ENGINEERING-43
Subtractive Oscilloscope Measurements
Lab-12
Lab Data Sheet – ENGR-43, Lab-12
Lab Logistics
Experimenter:
Recorder:
Date:
Equipment Used (maker, model, and serial no. if available)
Part-1 Directions
1. Check out
 a DMM
 an oscilloscope
 a Signal/Function Generator
 BNC↔ClipLead cables
2. Go to the side counter, collect resistors, “bread board”, and leads required to construct the
circuit shown in Figure 1. The hardware configuration should be similar to that shown in
Figure 3.
 Note that Scope and Signal-Generator are internally connected to Earth GND. The
“negative” lead for the scope will likely not need to be connected
3. Set the CFG250 Signal Generator knobs, dial and buttons in the same fashion as done in
the Scope Intro Lab Exercise. See Appendix 1
4. Turn on the TDS340 Oscope and clear the Turn-On notice by pressing the CLEAR MENU
button at the lower-right of the display.
5. Turn on the CFG250, and then on the TDS340 Oscope press the AUTOSET button to
obtain a scope trace similar to that shown in Figure 4.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 1
6. Set the time-period and voltage-amplitude with the CURSORS.
6.1. Press the front panel CURSOR button to call up the cursor menu. See Figure 5.
6.2. Press the V Bars side-menu button to select vertical (time and frequency) cursors. See
Figure 6.
6.3. Expand the trace horizontally using the HORIZONTAL SCALE knob as was done in
Lab-14.
6.4. Use the SELECT button and the GENERAL PURPOSE knob to position the cursors
Left-or-Right on the peaks of the trace. See Figure 7 and Figure 8.
6.5. Read the “Δ” measurement shown at the top-right corner of the display. See Figure 8. If
the Δ measure is not 1 ms, then adjust the CFG250 FREQUENCY knob, and then
remeasure until a 1 ms Δ-measurement is achieved.
6.6. Press the H Bars side-menu button and expand the trace vertically (if needed) using
the VERTICAL SCALE knob. See Figure 9.
6.7. Use the SELECT button and the GENERAL PURPOSE knob to position the cursors on
the peaks of the trace. Read the “Δ” measurement shown at the top-right corner of the
display. See Figure 9. If the Δ measure is not 14 V, then adjust the CFG250
AMPLITUDE knob, and then remeasure until a 14 V Δ-measurement is achieved.
6.7.1. Enter
6.8. Remove the cursor bars from the display by pressing the Off side-menu button
© Bruce Mayer, PE • Chabot College • 282219348 • Page 2
7. Verify the Frequency and Amplitude with the scope MEASURE function
7.1. Press the MEASURE button to activate the measure-trace function. See Figure 10.
7.2. On the bottom-menu press the Select Measrmnt for Ch1 button.
7.3. On the side-menu select Frequency. See Figure 11.
7.4. Press the –more– side menu button until the side menu contains the Pk-Pk
measurement (side menu 3 of 6). Press the Pk-Pk side-menu button. See Figure 11.
7.5. If the measurements are significantly different from 1kHz & 14V, then adjust the
CFG250 FREQUENCY & AMPLITUDE knobs to obtain the desired 1kHz & 14V
measurements. See Figure 12.
7.6. Enter the Pk-Pk Measrmnt for Ch1 as the value for “Scope Vpp” in Table I
8. Make the Scope Measurements needed to complete Table I
8.1. Note that scope can only measure voltage-waveforms relative to GND. Thus while the
scope can measure VR2, it cannot measure directly VR1. This value must be determined
by taking the difference between the CH1 and CH2 scope traces; i.e.,
o Vs = VR1 + VR2 (By KVL)
o or
o VR1 = Vs – VR2
9. Use the Scope to Measure VR2
9.1. Press the WAVEFORM OFF and CH2 buttons such that scope shows only the CH2
waveform associated with R2.
9.2. Press the MEASURE button to engage the measurement functions
9.3. On the bottom menu press the button for Select Measrmnt for Ch2. See Figure 13.
9.4. On the Side menu select the Pk-Pk measure, and read the Vpp for CH2 from the
display
10. Use the Scope MATH function to measure VR1
10.1.
Press the MATH button (located just below the CH 1 and CH 2 buttons) to bring
up the Math side menu. See Figure 14.
10.2.
Select Ch1- Ch2 using the side menu button. See Figure 15.
10.3.
Adjust the VERTICAL SCALE on all three traces so that all traces have the same
VOLTS/DIV scale
10.4.
Press the MEASURE button and press the Select Measrment for Math bottommenu button
10.5.
On the side menu select the Pk-Pk measurement, and read the Vpp for M1 (CH1
– CH2) as shown in Figure 16 . Enter the M1 PK_PK value in the VR1 Cell in
Table I.
11. Use the Scope Measurements to make the Calculations needed to complete Table I noting
that:
 VM = Vpp/2
 Vrms = VM/2
 Vavg = AVG(|Vs|) = 2VM/ = 0.6366VM
© Bruce Mayer, PE • Chabot College • 282219348 • Page 3
Figure 1 • Subtractive Scope Measurements. Vs = Sinusoid, 14.00 Vpp. f = 1 kHz.
R1 = 0.6-1.1 kΩ. R2 = 1.1-3.8 kΩ. R1:R2 Ratio Range = 1.5-2.5
 Actual (DMM) Values
R1 =
R2 =
Table I – Scope Resistor Measurements at f = 1 kHz
Value Determination
Vs
VR2
VR1
Scope Vpp
RMS Calc
DMM RMS
© Bruce Mayer, PE • Chabot College • 282219348 • Page 4
Figure 2 • Subtractive Scope Measurements. Vs = Sinusoid, 14.00 Vpp, f per Table II. R1
= 0.6-1.1 kΩ. R2 = 1.1-3.8 kΩ. R1:R2 Ratio Range = 1.5-2.5.
Part-2 Directions
12. Modify the circuit as indicated in Figure 2.
13. Make the Measurements and Calculations needed to complete Table II. Note that if the
scope MEASURE and MATH functions are properly set, then all needed measurements will
be shown on the display simultaneously (see Figure 16):
 Ch1 Freq (use to set the frequency in the frequency sweep)
 Ch1 Pk-Pk
 Ch2 Pk-Pk
 M1 Pk-Pk
14. The scope may be used for subtraction as in item 13.
15. Return all lab hardware to the “as-found” condition
© Bruce Mayer, PE • Chabot College • 282219348 • Page 5
16. QUESTION (5 points): For this practically-resistive circuit are the potentials
measured a function of the supply frequency? JUSTIFY your Answer.
Table II – Resistance v. Frequency Sweep
Frequency, f
Vs = CH1
VR2 = CH2
VR1 = CH1-CH2
100 Hz
500 Hz
1 kHz
2 kHz
5 kHz
10 kHz
50 kHz
Run Notes/Comments
Print Date/Time = 29-May-16/03:59
© Bruce Mayer, PE • Chabot College • 282219348 • Page 6
Figure 3 • Circuit Hardware Configuration, and initial Oscope trace.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 7
Figure 4 • Initial Oscope trace obtained after pressing the AUTOSET button.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 8
Figure 5 • Activate the Cursor Function by pressing the CURSOR button.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 9
Figure 6 • Use the Side-Menu button to select the Vertical-Bar (V Bars) Cursors.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 10
Figure 7 • Use SELECT button and the GENERAL PURPOSE knob to position the
vertical cursors Left-or-Right.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 11
Figure 8 • Vertical cursor bars positioned on the trace peaks. Cursor Difference (Δ)
measurement show in upper-right of display.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 12
Figure 9 • Activate the Horizontal cursor bars (H Bars) using the Side-Menu Button.
Position the cursor bars Up-or-Down using the SELECT button and GENERAL
PURPOSE knob. See Figure 7. With the cursor bars properly positioned across the trace
peak 7 vallyey, read the Cursor Difference (Δ) measurement show in the upper-right of
the display.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 13
Figure 10 • Activate the Measurement Function by pressing the MEASURE button.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 14
Figure 11 • Selecting and making Frequency and Peak-to-Peak Amplitude measurement
using the Measure Function.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 15
Figure 12 • Measurements for a properly adjusted 1 kHz, 14Vpp CFG250 output.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 16
Figure 13 • Determine VR2 using the Oscope’s MEASUREMENT functions.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 17
Figure 14 • Use the Oscope MATH function to perform the VR1 = Ch1 – Ch2 subtraction.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 18
Figure 15 • Select Ch1 – Ch2 on the MATH side menu to to display the results of the
subtraction that yields VR1 = Ch1 – Ch2.
© Bruce Mayer, PE • Chabot College • 282219348 • Page 19
Figure 16 • Making simultaneously all the measurements need to complete each
frequency line in Table II.
Appendix 1 - CFG250 Sigal Generator Initial Settings
Equipment
 TEK CFG250 Signal (function) Generator
o Settings
 Range (Hz)
 1kHz
 Frequency Modulation Dial
 1.0 Frequency (0-2 range)
 Function
 Sine wave 
 Main output (20Vpp)
 AMPLITUDE = Midway Between MIN and MAX (0-100% range)
 VOLTS OUT = 0-20 Vpp
© Bruce Mayer, PE • Chabot College • 282219348 • Page 20

Settings NOT used
 DC OFFSET (set to middle)
 DUTY (set to middle)
 INVERT (button OUT)
 SWEEP (button OUT)
 SWEEP RATE (set to middle)
 SWEEP WIDTH (set to middle)
Print Date/Time = 29-May-16/03:59
© Bruce Mayer, PE • Chabot College • 282219348 • Page 21