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DC AC Introduction Q3 2007 - Laboratory Experiments (Lab 7)
CAMOSUN COLLEGE
ELECTRONICS DEPARTMENT
DC / AC Introduction - LAB 7
Introduction to AC Measurements
OBJECTIVE
1.
To explore the operation of three pieces of lab equipment that are typically associated with
AC measurement: the oscilloscope, function generator and digital multimeter.
2.
To measure the voltage, frequency and period of an AC signal with an oscilloscope.
3.
To become familiar with the relationship between a waveform’s peak-to-peak value and its
Root-Mean-Square value. To measure the RMS voltage with digital multimeter.
BACKGROUND
Definitions:
 Waveform: The path traced by a quantity, such as voltage or current, plotted as a function of
some variable such as time, position, degrees, radians, temperature and so on.
 Instantaneous value v( t ): The voltage magnitude of a waveform at any instant of time.
 Peak amplitude ( Vp ) : The maximum voltage value of a waveform as measured from its
average value, or mean value.
 Peak value: The maximum instantaneous value of a function as measured from the zero level.
 Peak-to-peak value ( Vp-p ): The full voltage difference between positive and negative peaks
of the waveform.
 Periodic waveform: A wave that continually repeats itself after a certain time interval.
 Period ( T ): The time interval between successive repetitions of a periodic waveform, as long
as the same successive points of the periodic waveform are used in determining T.
 Cycle: The portion of a waveform contained in one period of time.
 Frequency ( f ): The number of cycles that occur in one second. (Hz)
 Average value ( VDC ) : The algebraic sum of all the areas under the curve of the waveform
taken for one cycle divided by the period of the waveform. The average value of any current or
voltage is the value indicated on a DC meter. In other words, over a complete cycle, the average
value is the equivalent DC value.
 Effective value ( Vrms ): The effective value of AC is also called root-mean-square (RMS)
value. The effective value, or RMS value, of any AC current or AC voltage is the value indicated
on an AC meter. The RMS value is also known as the “heating value”, since it produces the
same power dissipation as the equivalent DC value.
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DC AC Introduction Q3 2007 - Laboratory Experiments (Lab 7)
AC Measurements:
In general, when an oscilloscope is used, the peak-to-peak value is displayed. Many DSOs have
auxiliary digital readouts in peak, rms and average however. When an analog ( or digital )
multimeter is used, the RMS (root-mean-square) value is typically measured.
The relationship between peak-to-peak value ( Vp-p ), peak value (Vp or Vm) and root-mean-square
(Vrms) of three waveforms (sine, triangular, square) are listed below:
Sine Waveform:
Sine:
Vp-p = 2 Vp = 2 2 Vrms
or
Triangle: Vp-p = 2 Vp = 2 3 Vrms
or
Square:
or
Vrms 
Vm
Vrms 
Vm
Triangular Waveform:
Square Waveform:
Vp-p = 2 Vp = 2Vrms
2

Vp

Vp
2


Vpp
2 2
Vpp
3
3 2 3
Vm V p Vpp
Vrms 


2
2
2
LAB EQUIPMENT AND MATERIALS
1
1
1
1
1
1
Function generator
Oscilloscope
Digital multimeter
Breadboard
Resistor 10 
Resistor 1 k
Wires, leads and aligator clips
HP 33120A or equivalent
HP 54645A or equivalent
HP 34401A or equivalent
PROCEDURE
Part A. Watch a video tape about the oscilloscope ( optional)
1.
Watch a video tape about the oscilloscope if available on the day of the lab. (In library)
“ The Oscilloscope and Its Use (Lesson 3), BASIC AC CIRCUITS SERIES, TK-454-B262-No.3 c.1 ” This
video is a good introduction to what an oscilloscope is, and what it can do for us. (Length 29 minutes) There
are several such videos available for those that wish further information on CRO and DSO applications.
2.
Your instructor will discuss any variation in operation between the one on the video and the
one used in the lab.
3.
Refer to the background handout on the HP 54645A oscilloscope ( or equivalent ), or
equipment manuals if necessary.
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DC AC Introduction Q3 2007 - Laboratory Experiments (Lab 7)
Part B.
1.
Connect an oscilloscope probe to the oscilloscope.
2.
Set the horizontal time base control to 5 ms/div.
3.
Adjust the zero reference to be in the center of the screen (Horizontal line).
4.
Touch the probe tip with your fingers, and adjust the voltage/division control until a rough
looking sine wave is observed. Sometimes it helps to set the probe to X10 attenuation.
5.
Measure the period of the waveform by counting divisions. (You may use the “SINGLE”
storage button (at top right) on the scope to freeze the waveform on the screen first. Then
perform the measurement. On completion, switch back to the “RUN” position Only for HP)
T (period)
6.
Calculate the frequency of the waveform.
f
7.
= _________ (division) x 5 (ms/div) = _____________
(frequency)
=
1
T ( period )
Hz
= _____________________________
Attempt to explain where this waveform comes from.
Part C.
1.
Connect channel 1 (Ch1) of the oscilloscope to the function generator, as shown in
Figure 7-1.
Ch1
Figure 7-1
F.Generator
(HP33120A)
(Or Equivalent
+
GROUND
2.
Set the function generator’s Output Termination mode to HIGH-Z . (Press the following
keys: SHIFT, MENU,    (three right arrows) ,    (two down, one right arrow),
ENTER) Again take note: This is only for the HP equipment, ignore if other is used
3.
Set up waveforms according to Table 7-1. For each waveform, measure the period (T) and
the peak-peak voltage. Then calculate the frequencies (f = 1/T). Fill them in Table 7-1.
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DC AC Introduction Q3 2007 - Laboratory Experiments (Lab 7)
Table 7-1. The relationship between period and frequency
WAVEFORM
4.
FREQUENCY
VOLTAGE
(F.
GENERATOR)
(F. GENERATOR)
SQUARE
1 kHz
5 VP-P
TRIANGULAR
5 kHz
10 VP-P
SINE
20 kHz
12 VP-P
Vpeak-peak
(on the Scope)
FREQUENCY
(f)
Use the digital multimeter (DMM) to measure the values of R1 and R2.
R1 = ______________
5.
PERIOD
(T)
R2 = __________________
Now connect the circuit shown in Figure 7-2.
Ch1
Figure 7-2
6.
F.Generator
(HP33120A
)or equivalent
+ R1
1 kΩ
Ch2
-
R2
10 Ω
Gnd
Connect the oscilloscope probes to the points shown in Figure 7-2.
(The display on Ch1 will give the input voltage of the circuit. The display on Ch2 will give
the voltage across the 10 ohms “current sensing” resistor. If we divide this voltage (VR2) by
the resistance (R2) then we can calculate the magnitude of the current waveform in this
circuit. NOTE: It is important that you connect the ground leads of both probes to the
same reference point “Gnd” shown in the circuit diagram.)
7.
Set the output of the function generator according to Table 7-2 on the following page.
8.
Measure the peak-peak voltages on both channels. Ch1 gives the input voltage and Ch2 gives
the voltage across R2. Record them in Table 7-2 on the following page.
9.
According to the peak-peak voltage on Ch1, calculate relevant RMS voltage for each
waveform. Record these values in Table 7-2.
10.
Use a digital multimeter (DMM) to measure the RMS voltages across each channel (Ch1 and
Ch2). Record these values as well in Table 7-2.
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DC AC Introduction Q3 2007 - Laboratory Experiments (Lab 7)
TABLE 7-2
WAVEFORM
FREQUENCY
SQUARE
SINE
TRIANGULAR
(F. Generator)
5 kHz
5 kHz
5 kHz
SOURCE
VOLTAGE
10 Vp-p
10 Vp-p
10 Vp-p
(F. Generator)
Vp-p(Ch1)
(with
the scope)
Vrms(Ch1)
(calculated)
(see formulas on page 2)
Vrms(Ch1)
(with DMM)
Vp-p(Ch2)
(with the scope)
ITp-p
( ITpp 
VR 2 pp
)
R2
Vpp (Ch1)
Vpp (Ch 2 )
11.
Based on the data in Table 7-2, do the RMS voltages measured with the DMM agree with
the calculated RMS values from peak-peak voltages? Give the % difference.
12.
Based on the data in Table 7-2, do R1 and R2 constitute an AC voltage divider, i.e.
Vpp(CH 2 )
R2
does
?

Vpp(CH 1 ) R1  R 2
13.
Discuss why the input voltage Vp-p(Ch1) of the circuit would typically be smaller than Vs
(10 Vp-p) provided by the function generator? Would frequency affect this voltage?
]
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