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ECE 2006 LABORATORY 3
DIGITAL OSCILLOSCOPE SWEEP CONTROLS
Pre-Lab Calculations: Complete Table 2, Theoretical Frequency Response Calculations,
prior to performing lab experiments. See “RC Circuit Equations” below for help.
Objectives
The learning objectives in this laboratory give the student the ability to:
 Use the oscilloscope internal sweep generator.
 Use the oscilloscope trigger level and slope controls.
 Use the oscilloscope to display the difference of two signals.
 Use the oscilloscope for frequency, phase angle and voltage measurements.
References
TDS 3012 B Series User Manual
Background
Oscilloscope time mode:
In the last lab you learned how to use the oscilloscope in X-Y mode. In this lab you will
use the oscilloscope in time-based. In time (or sweep) mode the oscilloscope samples the
voltage levels of the input channels over time. The voltage on the input channels causes
the display to deflect vertically and the horizontal axis represents the time domain. You
can adjust the seconds per division to display more or less of the waveform and you can
adjust the vertical sensitivity to change the vertical volts per division.
Oscilloscope math operations:
This generation of oscilloscopes allows you to do mathematical operations on the input
waveforms. You can then display the results of these mathematical operations as another
input waveform. This allows you to observe waveforms that you could not measure
directly.
Oscilloscope trigger:
The oscilloscope is used to observe repetitive signals. If the screen is redrawn with the
display signal in a different part of its cycle each time the oscilloscope screen will show a
drifting wave shape. A trigger is used to catch the displayed signal in the same location
every time the screen is redrawn. The trigger controls include source selection, level
selection, slope selection, and time delay. The source can be set to channel 1, channel 2,
line (60 Hz power line), or external. The level control adjusts the voltage level of the
source at which the waveform will be drawn from the center of the screen. The slope
switch determines if the trigger will on the signals positive slope or the negative slope. It
is easy to see where the oscilloscope will trigger the waveform by looking at the trigger
level indicators on the screen.
Oscilloscope phase angle measurements:
Phase angle measurements include an angle and a sign. The phase angle can be measured
manually by reading off the screen or automatically by the oscilloscope. A phase angle
difference includes a sign as well as a magnitude. If Vr occurs, say 30o, earlier in time
than Vs, then Vr leads Vs by 30o or Vs lags Vr by 30o, and the phase angle difference is
Vr - Vs = +30o. If Vc occurs, say 30o, later in time than Vs, then Vs leads Vc by 30o or
Vc lags Vs by 30o, and the phase angle difference is Vc- Vs = -30o.
RC Circuit Equations:
For the RC circuit in Figure 1, the phase difference between Vr and Vs is
Vr - Vs = 90 o-tan-1(CR)
where
C is the capacitance in Farads, and  = 2f, 0 o tan-1(CR)  90 o
and the phase difference between Vc and Vs is
Vc- Vs = -tan-1(CR)
For the RC circuit in Figure 1, the magnitude ratio Vr/Vs can be found by using the
voltage divider equation given in class, Vr = Vs * R1 / (R1+R2). In this case, R1 is the
value of the resistor in the circuit, R. R2 is the impedance of the capacitor, (jωC),
which is dependent on the fruequency of excitation. The voltage divider can be
algebraically re-written to give the following result:
Likewise, the magnitude ratio Vc/Vs can be written as:
Equipment
Digital Oscilloscope TDS 3000 B Series
Function Generator
Resistor
Capacitor
Procedure
This lab deals with the time based mode of the oscilloscope. Last lab you learned how to
use the X, Y axes mode on the oscilloscope. Now both channel 1 and channel 2 will
represent vertical voltage deflections versus time.
Initial Setup:
A. Set the oscilloscope to time mode:
 (Display > X Y Display > Off (YT))
B. Turn on channel 1.
C. Set the channel 1 to ground reference and set level to center graticule line.
 (Ch1 > Coupling > GND)
D. Set channel 1 back to DC mode.
 (Ch1 > Coupling > DC)
1. Trigger.
1.1 Untriggered:
1.1.1
Connect the function generator to channel 1.
1.1.2
Set the function generator to 3 Vpp, Square Wave, 100 Hz.
1.1.3
Set the triggering mode of the oscilloscope to external.

1.1.4
Set sweep speed to 4mS/div.

1.1.5
(Trigger Menu > Source > Ext
(Turn Horizontal scale knob)
Have the lab instructor observe this response.
Lab Instructor _____________________________________
1.2 Triggering Automatic:
1.2.1
Now adjust the trigger to channel 1

1.2.2
(Trigger Menu > Source > Ch1)
Press the (Set to 50%) button. This will automatically set the trigger level.
Notice how the waveform stops drifting. The scope is now centering the
waveform at the same point every time.
1.2.3
Have the lab instructor observe this response and save to disk.
Lab Instructor _____________________________________
1.3 Triggering Manual Levels: Observe the change in the triggering level on the
center vertical graticule line.
1.3.1
Adjust the function generator to a 6 volt peak to peak 100 Hz sinusoidal wave.
1.3.2
Set the trigger to channel 1.
1.3.3
Set the trigger slope to trigger on the positive (rising) slope.

1.3.4
(Trigger Menu > Slope > Rising Edge )
Adjust the trigger level to 0 volts.

(Turn trigger level knob)
1.3.5
Adjust the trigger level to +1 volts.
1.3.6
Adjust the trigger level to +2 volts.
1.3.7
Adjust the trigger level to +4 volts.
1.3.8
Set the trigger slope to trigger on the negative (falling) slope.

1.3.9
(Trigger Menu > Slope > Falling Edge )
Adjust the trigger voltage to +1 volts.

Capture this output to disk.
1.3.10 Adjust the trigger voltage to +2 volts.
1.3.11 Adjust the trigger voltage to +4 volts.
1.3.12 Have the lab instructor observe this result.
Lab Instructor _____________________________________
2. Frequency Measurement.
2.1 Set the function generator to 150 Hz sinusoidal wave.
2.2 Using the oscilloscope, manually measure the period and record in Table 1.
Using this period data, calculate the frequency and record in Table 1.

Adjust the horizontal scale to obtain approximately one period on the
oscilloscope screen.

Count the number of divisions (vertical graticules) for one period.

Multiply this count by the number of seconds per division. Number
sec/div is given as M__#__ on the screen. This product is the period.
Record in Table 1.

Your frequency is 1/T. Record in Table 1.
2.3 Use the oscilloscope to automatically measure period and frequency.

(Ch 1 > Measure > Select Measurements for Ch 1 > Period)

(Ch 1 > Measure > Select Measurements for Ch 1 > Frequency)
Note: You may need to hit “-more-” if these options are not shown on
the right hand side of the display.
2.3.1
Record the automatic measurements shown on the screen in Table 1.
2.3.2
Save the output to disk.
2.3.3
Find the percent error of your period measurement versus the automatic
measurement and record in Table 1.
2.4 Repeat steps 2.2 and 2.3 for frequencies 400 Hz, 900 Hz, and 2400 Hz.
3. Frequency Response
3.1 Theoretical
3.1.1
Calculate the phase angle and magnitude ratio for each of the frequencies in
Table 2. Record your results in Table 2.
3.2 Measurement Setup:
3.2.1
Connect the circuit in Figure 1. Adjust the function generator for a sinusoidal
waveform with a magnitude of about 3 volts peak to neutral.
Figure 1: Simple RC Circuit
3.2.2
Display both channel 1 and channel 2.
3.2.3
Set triggering to channel 1 50%.
3.2.4
Ground both channels to zero divisions (center graticule line).
3.2.5
Set both channels to DC.
3.3 Show Vc: Vc is the difference between channel 1 and channel 2. Vc = Ch 1 –
Ch 2. To show this waveform we need to use the math operators.
3.3.1
Hit the (Math) button
3.3.2
Set 1st source to channel 1.
3.3.3
Set 2nd source to channel 2.
3.3.4
Set operator to minus. Now math operation is shown as red on the
oscilloscope screen and can be treated like channel 1 and channel 2
waveforms.
3.4 Magnitude Measurements: For each frequency in Table 3 measure Vs, Vr, and
Vc.
3.4.1
Manually measure the magnitudes of the voltages for 200 Hz and record in
Table 3.
3.4.2
3.4.3
Use the automatic measurements to measure magnitude.

(Measure > Ch 1 > Max for channel 1)

Repeat for Ch 2 and Math
Record the values shown on the screen to Table 4. (Save display to disk for
200 Hz.)
3.4.4
Repeat automatic measurements for each frequency.
3.4.5
Have the lab instructor verify your technique
Lab Instructor _____________________________________
3.5 Phase Measurement: For each frequency you will measure the phase angle
differences Vr-Vs and Vc-Vs.
3.5.1
Manual Measurement for 200 Hz. Before using the automatic phase
measurements you must learn how to manually measure phase difference.

Find the number of degrees per division

Find the number of divisions between the same points of the two
waveforms.

Phase difference is (deg/div )* (# of divisions).

For the sign, remember the leading waveform peaks first. (Leading wave
– trailing wave is positive sign; otherwise, negative).

Record measurements in Table 3
3.5.1.1 Have the lab instructor verify your technique
Lab Instructor _____________________________________
3.5.2
3.5.3
Automatic phase measurements using oscilloscope phase measurement

Press button of waveform to measure to (Ch 1, Ch 2, or Math).

Select Phase measurement from measurement menu.

Press button of waveform to measure from (Ch 1, Ch 2, or Math).

Select “OK Create Measurement” to show phase angle differences.
Repeat automatic measurement for each frequency in Table 4. (Save display
to disk for 200 Hz.)
Conclusions
Remember to save display to disk for every section that requires it and include
screen shots in your lab report.
Sample Calculations
None Required
TABLE 1
PERIOD MEASUREMENT AND
FREQUENCY CALCULATION
Automatic Scope
Measurements
Manual Measurements
Generator
Frequency
Hz
Calculated
Frequency
Hz
Period
msec
Period
msec
Frequency
Hz
% Error
Error in
period
measurement
150
400
900
2400
TABLE 2
RC CIRCUIT FREQUENCY RESPONSE
(THEORETICAL)
Frequency
Hz
f
200
350
500
700
1000
1500
2000
Magnitude
Ratio
Vr/Vs
Vc/Vs
Phase angle
Degrees
Vr-Vs
Vc-Vs
TABLE 3
RC CIRCUIT FREQUENCY RESPONSE
(MANUAL DATA)
Frequency
Hz
f
200
Magnitude
Vs
Volts
Vr
Vc
ratio
Vr/Vs Vc/Vs
Phase angle
Degrees
Vr-Vs
Vc-Vs
TABLE 4
RC CIRCUIT FREQUENCY RESPONSE
(AUTOMATIC MEASUREMENT DATA)
Frequency
Hz
f
200
350
500
700
1000
1500
2000
Magnitude
Vs
Volts
Vr
Vc
ratio
Vr/Vs Vc/Vs
Phase angle
Degrees
Vr-Vs
Vc-Vs