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ECE 291- LABORATORY V
AC MEASUREMENTS; AMPLITUDE AND PHASE
OBJECTIVES
Understanding reactance in electrical circuits and measurements
of the amplitude and phase differences of
two signals using a digital oscilloscope.
PRELAB
1.
Calculate current and voltage on all components of the
circuit shown below for
VS=1V,
f = 10 kHz, R = 1 kohms, C = 10 nF and L = 50 mH. It is best to calculate first the current and then
the voltage across each
element (current * impedance,
using complex numbers). Calculate also the
phase differences ÐVL
- ÐVR
and
ÐVs
- ÐVR
2. What are the phase differences between sinusoidal a.c.
voltage and current on a resistor, a
capacitor, an inductor, and on a power source supplying a circuit consisting of these three components connected in series (as
in Fig. 6, below)?
You may use PSPICE to find the answers.
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Fig. 6 Series RLC circuit
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LABORATORY
Equipment needed from the stockroom:
Oscilloscope
manual, parts kit, proto-board, AVM, leads.
Assemble a circuit consisting of an inductor L, a capacitor C
and a resistor R, connected in series, as
shown in Fig. 6. The voltage source Vs is a waveform generator supplying a sinewave
signal. Select C close to 10 nF, R close to 10 kW
and
use the 50 mH coil from your parts kit.
Measure the values of these components before assembly. The bridge for measurements
of L and C is to be sign out from the stock room by the instructor.
1. Set Frequency
Set the frequency of the generator to about 10 kHz.
Particular value of the frequency is not
critical, provided that you know what it is. Measure it precisely. Using an AVM, measure voltages across the three circuit
elements (R, L, and C) as well as across the
voltage source (waveform generator).. Note that you could not do these measurements
with an oscilloscope. The voltmeter works here because none of its terminals
is grounded. Note also that the relatively low frequency used in these measurements
is within the operating range of this instrument. Do
the measured voltage values agree
with Kirchoff’s voltage law? Explain!
2. Compare
Voltages
Compare voltages VS
and VR
by using two channels of the digital
oscilloscope, with probes attached between
points 0 - 1 and 0 - 3. Measure amplitudes and the phase difference
in degrees (use the digital scope cursors for phase measurements). After measurements
at about 10 kHz, repeat for another frequency (for example 5 kHz).
Note that your measurements determine the phase difference
between the source voltage VS,
and current, which is proportional to VR. (Here the bold letters of voltage
symbols indicate complex quantities, which give
information on both amplitudes and phases)
3. Measure Phase
Measure the phase difference between the inductor voltage and
current. Use two probes: one for voltage
proportional to current (points 0 and 1), the other for voltage across
both R and L (points 0 and 2). Using a convenient feature of the digital scope you can now subtract the two signals and view
simultaneously the voltage across L and
across R, and determine their phase difference.
REPORT
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