1 Oscilloscope Measurements
This experiment is designed to familiarize you with the oscilloscope and its use for
AC and DC measurements.
1.1 Apparatus
Kikusui oscilloscope, signal generator, humming box, rectier circuit, digital multimeter,
DC power supply,
step down transformer with variable and xed output (black unit), speaker, microphone (shared).
1.2 Introduction and control locations
An analog oscilloscope makes use of cathode ray tube in which a focused beam of electrons strikes the front
of the screen producing light. In contrast, a digital oscilloscope uses an analog to digital converter to sample
the test point's voltage and then stores the voltage values in digital form.
This data is then read out of
memory and displayed. Both scopes use the same basic controls since they are displaying a graph of voltage
as a function of time.
The advantage of an oscilloscope over a multimeter is that the voltages being tested can be functions of
time. Both repeating and non-repeating components may be captured, displayed and measured. Multimeters
have to be designed with certain assumptions and limitations and only give general information about a
voltage variation with time.
The basic controls are
•
the time-base control which sets the scale of the horizontal axis in
seconds/cm,
•
the two channel controls which set the scale of the vertical axis in
volts/cm,
•
the coupling switch for each channel, which selects what components of the waveform are displayed.
•
Lastly, trigger control can often be used in auto mode to stabilized the trace on the scopes display, but
in some cases manual adjustment is necessary.
These controls are shown in gures 1.1 , 1.2 and 1.3 for two models of oscilloscope.
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1 Oscilloscope Measurements
Figure 1.1: Kikusui COS5020 controls
Figure 1.2: Kikusui COS5021 controls
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1 Oscilloscope Measurements
Figure 1.3: Kikusui COS502x control legend
** Locate
the following controls: on/o switch, intensity, focus, horizontal positioning, time/div (time-
base), vertical position (ch1 and ch2), volt/div for ch1 and ch2 (vertical gain), coupling switches for ch1 and
ch2 (AC, GND,
DC ).
1.3 General Advice
1. The intensity control should be set to a level just bright enough for making measurements. Excessive
brightness can damage the phosphor screen.
2. Use the horizontal and vertical scale controls to spread the waveform out as much as possible when
making measurements. This will reduce your error.
3. The input voltage should be checked for any
DC oset using the coupling switch. This also means
that you must know where zero volts is. Use the coupling switch on the GND position and the vertical
position control to set this.
4. When stating the result of the measurement, make sure you put the correct units down.
Vp , Vpp , Vrms , VDC , s, ms, µs
etc.
1.4 Review of Waveform Parameters
A in V oltspeak , angular frequency ω
radians or degrees. If the oscillation has a nonzero average, then a DC voltage is
by B in V oltsDC . Mathematically, the form of equation 1.1 is used.
The parameters which describe a sinusoidal waveform are its amplitude
in
radians
second and phase
φ
in
present and represented
v(t) = B[VDC ] + Vm [Vp ] · sin(ωt + φ)
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(1.1)
1 Oscilloscope Measurements
These characteristics are measured using the oscilloscope and shown in gure 1.4. Hints on how to best
proceed are listed below.
2V m
0 volts
T
B
center
of
oscillation
φ
0 msec
Figure 1.4: Voltage as a function of time: Sinusoidal waveform.
1. It is simpler and more accurate to measure the waveform's amplitude by measuring the 'peak to peak'
voltage
[Vpp ]
and then divide by two.
2. The angular frequency
ω
is found by measuring the period
T
of the oscillating voltage and then using
equation 1.2.
ω = 2πf =
3. Measuring the phase shift
φ
2π
T
requires knowledge of where time
(1.2)
t=0
or some relevant point of interest
on the horizontal axis.
4. Measuring the
DC oset
B
requires knowledge of where
easily and accurately measured with a multimeter set to
v = 0V
(ground) is located. The
DC oset is
DC volts.
1.5 Vertical and Horizontal Controls
Note: One division (div) is one centimeter on the oscilloscopes display.
The scope has two input channels ch1 and ch2. The rst control to consider is the V/div dial. It allows
setting the vertical scale from
5 mV
div
to
V
5 div
per division or lower depending on the scope.
Also note that
there is a calibration dial located on the center portion of this control or in a nearby location. Use this dial
set fully clockwise, listen for the click. This means the channel is calibrated and voltage measurements will
be accurate. (Some measurements on the horizontal axis benet from taking the vertical axis o calibration,
and vice-versa.)
The other, the time axis, is the horizontal axis. Usually the time axis can be controlled in a range of scales
from 0.5
µs
s
div down to 0.2 div , depending on the bandwidth of the instrument. Again there is a calibration dial
located on the time/div dial. Check that it is in the calibrated position.
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1 Oscilloscope Measurements
Each horizontal and vertical control section has a position dial. These are used to to align parts of the
display waveform with the front grid for improving the ease and accuracy of measurements.
Liberal use of
the position dials and scaling dials is encouraged.
1.6 Coupling Switch
The scope's coupling switch has three positions.
1.
AC coupling: On AC, the scope will display only AC components of the test waveform. This is due to
DC
a capacitor being switched into series with the input line. This capacitor blocks and removes the
AC characteristics of a voltage which has a large DC
component. It is useful when trying to measure
component.
2.
DC coupling: On DC coupling, both AC and DC components are displayed. Nothing is removed from
the input waveform.
3. GND: Setting the coupling switch to GND removes the input voltage and places ground potential on
the input line of the channel. This allows the user to see and set where 0V is located on the screen.
1.7 RMS Voltage
The digital multimeters used in the lab display the rms voltage of a sinusoidal waveform when set to
AC,
which stand for root-mean-square. The general formula for calculating the rms value of a voltage waveform
v(t)
is shown in equation 1.3.
"
Vrms
For a sinusoidal voltage with no
frequency
ω , v(t) = Vm sin(ωt),
1
=
T
ˆ
#1
T
2
2
v(t) dt
(1.3)
0
DC component, amplitude
Vmaximum = Vm
in
V oltspeak = VP
and angular
the relation between the amplitude and the rms voltage can be show to be
equation 1.4.
Vm
Vrms = √ = 0.707 · Vm
2
Similarly the
AC current of
i(t) = Im sin(ωt)
(1.4)
can be quantied using the rms value:
Im
Irms = √ = 0.707 · AI
2
The rms values of voltage and current are known as the eective values. The goal for developing rms values
was to quantify an
AC voltage/current with one value that could be compared to a DC value and produce
the equivalent power through the load. Comparing the power
and an
AC current of
10Arms
P = V ·I
dissipated by a
DC current of
10ADC
through the same load, the heat dissipated by the load is the same amount.
Questions
1. Show that equation 1.4 is correct for the sinusoidal voltage
v(t) = Vm sin(ωt).
radians.
2. What is the advantage of a true rms meter? (Hint: google.com)
3. What is the frequency limits of the Fluke 77 meter for AC measurements?
5
Use a period of
2π
1 Oscilloscope Measurements
1.8 EXPERIMENT: Step-Down Transformer Waveform
Figure 1.5 shows the schematic diagram for the step-down transformer. Two output voltages are available.
These are a xed
∼ 3.3Vrms AC
voltage and a variable voltage output with a maximum of
∼ 7.5Vrms .
120Vrms
6.3Vrms CT
red
red
black
black
Figure 1.5: Step-Down transformer
1. Record the model and make of oscilloscope and DMM you have. The error values for time and voltage
measurements are both
2. Start with
±3%.
AC coupling. Connect the oscilloscope ch1 input to the red terminal which has the xed
voltage coming from the secondary center-tap. Adjust the scope's time/div dial to get between one and
6 wavelengths on the screen.
3. Measure the peak to peak voltage
[Vpp ]
of the waveform. Calculate the amplitude in volts peak
[Vp ].
4. Use the digital multimeter (DMM) to measure the same output voltage. What are the correct units for
this measurement?
5. Measure the period of the output waveform. Calculate the frequency of the waveform.
6. Check for and record the
DC oset using the oscilloscope and the DMM.
7. Measure and record the maximum and minimum amplitude of the voltage provided at the variable
output terminals.
Question
1. Show that the scope's peak voltage measurement and the DMM's ac voltage measurement agree.
1.9 EXPERIMENT: Signal Generator Waveforms
1. Connect the output (50Ω if stated on generator) to the channel one input of the scope. Examine the
triangle and square wave outputs. If a speaker is available, listen to the output and how it changes
with waveform shape, amplitude and frequency. Be kind to your ears! :-)
2. Set the output for a sine wave. Remove the speaker from the circuit.
3. Set the signal generators controls so that it produces the voltage waveform shown in equation 1.5. Show
this waveform to your lab instructor.
v(t) = 2.8Vp sin(2π × 7.2kHz · t)
4. Next add a
DC oset of
−1.7VDC
and show the instructor.
6
(1.5)
1 Oscilloscope Measurements
5. Remove the signal generator connection. Obtain the microphone and connect it to channel one. Set
the scope's volts/div control to its most sensitive setting. Speak or whistle into the microphone to see
the aect on the trace.
Question
1. In terms of frequency content, what is the dierence between a sine, a triangle and a square wave of
1kHz ?
1.10 Rectier Circuit
1.10.1 Description
Although electrical power is delivered as alternating current, the majority of electronic circuits utilize
voltages for operation. This requires conversion from
DC
AC to DC. A rectier circuit is one which performs all
or part of this conversion. The circuit provided is shown in gure 1.6.
AC
Source
v ac (t)
v(t)
R
Diode
Figure 1.6: Rectier circuit schematic
Current can only pass through a diode in one direction and the direction of the arrow shows which way
R
conventional current will pass. A voltage will develop across the resistor
when the diode conducts. No
voltage will appear across the resistor when the diode blocks current.
Freezing time at two instances (t1 and
t2 )
during a full cycle of the
AC voltage source helps to understand
the output voltage waveform. Consult gure 1.7. One way of thinking about a diode is to say that it has a
small resistance for current in one direction (a few
100 s
of
Ω),
and a very large resistance (M Ω
in the other direction.
vac (t)
t
0V
t1
2
t
Figure 1.7: Voltage applied to rectier circuit.
7
0 s) for current
1 Oscilloscope Measurements
1. At
t1 .
Conventional current ows from high potential to low.
When the
AC voltage is at its most
positive voltage on the source's upper terminal (gure 1.4), then a maximum current will ow though
the diode. This value of current is determined by Ohm's law using the resistance values of the resistor
and conducting diode.
2. At
t2 .
When the polarity of the
AC voltage source reaches its negative peak, then the diode blocks
current. This is due to the behavior of the junction of two doped semiconductor materials which make
up the diode. Applying a reverse bias to a diode only increases the barrier to current ow, and so the
voltage drop across the resistor becomes zero.
1.10.2 EXPERIMENT: Rectier Circuit
1. Connect the xed
AC output from the step-down transformer to the input terminals of the rectier
circuit. This will produce a circuit similar to gure 1.6. Connect the oscilloscope ch1 to the output
terminals to monitor the voltage dierence present across the resistor.
2. Use
DC coupling on the oscilloscope to measure and record the values shown in gure 1.8.
3. Indicate in your diagram where 0 volts is.
4. Use the DMM to measure the
DC voltage and rms voltage across R. Which measurement is correct
and which one is not correct? Explain why in both cases.
∆V p
∆t 1
∆t 2
Figure 1.8: Rectier waveform measurements
Question
1. Why is
∆t1 6= ∆t2 ?
1.11 Lissajous Figures
Lissajous gures are used to measure phase shift and frequency. They can yield accurate phase measurements
for small phase shifts which are dicult to directly measure. Their advantage in frequency measurement is
that a noisy signal's frequency can be accurately determined.
The oscilloscope can be congured to produce Lissajous gures by positioning all switches the XY designation. This changes the horizontal axis from a time axis to a voltage axis. Channel 2's input becomes the
voltage that sweeps the electron beam horizontally. The vertical channel is channel 1 as usual. We will use
this XY mode to measure the frequency of the signal generator at a few dierent frequencies.
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1 Oscilloscope Measurements
1.11.1 EXPERIMENT: Lissajous Figures
1. Set the scope up for XY mode. Put both ch1 and ch2 to 2V/div. Use
AC coupling on both channels.
Adjustment of channel sensitivity (V/div) will likely be necessary.
2. Connect the xed voltage terminals to the ch1 input. This provides a known 60Hz sine wave input for
the vertical or Y channel. Neither the frequency or amplitude of this input will not change.
3. Turn o the
DC oset of the signal generator if it is on. Set the signal generator to a sine wave output,
and the frequency to approximately 60Hz. We will consider the signal generator's frequency to be the
unknown frequency. Connect it to ch2.
4. Set the amplitude of the signal generator to give a noticeable deection horizontally (X). Once the
amplitude is about equal to the ch1's amplitude, a pattern similar to a rotating ring will be seen. Both
channels are being driven by sinusoidally varying voltages.
5. Adjust the signal generator's frequency to stabilize the ring. It is dicult to hold the pattern still due
to frequency mismatch. Once this pattern is nearly still, you have set the signal generator to a known
frequency of 60Hz. This is a 1:1 ratio, X:Y.
6. Use the frequency dial to go for a 2:1 ratio (X:Y), and then a 3:2 (X:Y) ratio. Draw both patterns for
the report. Explain and calculate how the diagrams indicate the frequency of the signal generator.
1.12 EXPERIMENT: Humming Box
A relay circuit inside the gray metal case uses
DC to produce an AC output voltage. This output is non-
sinusoidal in shape and audible.
1. Set the
DC power supply to provide
2. Connect the supply to the
2.0VDC .
The front panel meters are sucient in accuracy.
DC input terminals of the humming box. Adjustment of the DC voltage
supply may be necessary.
3. Replace the step-down transformer that is connected to the oscilloscope with the output of the humming
box. Adjust the signal generator's amplitude and frequency to get a 1:1 Lissajous pattern on the scope.
4. Measure and record the period of the signal generator's output by returning the scope to voltage versus
time mode. You may need to select ch2 as the trigger source. Use this period to determine the frequency
of the humming box.
5. Examine and record all relevant characteristics of the humming box output voltage waveform. Include
a drawing of the waveform in you report. Also include a measurement of the
DC level.
When you have nished, replace all wires back on the hanger near the sink. Turn o all components used in
the lab.
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