Experiment #1

Experiment #1
Physics 3330
Experiment #1
Fall 2005
Electronic Instruments
Purpose
This experiment introduces the three basic electronic instruments you will use in the course: the
oscilloscope, the function generator, and the digital multimeter. Spend enough time in the lab
this week to become familiar with the instruments. There are no pre-lab homework problems for
this first lab, and you do not have to write a report. In place of these, there are homework
problems on dc and ac circuit theory at the end of this chapter. Turn them in to your instructor
at the start of your second lab period.
Introduction
Any time you develop a new circuit, repair an electronic instrument, or measure the performance
of an electronic component or system, you will need to use one or more items of test equipment.
Almost every task requires an oscilloscope, the basic instrument for visualizing the time
dependence of electronic signals. A signal or function generator is used to produce periodic
signals of the frequency, amplitude, and waveform needed for input to the device under test. The
digital multimeter measures voltages, currents, resistance, and it can test silicon diodes and
transistors. Everything you do in an electronics laboratory depends upon your familiarity with
these instruments.
The instructions for this experiment are designed help you start seeing patterns on the
oscilloscope screen as soon as possible, and to familiarize you with the basic controls of each
instrument. They do not cover all of the capabilities of the instruments. For greater detail,
consult the manufacturer's manuals kept on the book shelf in the lab, or the links on our web site.
Readings
1.
In the first two weeks of the course read all Chapter 1 in Horowitz and Hill. For this first
lab the important sections are 1.01-1.04, 1.07, 1.11, and the first three appendices: The
Oscilloscope, Math Review, and The 5% Resistor Color Code. Before starting the
homework problems, read sections 1.12 and 1.18 (starting on page 29).
Experiment #1
1.1
Fall 2005
Experiment
TEKTRONIX 3014 OSCILLOSCOPE
The Tektronix 3014 oscilloscope is similar to the generic 'scope described in H&H, but is uses
digital technology and menus to provide many additional features. The 3014 has four
independent vertical channels instead of just two, so that four separate signals can be displayed
at once. We also have two separate horizontal sweeps (called A and B) which allow traces with
two different sweep speeds to be displayed at the same time. (The main use for this is to zoom in
and magnify a small part of a trace while still displaying the full trace.) Finally there are built-in
measurement and cursor capabilities that are useful for making measurements on the screen.
There are a few precautions to observe when operating the oscilloscope:
•
•
•
•
Avoid overheating the instrument. Do not block ventilation of the interior by laying books or
clothes on the case. This precaution applies to any instrument.
Do not apply more than 400 V to any input terminal.
Avoid serious or fatal injury from electrical shock. Voltages up to 14 kV occur inside the
unit. Do not remove the cover or insert anything metallic through the vent holes.
On older analog ‘scopes you could burn out the screen by setting the intensity too high. This
is not an issue with digital ‘scopes.
Otherwise, the instrument is robust and cannot be damaged by wrong settings. Don't worry—
turn it on and have fun.
We will begin by setting up the 'scope for basic one-channel operation. First, remove all cables
left by previous users, and turn the 'scope on by depressing the ON/OFF button. The settings are
remembered from the last user so you need to verify the state of the setup; refer to the manual if
you can’t spot the right control. Now set the following using the panel and menu controls as
follows (you may want to use this standard setup in the future if you have trouble):
Display Intensity
WAVEFORM INTENSITY 50%
B TRIG
Off
Cursor Controls
CURSOR Menu
Off option
Intensity of the A sweep.
Turn off B sweep.
Inactivates the digital measurement
system.
Experiment #1
1.2
Fall 2005
Vertical Controls
Horizontal Controls
Trigger Controls
CH 1, then Menu
Select channel 1 to display.
POSITION (CH 1) Midrange
Vertical position for Channel 1
SCALE Knob
0.2 V/div
Note value is displayed on screen.
COUPLING (CH 1) GND
Gives 0 V input for setting baseline
COUPLING
1 MΩ
Input Termination of 1 MΩ
BANDWIDTH
FULL
INVERT
OFF
POSITION Midrange
Horiz. position. (Knob)
SCALE
1 ms/div
Sweep speed. Value is displayed on screen.
MAGNIFY OFF
(Lets you expand the scale)
DELAY
OFF
MENU
B TRIG
MODE
COUPLING
Pick CH 1
OFF
AUTO
DC
Use CH 1 signal for triggering.
Selects A sweep only. (Green light off)
Sweep even when no signal present.
Send CH 1 to trigger with no filtering.
The baseline beam trace should appear as a horizontal straight line. Readjust Waveform
Intensity for desired brightness, move the trace vertically until it coincides with the X-axis using
the POSITION knob. Position the start of the trace so that it is visible at the left hand side of the
screen using the horizontal POSITION control.
You may need to turn CH 2, 3, or 4 off. Press the button CH #, then the off button under the
vertical position knob.
If no yellow trace is visible, check that the trigger MODE is on AUTO. If that fails, depress
AUTO SETUP. If you still don't see a trace, get help from an instructor.
Next we will try to display the signal from the scope’s calibration signal, a 5V p-p (peak-topeak) 1 kHz square wave. This output is labeled PROBE COMP because it can be used to adjust
the ‘compensation’ of oscilloscope probes. Connect the PROBE COMP signal to the CH 1 input
with a short coaxial cable and alligator clip probes. Note that the outside of the coax is always
ground, and the inner conductor carries the signal. The inner conductor is connected to the red
clip, so connect the red clip to the PROBE COMP output. Now set the CH 1 vertical
COUPLING to DC. A signal should appear on screen. Adjust the TRIGGER LEVEL (upper
right corner of panel) to give a stable and stationary waveform. Note the little arrow on the
display which indicates the value of the trigger level. Get some help if you can't see the
waveform.
Experiment #1
1.3
Fall 2005
Explore the effect of the following controls one at a time. Return each control to the original
setting before you change the next.
•
•
•
•
CH 1 VOLTS/DIV, both the calibrated control and FINE ADJUST (variable).
HORIZ SCALE, (sec/div)
CH 1 COUPLING: AC/DC/GND. Observe and understand the change in level.
TRIGGER LEVEL and SLOPE.
The trigger level controls the voltage at which the trace starts. Stability is lost when the trigger
level lies outside range of the displayed voltage. Change TRIGGER MODE to NORMAL. Note
that the trace now “freezes” when the trigger level is misadjusted. You can see whether the scope
is actually being triggered by looking for the small writing TRIG’D or TRIG? at the top of the
display.
Measure the peak-to-peak voltage and the period of the waveform using the screen scale. The
peak-to-peak voltage is the difference between the high and low extremes of the waveform. First
adjust the scale factors to give a large trace—between 50% and 95% of the screen in height and
about two periods horizontally. Finally measure the dc voltages of the lowest and highest part of
the waveform, and measure the time for a complete cycle. What is the frequency of the
waveform? Do the peak-to-peak amplitude and the frequency agree with what you expected?
Now try to get the same trace showing the PROBE COMP signal, but use the CH 2 vertical
input.
AGILENT 33120A FUNCTION GENERATOR
The 33120A provides sine, square, triangle, pulse and ramp waveforms over the frequencies
from 0.0001 Hz to 15 MHz in ten decades. The output amplitude is 50 mV to 10 V peak-to-peak
from an impedance of 50 Ω. The frequency may be controlled manually or swept automatically
between START and STOP frequency settings set in a menu. The symmetry of the waveform
may be varied, enabling square waves to be turned into pulses and triangle waves into ramps.
The output may be modulated either by amplitude modulation or frequency modulation.
Again there are a few precautions to keep in mind:
• Do not cover the perforated outer case—the instrument will overheat.
• Do not connect any output of the 33120A directly to dc power or to the output of any other
instrument. Doing so will burn out the output amplifier.
Experiment #1
1.4
Fall 2005
Except for these precautions the instrument cannot be damaged by incorrect settings.
We will set up the 33120A as a free running oscillator with manual control of frequency. This is
almost always the way we will use it. Remove any cables left by previous users, and turn the
function generator on. There are several methods of setting numbers for amplitude, freq and
offset; to become familiar with these, read the manual pp. 19-24 and the tutorial at pp. 33-40.
Then set the panel controls as follows to generate a 1 V p-p sine wave at 1 kHz with no dc offset
(you may want to use this standard setup in the future if you have trouble). You will need to set
the “output termination” to HIGH Z (see page 40) or the amplitude display will not read
correctly.
Frequency
1.000 kHz
(See p. 19)
Shape
Button
Select sine waveform.
Amplitude
1.0 VP-P
(See p. 20)
DC Level
OFFSET
0.0 VDC (p.21)
Frequency Sweep
SWEEP DURATION
OFF (button) Default is off at power up
Use a BNC coaxial cable to connect the function generator OUTPUT (lower right corner of
panel) to the CH 1 input of the oscilloscope. Set up the 'scope to view the signal on channel 1
with dc coupling. Adjust the 'scope trigger level to obtain a stable waveform. Observe the sine
wave and verify that the amplitude and frequency are as expected from the function generator
controls.
Explore the effect on the waveform of the following controls. Vary only one control at a time
and return it to its original value before changing to the next.
•
Depress the
•
•
•
waveforms.
Use the knob to change AMPL. Observe the amplitude changes.
Use the knob to change the OFFSET value. Note the change in dc level.
Set to
(square), use the knob to change the DUTY CYCLE
•
Return to
(square),
(triangle), and
(sine) buttons and observe the
(sine) and change the FREQUENCY using the knob; note how you need to
use the small arrow buttons to get the full range (100 µHz – 15 MHz).
Experiment #1
1.5
Fall 2005
Changing the function generator OFFSET control will change the ‘scope display in the same
way as the ‘scope’s vertical POSITION knob. But these two controls do very different things,
since the first changes the actual signal, while the second only changes the way it is displayed.
The function generator has a trigger output which can be used to trigger the 'scope (called
SYNC). Using the trigger output is more convenient than triggering the scope off of the
waveform itself because you avoid having to readjust the 'scope trigger every time you change
the waveform. To see how the trigger output works, first return the function generator to 1 V p-p
sine waves at 1 kHz with zero dc offset. Then setup to display both the CH 1 and the CH 4 on
the 'scope. Set the 'scope trigger SOURCE to CH 4, and connect a coaxial cable from the
function generator trigger output to the 'scope CH 4 input. Observe both the sine wave and the
trigger signal on the 'scope. Adjust the 'scope trigger level for a stable display. Now change the
amplitude and frequency of the sine wave, and notice how the 'scope remains nicely triggered.
The function generator you are using is actually a synthesizer, which means that its output is
derived from a highly stable quartz oscillator. We have some instruments in the lab called
counter/timers, which can be used to make very accurate frequency and time interval
measurements. There is not much point in using a counter/timer to measure the frequency of a
synthesizer, except to check that it is not broken. Counter/timers are useful when studying
signals generated by a circuit you build or that come from an outside source of unknown or
unstable frequency.
FLUKE 77 DIGITAL MULTIMETER
Most measurements that are done with the digital multimeter could also be made with the 'scope,
but the multimeter is usually more accurate, and it is very convenient for continuously
monitoring steady voltages or currents.
Check the accuracy of the ohms range by measuring precision resistors of 1 Ω, 1 kΩ, and 1 MΩ.
Identify an ordinary 1 kΩ 1/4-watt resistor by its color code and measure it with the multimeter.
Determine the frequency range for which the multimeter can be used to measure ac voltage.
Generate 500 Hz sine waves of 1 V p-p amplitude with the function generator, and arrange that
the oscilloscope and multimeter both measure the same signal. Be sure that the 'scope input is dc
coupled. Determine the amplitude from the oscilloscope trace and compare with the multimeter
reading. Note that the multimeter measures rms amplitudes and recall that p-p = 2 2 rms. Now
Experiment #1
1.6
Fall 2005
vary the frequency from 10 Hz to 10 kHz. What is the frequency range over which the
multimeter reading is constant to within 2%? Remember in the future that this is the usable
frequency range for the ac setting of the multimeter.
CURSOR MEASUREMENTS WITH THE OSCILLOSCOPE
Digital measurements of voltage, time, and frequency can be made for signals of the
oscilloscope. There are several modes of operation: a manual mode using CURSORS and an
automatic mode called up with the measurement menu. The CURSORS are more accurate but
the menu buttons are easier to use. A menu system is used to access the functions. The cursor
measurements will be most accurate when the displayed signal nearly fills the oscilloscope
screen. Set the amplitude to cover about 5 divisions vertically and the sweep speed to give about
2 periods horizontally (An example is in the manual, p. 2-12).
Try using the CURSORS to measure the p-p amplitude of a 1 kHz sine wave from the function
generator. If you get lost in the menu system, you can always get back to the beginning by
pressing the MENU OFF button.
Next try out the MEASURE system using the same 1 kHz sine wave. Measure the peak-to-peak
amplitude and the dc level. Then introduce some dc offset from the function generator and note
the changed readings. Try all the different measurements available.
Note that the automatic mode searches for the very top and bottom of the signals. For example,
in the schematic of a noisy sine wave below, automatic mode will return the upper and lower
most lines which will include the noise contribution. A careful manual measurement indicated
by the inner (shorter) lines can remove the noise from the amplitude measurement. With small
or noisy signals, the automatic mode will give very poor results.
Experiment #1
1.7
Fall 2005
SET UP TEST WAVEFORMS
This is an important check point to evaluate your understanding and skill. If you are having
trouble, be sure to get help from an instructor.
Set the oscilloscope vertical and horizontal controls such that the display covers about 5 vertical
divisions and about 2 periods across the screen. Use the counter-timer and the digital
measurements on the oscilloscope to verify that you have obtained the signal you are seeking.
1. 5 Hz triangle waves with a peak-to-peak amplitude of 3 V. (You will have to use NORMAL
triggering, not AUTO. Why?)
2. 15 MHz sine waves with an amplitude of 500 mV. (If you have trouble getting the ‘scope to
trigger at high frequencies, try pushing AUTO SETUP to restore the default settings.)
3. 10 kHz square waves with low value at 0 V and high value at + 5.0 V. Do the scope scales
agree with the output of the function generator?
4. Pulses at a frequency of 1 kHz, with an 800 µs low level at 0 V and a 200 µs high level at +
5.0 V. (This could be used as a logic signal).
Experiment #1
1.8
Fall 2005
Homework Problems
Turn these problems in at the start of your second lab session. This assignment takes the place
of the first pre-lab homework and lab report and is worth 50 points.
1.
Learn the color code for resistors (See H&H Appendix C). The sequence of colors
black brown red orange yellow green blue violet gray white
can be learned by memorizing the backpacker's lament:
Black bears resting on your gear bring very gray weather.
brown
black
orange
sliver
orange
orange
brown
gold
Figure 1.1 Color Coded
green
brown
black
gold
What are the values and uncertainties of the resistors in Figure 1.1? Write the resistance
using the most appropriate units of Ω, kΩ or MΩ, and include the % tolerance. (For
example, 1.8 kΩ ± 5%.) Resistors labeled with color bands are usually carbon
composition and 5% or 10% tolerance. 1% metal-film resistors are labeled by a different
scheme (H&H Appendix D).
2.
There is a three-digit number code for labeling capacitors. The label 224 means 22 with
4 zeros, interpreted as a capacitance value in picofarads. (224 = 220,000 pF = 220 nF.)
You also will see labels like 22 µF, 22 uF, 22 MF, and 22 MFD, all of which mean 22
microfarads. Another notation is 220 uuF, which means 220 picofarads (uu = micro
micro). Write down the values of the capacitors shown in Figure 1.2. Use the most
appropriate unit: pF, nF, or µF.
103
332
475
Figure 1.2 Number Coded Capacitors
Experiment #1
1.9
Fall 2005
3.
H&H exercises 1.1, 1.2, 1.3 (on page 6)
4.
H&H exercise 1.16 (on page 33)
5.
Do H&H exercise 1.16 again, but for inductors.
6.
Find a formula for the impedance Z1 in Fig. 1.3. Now evaluate numerically the real and
imaginary parts of the impedance for R = 1 kΩ, C=2 µF, L=1.5 mH, and f=2 kHz.
Express your results in Ohms with the appropriate SI prefix (k, µ ,etc.) and don’t forget
about the difference between ω and f. In this class it is essential to be able to get the
correct numerical results for problems like this.
7.
Find a formula for the impedance Z2 in Fig. 1.3.
8
Find a formula for the impedance Z3 in Fig. 1.3.
C
L
R
C
L
R1
Z1
Z2
R2
C
Z3
L
R1
R2
Figure
Experiment #1
1.10
Fall 2005
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