PHYS 3322 Modern Laboratory Methods I
Test Instruments
Purpose
In this experiment, you will learn how to use some common test instruments that will be
encountered in a research laboratory. These instruments include the oscilloscope, the LCR
digital multimeter (DMM), the frequency counter, and the function generator.
Equipment
•
HP 54603B oscilloscope
• Resistors
•
•
HP 3311A function generator
o 100 Ω
o 0.1 µF
•
Tektronix DC504 counter/timer
o 470 Ω
o 10 µF
Capacitors
• LCR DMM
Background
THE OSCILLOSCOPE
electron gun
vertical
amplifier
vertical
input
horizontal
amplifier
sawtooth
voltage
generator
horizontal
input
Figure 1. A diagram of an 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 like that
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 that emits light
where an electron strikes it, this 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 of plates for deflecting
the electron beam in the vertical direction. When an input signal to be measured is applied to the
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Test Instruments
vertical input, the vertical distance that the actual beam spot moves from the center of the screen
is proportional to the applied voltage V. The saw tooth voltage generator outputs a periodic
voltage signal that linearly increases with time and very quickly drops back to zero. The period
of the saw tooth signal is adjustable and provides a convenient method to deflect the electrons
horizontally and may be synchronized with the vertical input thus displaying the input waveform
as a function of time. The horizontal input of the oscilloscope may also be used to operate the
scope in the “xy” mode.
The 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.
The digital multimeter (DMM)
The digital multimeter is a versatile instrument that is used for measuring voltage, current, and
resistance. This instrument has very high input impedance so that the circuit under test does not
impose a significant load on the circuit; it is virtually unseen by the circuit under test. There are
several digital multimeters available in the laboratory and each differs in the sensitivity and
accuracy for the voltage, current, and resistance measurements.
The 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 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
capacitance of a capacitor and the inductance of an inductor. This instrument measures the
actual (as opposed to nominal) value of fixed capacitors or inductors before inserting them into a
circuit. It can also be used for resistance measurements.
Procedure
Listed below is a set of laboratory exercises to be carried out in this experiment. You are
expected to keep detailed notes of these exercises in the lab notebook.
Exercise 1. In this part of the experiment the function generator will be used as a source for the
oscilloscope and the frequency counter. Place the amplitude knob of the function generator at
one-fourth of its full range. Connect a BNC tee to the MAIN OUTPUT of the function
generator. Connect CHANNEL 1 of the oscilloscope and the frequency counter input to the tee.
Set the function generator to output a sine wave at a frequency of 1 kHz as shown by the
function generator and not the frequency counter. [Method 1 of measuring frequency] Press
[Auto scale] on the oscilloscope. The o-scope screen should now display the sine wave output of
the function generator. Note that pressing [Autoscale] on the o-scope should automatically
display the scope input provided the amplitude is high enough to trigger it. Now, adjust the
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Test Instruments
amplitude of the wave using the knob on the function generator, vary the frequency, and change
the output of the function generator to sine, square, triangular, etc., waves to get an idea on the
operation of the function generator. When finished, set the function generator to output a sine
wave at a frequency of 1 kHz.
The vertical position as well as the horizontal position of the displayed signal can be adjusted
using knobs located on the front panel. Find and adjust these knobs to learn their effects on the
displayed signal. You should note that these position adjustments do not change the shape of the
displayed signal but allow the user to place the displayed signal at a convenient location on the
screen within limits.
Next, press [Auto scale] and adjust the Volts/Div (volts per division) of channel 1 to several
different values and note the effects this has on the displayed signal. The setting of this
adjustment can be seen at the top of the display. This adjustment only affects the vertical
dimension of the displayed signal. Counting the number of vertical divisions between a peak
and a trough of the displayed signal and multiplying by the Volts/Div setting yields the peak-topeak amplitude of the signal in units of volts. Any voltage amplitude of the displayed signal can
be determined in this fashion; counting the number of vertical divisions and multiplying by the
Volts/Div setting.
Next, press [Auto scale] and adjust the Time/Div (time per division) knob to several different
values and note the effects this has on the displayed signal. The setting of this adjustment can be
seen at the top of the display. Adjusting the time base only affects the horizontal dimension of
the displayed signal, which in this case is time. Counting the number of divisions it takes to
complete a full cycle and multiplying by the time per division setting will determine the period
of the signal. Any time elapse of the displayed signal can be determined in this fashion;
counting the number of horizontal divisions and multiplying by the Time/Div setting.
Using the [Auto scale] function of the oscilloscope is the easiest and quickest method to display
a signal on the o-scope. This function does have its drawbacks when both CHANNEL 1 and
CHANNEL 2 inputs are being used to display different input signals. Note that instead of using
the [Auto scale] function, manual adjustment of Volts/Div and Time/Div for a particular channel
can also be used to conveniently display an input signal.
One method of determining the frequency of a sinusoidal input signal to the o-scope is to use the
method described above; counting the number of divisions it takes to complete a full cycle and
multiplying by the Time/Div setting. This yields the period T of the signal from which the
frequency f can be determined since, f=1/T. [Method 2 of measuring frequency] Another
method is to use the [Cursors] menu and utilize the time cursors. Press the [Cursors] button and
notice on the display that vertical bars appear across the screen and a menu appears at the bottom
of the screen. One item on the menu is the [t1t2] button. Pressing the [t1t2] button sequentially
highlights t1 or t2 or both t1 and t2. Now turn the knob located near the [Cursors] button and
note the effects it has on the vertical bars on the screen when t1, t2 and both t1and t2 are
highlighted. Also note that the time position of each vertical bar is displayed on the screen as
well as the time difference between t1 and t2 (∆t), and the inverse of this time difference (1/∆t).
Placing t1 and t2 at conveniently locations on the display will yield the period and frequency of
the input signal on the display. [Method 3 of measuring frequency]
And yet a third method can be used to determine the frequency an input signal utilizes the builtin circuitry of the o-scope. Press [Auto scale], and then press the [time] button. Pressing [time]
causes a different menu to appear at the bottom of the screen. One of the menu buttons is [Freq]
and another is [Period]. Press each and numerical values for both the frequency and period will
appear on the display as well as vertical bars denoting time cursors on the display. [Method 4 of
measuring frequency]
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Test Instruments
A fourth method of measuring the output frequency of the function generator is to use the
frequency counter. [Method 5 of measuring frequency] Familiarize yourself with the
capabilities of the frequency counter.
For this exercise you will measure the measure the output frequency of the function generator
using the first four different methods describe above. As a control we will assume the frequency
counter measures the most accurate frequency, the fifth method of measuring frequency.
Unhook the frequency counter from the experimental setup. Set the function generator to output
a sine wave at 100 Hz using only the dial on the function generator to do this. We want to
get an ideal on what frequency you think the function generator is outputting [Method 1] and
what you measure on the o-scope [Methods 2-3] before actually measuring the frequency on the
frequency counter [Method 4, which we are taking to be exact]. Next used methods 2-3 to
measure the frequency. Finally hook the function generator to the frequency counter to measure
the actual frequency and determine any errors. Repeat the above for the frequencies 350 Hz and
625 kHz.
Exercise 2. Next, we study the triggering mode on the scope. Select the triangle waveform on
the function generator, 1 kHz, and set the amplitude to about mid-range of full scale. Adjust the
o-scope to obtain the waveform on the display. On the trigger menu of the o-scope press
[Source], then toggle each of the soft keys that appear as a display menu. Notice the changes
pressing each key makes and explain.
Press [Auto scale], then press the [Mode] key and toggle each of the soft keys on the display
menu. Press [Auto scale], then vary the triggering Level, the turn knob next to the [Source]
button, for each [Auto Lvl], [Auto], [Normal], and [Single] mode in turn. Watch the changes on
the screen and on the status line. What happens if the scope loses trigger?
Press [Auto scale], then press [Slope/Coupling] key and note the effect of switching the
[Coupling] mode from AC to DC by varying the DC offset on the function generator. Set the
[Coupling] to DC and note the effect of varying the Vertical Position control. Do you see the
same effect if the [Coupling] is set to AC? Why? What happens when you change the [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. 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 do 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.
Using the LCR meter measure the exact value of the 0.1 µF capacitor. Using both the DMM and
LCR meter the exact values of the 470 Ω resistor. Which meter would you say is more accurate?
Connect the square wave signal to a simple RC circuit shown below with R=470 Ω and C= 0.1
µF. Measure the voltage across the capacitor and compare the rise time with the time constant
RC of the circuit. Explain your results.
function
generator VIN
ch 2 – VIN
ch 1 – VOUT
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Test Instruments
Exercise 4. In this part of the experiment you will use three methods to make phase difference
measurements. The first method uses the scope's dual trace capability to measure the phase
difference, the second uses the internal circuitry of the o-scope, and the third utilizes the X-Y
capabilities of the o-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 the voltage across the capacitor is phase
shifted with respect to the applied or driving voltage. Therefore, by comparing the driving
voltage to the voltage across the capacitor in a RC circuit we can determine the phase difference
between the two. Set-up a simple RC circuit shown below with R=100 Ω and C=10 µF. The
function generator is used to deliver a sine wave of 140 Hz. The oscilloscope is to be connected
so that Ch1 measures the voltage across the capacitor and Ch2 measures the voltage across the
function generator.
Display both traces on the screen and measure the phase difference between the two traces off
the screen. Next press [Time], then press the [Next Menu] button twice, then [Phase]. This
displays the phase difference between the two signals. Repeat the experiment using 500 Hz.
What happens to the phase difference between the two signals when the frequency in increased?
The third method uses the concept of the Lissajous figure to measure the phase shift. Press the
[Main/Delayed] button on the front panel then press [XY] on the menu that appears on the
display menu. You should see a trace on the o-scope that looks similar to any one of those
shown below. The trace will be canted if the phase on the two sine waves is different. You
should measure A and B and use the equation sinφ =B/A to calculate the phase shift φ. Compare
the result with the measurements obtained above.
B
A
sin ϕ = B/A
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