EXPERIMENT
7A: MICROPHONES, FILTERS, OSCILLOSCOPES, AND AMPLIFIERS
Objectives
Discover basic oscilloscope
functions and controls
Compare oscilloscopes with
the LabVIEW computer data acquisition system
Discover low-, high-, and
band-pass filters and develop their transfer functions
Graph Bode Plots of
predicted and measured filter data
Predict and measure
amplifier gains
Equipment
and Supplies
Protoboard
Resistors (100W)
Capacitors (0.1 mF)
Inductors (10 mH)
Operational Amplifier
Global Specialties 2001A
Function Generator
Tektronix TDS210
Oscilloscope
DAS (Acquire Waveforms and
Graph.vi)
Reference Pages
FUNCTION GENERATOR p.
135
Additional
Reference Material
Any of the traditional
introductory electrical engineering principles and applications texts like:
Hambley, A.R., Electrical Engineering Principles and Applications, 1997,
Prentice Hill, Chapter 6 “Frequency Response, Bode Plots, and Resonance” and
Chapter 8 “Amplifiers: Specifications and External Characteristics”. Or, Rizzoni, G., Principles and
Applications of Electrical Engineering, 3rd Ed., McGraw Hill,
Chapter 6 “Frequency Response and System Concepts,” and Chapter 9, “Transistor
Fundamentals.”
Function Generator
Description and Specifications at: http://www.globalspecialties.com/2001a.html
Hint: go to the MEL Web
Site and click on these links
The Problem
The problem for experiments
7A, B, and C is that your company wants to develop a new product composed of an
inexpensive microphone and a signal conditioner. The inexpensive system might be used as a
security device or to detect unusual noises in manufacturing machinery for
machine health monitoring. Your job is to evaluate the proposed product that is
shown in Figure 7-5. You will create a
set of signals with a function generator and speaker to produce a known sound
input and you will evaluate the system response to these signals. First you need to learn about oscilloscopes,
filters, and amplifiers, so there are several preliminary steps in the project.
An Oscilloscope
Primer
In previous experiments, you
gained experience with power supplies, digital voltmeters, signal generators,
and computer data acquisition. This
experiment provides experience with an oscilloscope, the Tektronix TDS210. Begin by carefully studying the Basic
Concepts, Operating Basics, and Application Examples sections of the TDS200
Series manual accessed via the web URL listed above as Additional Reference
Material. Further information can be
found in the “XYZ’s of Oscilloscopes” reference, also on the Tektronix web
site.
There are two types of oscilloscopes, analog and
digital. As previously discussed, analog
instruments display continuously variable signals and digital instruments
sample the signals and display and store discrete values. The TDS210 is a digital storage
oscilloscope. Therefore, it has a
digitizing capability similar to the DAS.
So, if it is similar to the DAS, why should you learn about both
instruments? Oscilloscopes have much
higher resolution and bandwidth than our DAS.
For example, the TDS 200 series has 100 MHz (TDS 220 or TDS 224) or 60 MHz (TDS 210)
bandwidth with selectable 20 MHz bandwidth limit and 1 GS/s sample rate and
2,500 point record length for each channel.
Therefore they can capture
signals more accurately than the DAS.
Furthermore, oscilloscopes are widely used by
engineers. The XYZ’s of Oscilloscopes
Manual from Tektronix states, “Oscilloscopes are used by everyone from
physicists to television repair technicians. An automotive engineer uses an
oscilloscope to measure engine vibrations. A medical researcher uses an
oscilloscope to measure brain waves.” It
is important for all engineers and scientists to be familiar with
oscilloscopes.
Displaying
Waveforms
Before you can view an input
signal with the oscilloscope, you must connect it to a known input signal,
so set the Function Generator to
produce a 120 Hz repeating sinusoid with 5.0 V peak-to-peak amplitude.
Set the oscilloscope up to
trigger automatically, dc-coupled on the channel containing the signal. Find a time scale that displays 3-5 cycles on
the screen. Display 3-5 divisions of voltage amplitude on the screen. Note that
if a signal has an amplitude of 5 V,
very little will be learned about the signal by displaying it using a 10 V/div
scale setting.
Read about and experiment
with the vertical and horizontal position controls to determine their function.
Triggering
Many times, we want the
display to start at a specific point on the signal and to start at that point
on every sweep. This produces an invariant or stable pattern that is especially
useful when comparing two signals, say on channels A and B. The TDS210 has a triggering feature (similar
to the triggering used previously with the DAS) to facilitate this type of
display. Triggering automatically controls the start of a “scan” at exactly the
same point on a repetitive waveform.
Note that auto mode simply triggers the scope every time it has traced
out a waveform. Set the trigger mode to
trigger and the trigger source to line. Display a signal generator output of a
60-Hz waveform. Vary the frequency of
the signal generator to determine what other frequencies, if any, will remain
stationary on the screen.
Many times, the signal of
interest will not be at a convenient frequency that is easily displayed by
triggering with the line. For these
cases, one can use the signal itself to trigger the oscilloscope. To accomplish this, set the trigger source to
the channel being used to display the signal.
You can control the coupling, slope, and level at which the scope is
triggered. Using a 1-kHz sine wave
output from the function generator, determine how the position, slope, and
level controls can be used to control the manner in which the waveform is
displayed. Sketch in your notebook the
effect of changing each of these parameters.
Experiment with the coupling
controls.
Verify that you can display
a single waveform.
After all members of the
team have experimented with the oscilloscope, have one of your lab partners set
up the signal generator to output a signal unknown to you. Test your ability to use the scope by
displaying the correct signal and reporting frequency, shape and Vpp
back to the person who set up the generator.
Rotate until all members of your group display and report
characteristics of an unknown signal.
Write a description in your
notebook for future reference of the function of the controls you used in this
experiment, because you will use them in future activities.
Filters
Now we will begin to build
the circuits that will be important for evaluating the microphone and signal
conditioner. In all systems, the output
from one component must match the input requirements for the next component in
line. Sometimes this requires intermediate,
signal-conditioning components. One
example is the need to select a radio station.
The broadcast signal contains many stations and your antenna or cable
supplies this signal to your radio. Your
radio must contain an intermediate component that will play the station of
interest. You need to block the unwanted
frequencies and pass the frequency of interest.
Filter circuits accomplish this.
You should remember from
Physics that we use decibel units to measure large frequency ranges of sound
intensities. Decibels are also used in
this application to evaluate filters.
The transfer function, H(f)
of a filter is the ratio of the output to input voltage, or H(f) =
Vout/Vin. A low value for the transfer
function is required to block unwanted frequencies. We use Bode Plots to express the effective
action of a filter of blocking some frequencies and passing others. A Bode Plot
of frequency response is magnitude of the transfer function in decibels on the
Y-axis versus frequency in a log scale on the X axis. To convert transfer
function magnitude to decibels multiply the base 10 logarithm of the transfer
function by 20, or |H(f)| dB = 20 log|H(f)|.
Three filter circuits are
shown below in Figure 7-1 a, b, and c .
Predict which circuit is low pass, which is band pass, and which is high
pass. Write the equations for the output
voltage of each of the filters. Sketch the expected behavior of the
circuits. Refer to an introductory
electrical engineering principles and application text like those listed in the
Other Reference Material section of this experiment.
Figure 7-1 a. Filter Circuit
Schematic (R = 100 W, C = 0.1 mF, L = 10 mH for Figures 7a, b, and c.)
Figure 7-1 b. Filter Circuit
Schematic
Figure 7-1 c. Filter Circuit
Schematic
Wire each of the three circuits (low-pass filter, high-pass filter, band-pass
filter) on a protoboard so that you can easily connect the signal generator and
oscilloscope. Set the function generator
to output 5Vpp.
Higher values might damage components in the circuit. Each component has a specified rating within
which it will perform. Outside of this
rating range, the component will be damaged.
Use two channels on the
oscilloscope to display the input (the signal generator output) and the output
signals simultaneously. As a means of
“exploring” the behavior of these circuits, overlay the signals on the screen
so that the amplitude of the input signal occupies the 0 to 100 % scale. Vary the frequency of the input signal to
evaluate the predicted Vout at various frequencies. Does the input signal stay the same in terms
of amplitude and the output signal change in amplitude at various frequencies
as predicted? By cleverly selecting your
scale settings for channel B, you should be able to read the display in terms
of the fractional output of the filter (for example, a low-pass filter will
pass about 100 % of the signal amplitude at low frequencies).
Make 3 short tables (six to
eight points) from the measured and predicted transmission characteristics of
each of the three filters. Each table
should list the frequency, the measured transmittance (transmittance = Vout/Vin
= amplitude fraction passed), and the predicted transmittance (from your
equations). Note that you should choose
the frequencies carefully so that at least three points on either side of the
half-power frequency of the filter are included. Highlight the half-power frequency and the
band-pass resonant frequencies in the tables.
Bode Plots
To display the effectiveness
of the filters, make two Bode plots of the high-pass filter. One plot is from your calculated, or
predicted, frequency response from the Vout equation. The other is from your measured data. What is
the slope of the low-frequency and high-frequency asymptotes in dB/decade for
both plots? Compare the intersection point of the asymptotes with the half
power frequencies calculated and measured.
Experiment 7a
Preparation Results
DATE___________ NAME___________________________________
GROUP MEMBER
NAMES__________________________________________________________
LABORATORY BENCH NUMBER_____________
1. Identify which circuit is low, high, and band
pass: (15 pts)
Figure Filter
Type
7-1a. ___________________
7-1b. ___________________
7-1c. ___________________
2. Develop the equation for the transfer function and
magnitude of the transfer function of the low pass filter. (5 pts)
3. Sketch the magnitude of the transfer function
versus frequency for the low pass filter. Put Vout/Vin (in dB) on the Y-axis
and frequency on the X-axis. (5 pts)
4. Calculate the half power frequency for the
low-pass filter. (5 pts)
5. Write the equation for the transfer function and
magnitude of the transfer function of the high-pass filter. (5 pts)
6. Sketch the magnitude of the transfer function
versus frequency for the high-pass filter. Put Vout/Vin (in dB) on the Y-axis
and frequency on the X-axis. (5 pts)
7. Calculate the half power frequency for the
high-pass filter. (5 pts)
8. Write the equation for
the transfer function and magnitude of the transfer function of the band-pass
filter. (5 pts)
9. Sketch the magnitude of the transfer function versus
frequency for the band pass filter. Put Vout/Vin (in dB) on the Y-axis and
frequency on the X-axis. (5 pts)
10. Calculate the resonance frequency for the band
pass filter. (5 pts)
11. Attach copies of the procedures that you plan to
use for Experiment 7a. (20 pts)
12. Attach copies of the circuit and wiring diagrams
that you plan to use for Experiment 7a. (20 pts)
Experiment 7A
Report
DATE___________ NAME___________________________________
GROUP MEMBER NAMES__________________________________________________________
LABORATORY BENCH NUMBER_____________
* identifies
questions that require the same answer for the entire group. All other questions require individual
answers.
OSCILLOSCOPE
1. Sketch a 20 Hz signal using 10ms/div time
scale. (2 pts)
2. a. What exceptionally common waveform is used for
the trigger in line mode? (2 pts)
b. What other frequencies will stay
stationary on the screen? (2 pts)
3. Sketch the shape of the unknown signal given by
your lab partner. Put the time scale and
amplitude (and write the frequency on the sketch). (2 pts)
4. Describe the function generator and oscilloscope
settings that were used in 4. (2 pts)
FILTERS AND
AMPLIFIERS
5. *Compare the
low, high, and band pass filters that you built with your predicted
values. To do this make three tables
(one for each filter type) with the following column headings. Determine how many frequencies are necessary
to completely evaluate each filter. This will determine the number of rows in
the table. Choose the frequencies
carefully so that points on either side of the half power frequency of the
filter are included. (15 pts)
Low Pass Filter
|
Frequency |
Predicted Transmittance |
Fractional Measured Transmittance |
6. *Attach a print out of the high pass filter Bode
plot from your calculations. Format the
graph properly. (3 pts)
7. *Attach a print out of the high pass filter Bode
plot from your measurements (you may place this plot on the same graph as 8).
(3 pts)
8. *Complete the following table from the high pass
filter calculations and measurements:
(5
pts)
|
|
Calculated Data Plot |
Measured Data Plot |
|
Low Frequency Asymptote
Slope ** (dB/Decade) |
|
|
|
High Frequency Asymptote
Slope ** (dB/Decade) |
|
|
**See a traditional
electrical engineering principles and applications text for definition of
asymptote.
9. *Identify the half power
frequency as fB on both calculated and measured high pass filter
Bode plots. (2 pts)