capacitances
|
|
Measured
Using LCR Meter |
Calculated
From Trace Seen On Oscilloscope |
|
Cable
Capacitance |
173
pF |
|
Figure 19
Cable
Length and Capacitance
In the pre-laboratory lecture, Dr. Baker explained
that a good estimate for the capacitance of a cable is 100 pF/m. In our experiment
we used a 1.8 meter (~6 feet) long coaxial cable. According, to the estimate
(100 pF/m), our cable should have a capacitance of ~180 pF. Our, measured and calculated results are in
agreement with the estimate of 100 pF/m (See Figure 19).
Probing a Voltage Divider with a
Compensated Scope Probe and a Coaxial Cable
Figure 20
We then constructed a voltage divider as described in
Figure 20. We then probed the circuit at the ProbingPoint with both a compensated oscilloscope probe and merely
a coaxial cable.
Compensated
Scope Probe
Figure 21 Voltage divider probed with a
compensated scope probe.
Figure 22: Waveform of the voltage divider probed with a compensated scope probe.
Probing the voltage divider with a compensated scope
probe compensates for the large cable capacitance and allows for a smaller RC
time constant. Thereby, we can see the output of the voltage divider beginning
to charge the capacitance and partially discharging. Figure 23 and 24 show this
case in LTspice.
Figure 23
Figure 24
Coaxial
Cable
Figure 25: Voltage divider probed with
a coaxial cable.
Figure 26: Waveform of the voltage divider probed with a coaxial cable.
Using a normal coaxial cable, a large capacitance is
applied to the test point. This large capacitance coupled with the impedance of
the oscilloscope, leads to a large RC time constant. A frequency of 1 MHz is so
large that, the signal is never able to ever charge fully. Thereby, giving us
what appears to be a flat voltage trace. Figure 27 and 28 shows the case of
probing the voltage divider with merely a coaxial cable in LTspice.
Figure 27
Figure 28
Probing a PCB Test Point: Multi-Signal
& Multi-Attenuation (MSMA) PCB Probing System
The MSMA PCB Probing System is how I would implement a
test point on a PCB so that a known length of cable could be could be connected
directly to the board without loading the circuitry on the board. Figure 29
explains the probing system. We have n number
of onboard test signals. These test signals would be connected to ports, in
which standard compensated oscilloscope probes could be connected to. However,
this system also allows the ability to connect ordinary cables to the board
without loading the circuitry on the board. A switch or a jumper cable would be
used to select the test signal to be observed. Then another switch or jumper
would be used to select the desired attenuation factor (created by n pairs of resistor-capacitor cascodes).
The capacitor contained within each resistor-capacitor cascode will be a
variable capacitor within a certain range. The variable capacitors would then
be adjusted based upon the length of the cable connected to the PCB Cable
Terminal.
Figure 29
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