PHYSC 3322
Experiment 1.1
17 February, 2016
Test Instruments
Purpose
In this experiment, you will learn how to use some common test instruments, which
you will encounter in a research laboratory. These instruments include oscilloscope,
digital multimeter, frequency counter, function generator, LCR meter, and power
supplies.
Equipment
HP 54603B oscilloscope, Ebclo LCR meter, HP 3311A function generator, Fluke 29
multimeter, HP 34401A multimeter, Tektronix DC504 counter/timer, and various
power supplies.
Background
Oscilloscope
The oscilloscope is one of the most used test and measurement tools in laboratory. It is
really a type of analog voltmeter with an arbitrary zero. It can read DC voltages as an
offset voltage as well as AC voltages by displaying the true waveform. Most modern
oscilloscopes are capable of measuring AC signals over a wide range of frequencies and
you should check your oscilloscope for its frequency response. The key component of
the oscilloscope is a cathode ray tube shown in Fig. 1. Here a beam of electrons is
ejected from the "electron gun" and is directed to the center of the screen. The screen is
covered with a luminescent material emitting light where the electron beam strikes it.
The light emission lasts a fraction of a second after the impact (an effect known as
"afterglow"). The oscilloscope is equipped with a vertical pair of plates for deflecting
the electron beam in the horizontal direction, and with a horizontal pair for deflecting in
the vertical direction. When an input signal to be measured is applied to the horizontal
plates, the vertical distance that the actual beam spot moves from the center of the
screen becomes proportional to the applied voltage V. The vertical plates are often
connected to a known time varying voltage. An example is the ‘’saw-tooth” signal,
which linearly increases with time up to a set limit and then very quickly drops back to
zero. In this way, one can display the waveform of the input signal as a function of
time.
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PHYSC 3322
Experiment 1.1
17 February, 2016
Function Generator
This device produces sine, square, and triangle waves plus a few others that you will
not need for this experiment. The frequency range varies from less than 1 Hz to many
MHz. You will use this generator to provide an AC signal to your circuits and the
oscilloscope. This will enable you to study different waveforms and to get an
understanding of some basic AC signals. There are several function generators
available in the laboratory. Some of them have a synchronous pulse or square wave
output useful for triggering purpose and a voltage-controlled oscillator VCO, which
allows one to vary the frequency by applying a controlled-voltage to a connector on the
instrument.
Digital Multimeter (DMM)
Digital multimeter is a versatile instrument used for measuring voltage, current, and
resistance as well as the circuit continuity. The instrument has very high input
impedance so that it is virtually not seen by the circuit under test. That is, it does not
impose any significant load on the circuit. There are several digital multimeters
available in the laboratory. They differ in the sensitivity and accuracy for the voltage,
current, and resistance measurements.
Frequency Counter/Timer
Frequency counter/timer is used for measuring and analyzing frequency, phase, and
time-interval signal characteristics. If you are dealing with a source of unknown
frequency, it is wise to check the frequency from the counter against a crude
measurement of the period of the waveform on the oscilloscope. The frequency counter
DC 504 used in the laboratory is easy to use and it works reliably if it is triggering
properly. If a frequency counter is not triggering correctly, the reading will be very
wrong (e.g., the frequency is off by a factor of exactly two) and unstable.
LRC Meter
This instrument is essentially an impedance bridge, which can be used to measure the
values of reactive components, such as a capacitor and an inductor. It can also be used
for the resistance measurements, but they are more conveniently done by a digital
multimeter. The LRC meter is used to measure the actual (as opposed to nominal)
value of fixed capacitors or inductors before inserting them into a circuit.
Procedure
Listed below are a set of laboratory exercises to be carried out in this experiment. You
are expected to keep detailed notes of these exercises in you lab notebook.
Exercise 1. In this part of the experiment you will use the function generator directly
as a source for the oscilloscope. Connect the function generator to the vertical input of
the scope and use the time base to reproduce the sine or square waves generated by the
source. Use three different frequencies, 1 kHz, 10 kHz and 1 MHz, for both the sine and
square wave functions. Sketch what you see on the screen and indicate under each
sketch the settings for the scope and the generator. This is critical because you want
quantitative information from these traces. To display the waveform properly, you
need to set the Triggering Mode to Auto. You may also need to adjust Vertical
Amplitude (volt/div), Vertical Position, Time Base (time/div), and Horizontal Position.
Measure the frequency of each signal from the oscilloscope trace and compare this
value to that indicated on the function generator. You can read the frequency more
accurately by using a frequency counter. You can also make frequency measurements
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PHYSC 3322
Experiment 1.1
17 February, 2016
automatically using the Time key on the scope. Compare your results obtained with
different methods.
Exercise 2. Next, we study the triggering mode on the scope. Select the triangle
waveform from the source, and adjust the peak voltage to obtain a convenient
waveform. Select Source key and toggle each of the soft keys and notice the changes
each key makes and explain why. Select Mode key and toggle each of the soft keys.
Vary the Triggering Level and watch the changes on the screen and on the status line.
What happens if the scope loses trigger? Select Slope/Coupling key and note the effect
of switching the Triggering Mode from AC to DC by varying the DC offset on the
function generator. Set the Triggering Mode to DC and note the effect of varying the
Vertical Position control. Do you see the same effect if the Triggering Mode is set to
AC? Why? What happens when you change the Triggering Slope from Positive to
Negative? Do you know why?
Exercise 3. We now measure the following time parameters with the oscilloscope:
frequency, period, duty cycle, width, rise time, and fall time. You may read the
instructions for the scope to find the definitions of these parameters. Set the function
generator to about 1kHz (square wave) and connect its output to the input of the scope.
From the scope trace, measure the frequency, period, duty cycle, width, rise time, and
fall time of the signal. Record the values in your notebook. How does your results
compare with those obtained automatically using the Time key on the scope. Vary the
duty cycle (dead time) of the square wave using the control on the function generator.
From the scope trace, verify that the duty cycle is indeed varied and sketch several
examples of different duty cycles. Connect the square wave signal to a simple RC
circuit shown below with R=470 ohm and C= 0.1 F. Measure the voltage across the
capacitor and compare the rise time with the time constant RC of the circuit. Explain
your results.
Exercise 4. In this part of the experiment you will use two methods to make frequency
and phase difference measurements. The first method uses the scope's dual trace
capability to measure the phase difference and the second treats the scope as an X-Y
scope. A simple method to observe a signal, which is shifted in phase from another, is
to use a RC circuit. You have learned that a capacitor has an unusual voltage charging
effect. This is a result of the voltage across the capacitor "bucking" the input voltage
with the result that the capacitor takes time to charge. For an AC signal, this translates
into a phase shift of the signal through a capacitor compared to that through a resistor.
Therefore, by comparing a signal across the resistor and a signal across the capacitor in
a RC circuit we can see the phase difference. Set-up a simple RC circuit shown below
with R=100 ohm and C=10 F. The function generator is used to deliver a sine wave of
1 kHz. The oscilloscope is to be connected so that Ch1 measures the voltage across the
function generator and Ch2 measures the voltage across the capacitor. The math
function Ch1-Ch2 on the scope gives the voltage across the resistor.
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PHYSC 3322
Experiment 1.1
17 February, 2016
The first method uses the dual channel capability of the scope to compare the two signals
and thus to determine the phase shift. You should observe the dual trace for these sine
waves and then be able to measure the phase difference between the two signals by
measuring the time delay between the leading and lagging peaks of the two signals.
Repeat the experiment using 100 kHz. What happens to the phases of the two signals?
The second method uses the concept of the Lissajous figure to measure the phase shift.
Apply one signal to the horizontal input and the other to the vertical input. You should
see a circle if the two frequencies are equal and the phase delay is zero. As shown below,
the circle will be canted if the phase on the two sine waves is different. You should
measure A and B and use the equation sin =B/A to calculate the phase shift . Compare
the result with the measurement using the dual trace method. Which method is more
reliable?
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