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Southern Illinois University - Carbondale
Department of Technology
Electrical Engineering Technology Program
ET 438b
Sequential Digital Control and Data Acquisition
Automated Frequency Response Testing Project Design Document
Table on Contents
Project Overview ............................................................................................................. 6
Organization, Management, and Evaluation of the Project ............................................. 6
Task List .......................................................................................................................... 7
Technical Details of the Project ....................................................................................... 8
Graphical User Interface (GUI) and Displayed Variables: ............................................... 9
Data Acquisition Board General Specifications ............................................................. 10
Task 1 - Function Generator Voltage-to-Frequency Characteristic ............................... 11
Objective ....................................................................................................................... 11
Procedure...................................................................................................................... 11
Linear Least-Squares Curve Fitting ............................................................................... 12
Desired Results ............................................................................................................. 12
Table 1-1 Range 1 Data ................................................................................................ 14
Table 1-2 Range 2 Data ................................................................................................ 15
Table 1-3 Range 3 Data ................................................................................................ 16
Task 2 -Construction of Test Amplifier .......................................................................... 17
Objective ....................................................................................................................... 17
Procedure...................................................................................................................... 17
Task 2 Amplifier Measurements .................................................................................... 19
Task 3-Input Voltage RMS Conversion and Scaling ..................................................... 20
Objective ....................................................................................................................... 20
Design Criteria .............................................................................................................. 20
Task 3 Lab Measurements ............................................................................................ 24
Table 3-1 Input Voltage Scaling Test ............................................................................ 24
Table 3-2 Input Voltage Frequency Response .............................................................. 24
Task 4-Output Voltage Scaling and RMS Conversion .................................................. 25
Objective ....................................................................................................................... 25
Design Criteria .............................................................................................................. 25
Desired Results ............................................................................................................. 28
What to Present for Evaluation ..................................................................................... 28
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Task 4 Lab Measurements ............................................................................................ 29
Table 4-1 Output Voltage Scaling Test ......................................................................... 29
Table 4-2 Input Voltage Frequency Response .............................................................. 29
Table 4-3 Input Voltage Frequency Response .............................................................. 30
Table 4-4 Input Voltage Frequency Response .............................................................. 30
Task 5 – Design of a Sinusoidal Voltage-Controlled Oscillator (VCO) .......................... 31
Technical Details and Desired Results.......................................................................... 31
Task 5 Supplement ....................................................................................................... 35
Task 5 Lab Measurements ............................................................................................ 38
Table 5-1 Harmonic Distortion Levels ........................................................................... 38
Table 5-2 Total Harmonic Distortion Calculation .......................................................... 39
Task 6-Frequency-to-Voltage Conversion of Input Signal ............................................. 40
Objective ....................................................................................................................... 40
Prelab Preparation ........................................................................................................ 40
Technical Specifications and Design Alternatives ......................................................... 40
Analog Frequency-to-Voltage Conversion..................................................................... 41
Design Note Concerning Accuracy ............................................................................... 44
For Alternative 1: Analog Frequency-to-Voltage Conversion ....................................... 46
Desired Results ............................................................................................................. 46
What to Present for Evaluation ..................................................................................... 46
For Alternative 2: Frequency Measurement Using Digital Counters............................. 46
Desired Results ............................................................................................................. 46
Task 6 Lab Measurements ............................................................................................ 49
Table 6-1 – Range 1 Measurements ............................................................................. 49
Table 6-2 – Range 2 Measurements ............................................................................. 50
Table 6-3 – Range 3 Measurements ............................................................................. 51
Task 6 Lab Measurements ............................................................................................ 52
Table 6-1 – Range 1 Measurements ............................................................................. 52
Task 6 Lab Measurements ............................................................................................ 53
Table 6-2 – Range 2 Measurements ............................................................................. 53
Table 6-3 – Range 3 Measurements ............................................................................. 54
Task 7-User Interface and Data Conversion Using LabVIEW....................................... 55
Objective ....................................................................................................................... 55
Prelab Preparation ........................................................................................................ 55
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Basic Control Process Steps for Data Acquisition Software ......................................... 55
Desired Results ............................................................................................................. 57
What to Present for Evaluation ..................................................................................... 57
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Introduction
The first half of the semester will be the study of physical variable measurements, the
conversion of these measurements to digital signals, and the recording of these
measurements. A computer-based data collection system will be used to demonstrate
the concepts presented in the lecture. This system consists of computer hardware and
software that allows the developer to input both analog and digital information and then
output digital control signals to implement on/off control. The digital outputs can also be
used to drive a digital-to-analog converter chip for reproduction of analog signals.
The project selected for implementation is a automated frequency response testing
system. This system can be used to check the frequency response of small-signal
amplifiers and audio power amplifiers. The project will make use of commercial test
instruments and custom designed analog and digital circuits to bring signals into the
system and output control signals to the equipment under test (EUT). A high level data
flow programming language will implement the user interface and computations
required. Figure 1 shows the basic block diagram of the desired system.
Figure 1. Block Diagram of the Frequency Response Testing System.
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Project Overview
A single chip voltage control oscillator (VCO) is the signal source for the frequency
response testing system. This device will be provided along with application notes that
show typical designs. The VCO input can take either an ac or dc voltage that will
change the frequency of the output voltage depending on the magnitude of the VCO
input. An analog control signal will be sent from the controlling PC to the VCO such
that the VCO frequency will vary over a range of 20 Hz - 20 kHz. The input voltage
level from the VCO is set manually, but must be monitored by the data acquisition
system. This is a sinusoidal signal that must be scaled and modified to within the range
of the acquisition hardware analog inputs. The output voltage of the equipment under
test is also monitored by the system. This signal must also be scaled to be within the
physical limits of the data acquisition hardware provided. The frequency from the
function generator must be measured. Additional signal conditioning will be necessary
to convert this variable into a range of voltages that is compatible with the hardware.
The frequency measurement will be done on the input of the equipment under test
(EUT).
Software will be written so that the incoming signals will correctly represent the actual
physical measurements. It is necessary to convert the sine input signals into RMS
voltage levels. This can be done using a combination of analog signal conditioning and
software, or be done totally in software. The software should convert the input and
output voltage readings into a gain, given in decibels, for the y-axis of the frequency
response plots. The values of frequency and decibels collected from the test will be
stored to a file on the PC as well as being displayed both graphically and numerically on
the tester's user interface. Simple on/off controls will allow the user to start the test.
This control will be implemented in software.
Organization, Management, and Evaluation of the Project
This project’s size dictates that it be done as a group effort over several weeks during
the semester. The groups will consist of 3 to 4 people. The initial schedule for the
construction, testing and documentation of this project will be 10 weeks. This is not a
long time! The only way that this project can be completed satisfactorily is for project to
be divided into subsystems with different members of the group working on the parts.
The course instructor has identified different tasks and design milestones that must be
finished to complete the project. These subsystems can be considered as individual
labs, but will not be reported in the traditional way since the outcome of this exercise is
the design and documentation of the overall system. The group members are
responsible for the division of the tasks among themselves. These assignments will
then be given to the course instructor and laboratory T.A. These assignments must
be turned in by the second lab meeting of the semester. Individuals assigned these
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subtasks will be required to provide documentation to demonstrate their progress at
regular intervals throughout the semester. The type of documentation and
demonstration will depend on the task. The requirements for the successful
completion of the task will be described later. The individual and group grade will be
determined by a combination of overall group performance and the individual's
performance within the group. How well the individuals and groups meet the
requirements for each of the task and integrate these stages into a complete system
will also determine the student's grade.
Task List
The major subtasks for the completion of the automated testing project are listed below.
These tasks outline one method for the completion of the design. The technical details
for the tasks are given later. Material presented in the lecture and during discussions in
lab meetings will provide the group members with the background knowledge
necessary to complete these tasks. Some of the tasks can be completed with the
knowledge and skills that are expected for a course of this level. It may also be
necessary for the persons of the group to use their own initiative to find solutions to
problems that are not listed here. This may require using their own design ideas and
researching topics. If a group or one of its members has excessive difficulty in
completing the task(s), they should notify the course instructor or lab T.A. This is a
design project, so asking people who have more experience to troubleshoot or help
with an unforeseen problem of the design is encouraged.
Task 1-Voltage Control Oscillator Voltage-to-Frequency Characteristic: Use
multimeters, dc power supplies, digital scopes, and the lab designed VCO to determine
the voltage/frequency characteristic of the sinusoidal sources. This information will be
used to determine the gain of the function generator so that the control level from the
automated test system can be determined. Use least-squares curve fitting to find the
equation that describes the data. Develop a LabVIEW virtual instrument to control the
analog output and send a voltage to the VCO.
Task 2-Construction of Test Amplifier: Design and build the equipment under test
for use with the automated test setup. The EUT for the design will be a simple two
stage OP AMP circuit that will have a low frequency cut off, high frequency cut off and a
mid-band gain given as design parameters. A circuit simulation or actual lab test
should be performed on the design to determine its operation before it is used in the
automated test setup. Test data taken in this task are compared to the automated
system.
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Task 3-Input Voltage RMS Conversion and Scaling: Design a scaling circuit for the
acquisition of the input voltage ac signal. Signal conditioning will include the hardware
and/or software necessary to convert the voltage into a value that can be used in a
decibel calculation. A LabVIEW program will display of the circuits output on a PC.
Task 4-Output Voltage RMS Conversion and Scaling: Design a scaling circuit for
the acquisition of the output voltage ac signal. Signal conditioning will include the
hardware and/or software necessary to convert the voltage into a value that can be
used in a decibel calculation. A LabVIEW program will display of the circuits output on
a PC.
Task 5-Design and Test of Voltage Control Oscillator: Design a VCO that takes a
dc voltage input within the range of the data acquisition board's analog output and has
an output frequency of 20 Hz to 20 kHz in three ranges. These ranges are: 15-400 Hz,
150-4000 Hz, and 2000-40,000 Hz. The VCO output should be a low distortion sine
wave that has a range of 10 mV to 10 V peak.
Task 6-Input Frequency Measurement: Determine how to make frequency
measurements using the available inputs and outputs of the data acquisition hardware.
Option 1: Consider using a linear IC that converts the frequency to an analog voltage
(LM2907 or equivalent). The design should include three ranges that accurately cover
the output ranges of the VCO design in Task 5. A LabVIEW program will control the
ranges and display the measured frequency on a PC.
Option 2: Use the digital counters in the data acquisition board to measure frequency.
This option requires signal conditioning of a sine or square wave into a TTL signal
level. It also requires the creation of LabVIEW software to access the hardware and
convert the counter readings into frequency.
Task 7-User Interface Design: Design a program in LabVIEW that implements the
functions of the automated test system. Develop the program that will collect the data,
display the information on the user interface, and save the results to disk using the data
collection software provided. Create the user interface based on given specifications.
This task must use scaling information from all the tasks above to display correctly the
information to the user. A working knowledge of the data collection software must be
developed.
Technical Details of the Project
Generate frequency response curves for an electronic amplifier circuit by using an
automated test setup to change the frequency of a function generator chip while
measuring the input and output voltages. The frequency response tester will produce a
graph of the results as the measurements are being made. Save the data to a disk file
for further processing.
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Input Maximum Voltage:
250 mV peak ac (set manually to 200 mV before testing
starts)
Input Voltage Tolerance: ± 5%
Output Maximum Voltage: 12.5 V peak ac (varies with frequency response of EUT, 10
V peak is the maximum mid-band value)
Output Voltage Tolerance: ± 5%
Power Supply minimum voltages: 15 V dc
Frequency Range: 20 - 20 kHz
Desired frequency test points ( ± 10%)
20 Hz, 40 Hz, 80 Hz, 160 Hz, 200 Hz, 400 Hz, 800 Hz, 1,200 Hz, 1,600 Hz,
2,000 Hz, 4,000 Hz, 8,000 Hz, 10,000 Hz 12,000 Hz, 16,000 Hz, 20,000 Hz.
Graphical User Interface (GUI) and Displayed Variables:
The GUI for the frequency tester is shown in Figure 2. This interface can be easily
constructed using the LabVIEW software installed on the computers with the data
acquisition systems.
Figure 2. GUI for Frequency Response Tester.
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The testing will start when the on/off switch is turned to the "on" position. The data
collection program will then run until the last data point, 20,000 Hz, is collected. The
RMS value of the input and the output voltage will be displayed on the screen.
The current test frequency will also be displayed on a digital display. The desired and
the measured frequencies will be displayed for each test frequency. A percent error
calculation will be made and displayed at each test frequency. As the frequency
increases through all the ranges, an indicator will light to show the frequency range
used.
When the test is completed the, frequency and dB will be displayed on graph. An LED
will be activated to show that the test is completed. The frequency axis (x-axis) must
have a logarithmic scale. The gain, in dB, and frequency data will also be saved to
disk.
Data Acquisition Board General Specifications
Two models of data acquisition boards exist in the laboratory computers. Check the
model that in on the computer used for the project. They are both made by National
Instruments (NI)
Model: NI 6024E
8 digital input/output points
16 channels of single-ended 12 bit analog input 250 kS/s
(Programmable input ranges ±0.05 to ±10 V)
200 kHz maximum sampling rate
2 analog outputs (±10 V dc limits)
Model: NI 6221
24 digital input/output points
16 channels of single-ended 12 bit analog input 250 kS/s
(Programmable input ranges ±0.05 to ±10 V)
200 kHz maximum sampling rate
2 analog outputs (±10 V dc limits)
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Task 1 - Function Generator Voltage-to-Frequency Characteristic
Technical Details and Desired Results
Objective
Use multimeters, digital scopes,dc power supplies and a single chip VCO function
generator prototype to determine the voltage/frequency characteristic of a function
generator.
Procedure
1.)
2.)
3.)
4.)
5.)
6.)
7.)
Connect the VCO prototype to the power supply and check its functionality.
Activate the lowest range by placing a voltage signal on the coil of the ranging
relay. Connect a potentiometer to the input such that a variable dc voltage is
developed across the VCO input. Adjust the frequency to a midrange value.
Connect a multimeter to the output of the VCO prototype. With the multimeter
set to ac, set the output level of the VCO to 141.4 mV RMS. Change the
function of the multimeter to the frequency measurement mode or connect a
scope with frequency measurement capabilities to measure the VCO output
frequency
Connect a multimeter to the wiper arm of the potentiometer so that the input
voltage can be measured.
Adjust the dc input at the wiper arm until the frequency reading matches each of
the values listed in Table 1-1. Record the value of dc voltage that gives the
desired frequency for every value in the table. Perform this experiment twice
once for frequencies that start at the maximum value and are decreased by the
VCO input, then for increasing values of frequencies.
Adjust the range by disconnecting the voltage signal applied to the lowest range
relay coil and reconnect it to the middle range. Repeat steps 2-4 and record the
results in Table 1-2.
Set the VCO to the highest range by disconnecting the voltage signal applied to
the middle range relay coil and connecting it to the high range coil. Repeat steps
2-4 and record the results in Table 1-3.
Write a LabVIEW program that will activate a digital output to energize the
correct range and then output an analog voltage to control the frequency.
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Linear Least-Squares Curve Fitting
Experimental data inherently has error. This error can be all associated with the
dependent variable (y) of a graph. The actual relationship between the independent (x)
and dependent variables is approximated by the measurements. A least-squares curve
fit of the data estimates the actual relationship by minimizing the sum of the square of
the errors between the measurements and the actual relationship. The equation for
linear relationships in general form is:
y  mx b
where:
m is the slope of the line
b is the y-intercept of the line
The following two equations find estimates of these parameters using the measured x
and y values of the data set.
N
N
b  N  m   X i   Yi
i 1
i 1
N
N
N
i 1
i 1
i 1
b   X i  m   X i2   X i  Yi
where
Xi = the independent variable values of the data set,
Yi = the dependent variable values of the data set,
N = the number of measurements in the data set.
This gives two equations for the two unknowns m and b that can be solved using
calculators or computer programs.
Desired Results
1.)
Using the results in Tables 1-1, 1-2 and 1-3, construct three graphs of the
voltage-frequency response of the function generator tested by plotting both the
increasing and decreasing frequency voltage values on the same plot. Use
Excel or MathCAD to produce the graphs. Place measured frequency on the yaxis and VCO voltage on the x-axis.
2.)
Determine the maximum hysteresis of the readings by taking the difference
between the decreasing frequency readings and the increasing frequency
readings at the test frequencies.
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3.)
Take the average values of the decreasing and increasing f VCO readings for
each of the three ranges. Produce three plots using Excel or MathCAD that
have the average VCO voltage readings on the x-axis and the measured
frequency on the y-axis. Do not connect the test points with lines on these plots.
4.)
Use the least-squares curve fit to “best fit" a straight line through the data points
from part 3. Graph the "best fit" lines on the same axis as the average voltage
for each of the three ranges. Compute the values of m and b for each range
5.)
Write the best fit equations for each range with VCO voltage as independent (x)
value and VCO frequency as the dependent (y) variable. Report and record
these three equations for later use.
6.)
Solve the equations in the previous step for the VCO voltage. These equations
will give the VCO input voltage (y) for a give frequency (x).
What to Present for Evaluation
1.)
2.)
3.)
4.)
5.)
6.)
Data Tables 1-1, 1-2, and 1-3 with lab measurements, signed and dated.
Graphs of frequency vs. increasing and decreasing VCO voltage for each range.
Calculation of maximum hysteresis for each range
Graph of average VCO voltage vs. frequency for each range
Best fit equations for all three ranges
LabVIEW program that controls VCO frequency and changes ranges
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Table 1-1 Range 1 Data
Desired
Frequency (Hz)
Measured
Frequency (Hz)
VCO voltage (Vdc)
(decreasing f)
VCO voltage (Vdc)
(increasing f)
20 Hz
40 Hz
60 Hz
80 Hz
90 Hz
100 Hz
125 Hz
150 Hz
160 Hz
200 Hz
225 Hz
250 Hz
275 Hz
300 Hz
350 Hz
400 Hz
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Table 1-2 Range 2 Data
Desired
Frequency (Hz)
Measured
Frequency (Hz)
VCO voltage (Vdc)
(decreasing f)
VCO voltage (Vdc)
(increasing f)
400 Hz
500 Hz
600 Hz
800 Hz
1000 Hz
1500 Hz
2000 Hz
2250Hz
2500 Hz
3000 Hz
3250 Hz
3500 Hz
3750 Hz
4000 Hz
5000 Hz
6000 Hz
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Table 1-3 Range 3 Data
Desired
Frequency (Hz)
Measured
Frequency (Hz)
VCO voltage (Vdc)
(decreasing f)
VCO voltage (Vdc)
(increasing f)
2000 Hz
3000 Hz
4000 Hz
5000 Hz
7000 Hz
8000 Hz
10000 Hz
12000Hz
14000 Hz
16000 Hz
18000 Hz
20000 Hz
24000 Hz
28000Hz
32000 Hz
36000 Hz
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Task 2 -Construction of Test Amplifier
Technical Details and Desired Results
Objective
Design and test a two stage OP AMP amplifier circuit for use with the automated test
setup. The circuit will have a low frequency cut off, high frequency cut off and a
midband gain given as design parameters. Verify the circuit operation by measuring
the gain of the circuit at several test points.
Procedure
1.)
Find values of resistors R1, R2, R3, and R4 in Figure 3 such that the mid-band
gain of the two stage amplifier shown below is 50. The values of C1 and C2 are
given as:
C2
R2
R4
C1
R3
Vi
R1
U1
U2
Vo
Figure 3. Two-Stage Amplifier: Automated Test System Test Circuit.
C1 = 0.1 F
C2 = 470 pF
2.)
3.)
The values of R1 and R4 should also be selected to give a cutoff frequency of
100 Hz for the U1 circuit (high pass filter) and 10 kHz for the U2 circuit (low pass
filter). If the results of the calculations result in non-standard values, add a
trimming potentiometer to get the exact value.
Draw a schematic for the circuit design and place the values computed on it.
This can be done with a simulation package, which can also verify the design
performance before construction.
Construct the circuit designed in step 1 using the component values and ± 17.5
Vdc power supplies. Test the circuit by applying a 200 mV peak sinusoidal
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4.)
5.)
signal to the input and measuring the peak output voltage at the frequencies
shown in the table. Take these measurements and compute the decibel gain of
the circuit for each of the applied frequencies.
Derive the transfer function of the overall circuit. Find the magnitude of the
transfer function and compute the theoretical value of gain, in dB for each
frequency were test data was taken
Use Excel or MathCAD to produce a frequency response plot of the
measurements taken from the circuit test and the theoretical values computed in
step 4.
What to Present for Evaluation
1.)
2.)
3.)
4.)
5.)
6.)
Table with lab measurements, signed and dated.
Calculations for the values of resistors computed in procedure (neatly done).
Schematic of the design.
Sample calculation for the decibel calculation.
Graph of the circuit frequency response.
Short discussion (1- 1.5 pages double spaced) of the circuit theory of operation
that includes the formulas used to compute the component values and a
explanation of the frequency response of the circuit stages.
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Task 2 Amplifier Measurements
Frequency (Hz)
Input Voltage
(ac peak)
20
200 mV
40
200 mV
80
200 mV
90
200 mV
100
200 mV
160
200 mV
200
200 mV
400
200 mV
800
200 mV
1200
200 mV
1600
200 mV
2000
200 mV
4000
200 mV
8000
200 mV
9000
200 mV
10,000
200 mV
12,000
200 mV
16,000
200 mV
20,000
200 mV
Output Voltage
(ac peak)
Circuit Gain
(db)
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Task 3-Input Voltage RMS Conversion and Scaling
Technical Details and Desired Results
Objective
Design and test an input voltage measurement circuit for the acquisition of the input
voltage ac signal. Signal conditioning and scaling will include the hardware and
software necessary to convert the ac voltage into a value that can be used in a decibel
calculation. Write a LabVIEW program that displays the output of the circuit scale to
show the measured value correctly.
Design Criteria
Figure 4 shows a block diagram of the input voltage scaling circuit with the desired
voltage ranges shown. This circuit takes a 0-250 mV peak ac (176.75 mV ac RMS)
signal from the input of the test amplifier and converts it to a dc voltage with a range of
5 Vdc. This is the range of the analog inputs on the data acquisition card.
Ac Amp
Input Scaling
0-200 mV
V ac RMS
0- 176.75
Vac RMS
Input
Z Buffer &
Scaling
From
Test Amp
Input
Vin
Zin >1 M
True
RMS
Converter
0- -200 mV
Vdc
0- 200 mV
Vdc
Sign Changer
Av = -1
VRMS
Zout <50 
- 5 to +5 Vdc
Vdc
OP AMP
Scaling CKt.
V Scaler
To Data
Acquisiton
Card Analog Input
Figure 4. Block Diagram of the Input Scaling and RMS Conversion Circuit.
The signal from the test amplifier will be a constant 200 mV peak ac (141.4 mV ac
RMS). The scaling in Figure 4 should give a value of 80% of the fullscale value, +3 Vdc,
when the test amplifier signal is applied.
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The input of the voltage measurement circuit requires a high impedance to minimize the
loading effects of the voltage sensor on the circuit under test. A OP AMP voltage
follower circuit provides the necessary Zin and Zout specified. The ideal follower circuit
will have infinite Zin and zero Zout. Check the impedance characteristics of the actual OP
AMP used in the design to verify that it satisfies the values shown in Figure 4.
The true RMS converter chips provide one design solution for this task. Check the
input impedance specification of the chips to determine if it meets or exceeds the
design requirements. High Z input to the RMS converters eliminates the need for a
High Z buffer stage, simplifying the design. Check the chip input specifications to
determine if the required input exceeds the devices capabilities. A further refinement of
the circuit would be to use an active low-pass filter circuit for this stage. The active filter
cutoff frequency will eliminate high frequency noise from the test signals. If this circuit
is used, set the cutoff frequency to a value at least 10 times greater than the highest
test frequency.
IC’s manufactured by Analog Devices performs a true RMS conversion of the ac wave.
The chips are the AD737A and AD736. Data sheets for these chips are available in pdf
format. They show typical application circuits and gives chip specifications and
limitations. Verify that they will satisfy the design specifications before starting
construction of the application circuits. The chip outputs a negative dc voltage that has
the same numerical value as the RMS ac input. A sign change stage produces a
positive value for the scaling circuit.
The following equation gives the mathematical definition of RMS.
v rms 
where
1 T
2
0 vt  dt ,
T
vrms = the root mean square value of the voltage wave,
v(t) = the time varying voltage waveform,
T = the period of the waveform.
To find the true RMS value of an ac wave, the waveform function must be squared.
The resulting function is integrated over a single period of the wave and divided by the
reciprocal of the period. This is the average value of the squared function. Taking the
square root gives the RMS value of the wave. Using this mathematical definition on
any waveform will always give the correct RMS value. The AD737/AD736 implement
this equation using integrated analog electronics. Read the circuit operation section in
the data sheets and summarize it for your circuit description.
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These design requires output scaling to take full advantage of the input range of the
data acquisition analog input. The analog input range is -5 to +5 Vdc. The AD737/736
has an output range of 0- -200 mV Vdc for the given input range. Figure 5 shows the
scaling circuits that should give the desired range. Note that the upper circuit is for the
AD 736 converter and the lower is for the AD737 device.
+12V
R8
R6
R3
R1
R2
R9
D1
R5
U2
U1
+12V
Vin
Vout
R4
R7
R14
Vrms
R11
R15
R10
1
U3
Vout2
R12 3
2
D2
Figure 5. Scaling Circuit for the Input Voltage Signal.
The output of this circuit connects to an analog input channel of the data acquisition
card.
The input voltage circuit should be calibrated and tested before adding it to the system.
Calibration requires performing two operations: zeroing and spanning. To zero the
overall circuit:
a.)
b.)
Ground the input voltage, Vin in Figure 4.
Measure the outputs from each OP AMP. Each output should be at zero
except the scalar circuit output, Vscaler in Figure 5, which should read
–5 Vdc.
if the Z-buffer or input scaling stages are not zero with a grounded input,
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c.)
d.)
add an offset null circuit to the appropriate stage. The OP AMP data
sheets give the connections for adding offset null to the IC.
Apply the full-scale voltage to Vin and measure the voltages at each stage
in the circuit. If the measured values do not match the theoretical values,
adjust the variable resistors in each stage to eliminate the differences.
Repeat the above steps until the system is values match the theoretical
values to within the measurement tolerances.
Desired Results
1.)
2.)
3.)
5.)
6.)
A working circuit that uses hardware to produce a scaled dc value proportional to
the RMS value of the ac input voltage.
Schematic drawing of the hardware used in the circuit with all component values
shown.
A block diagram, similar to Figure 4 that shows an overview of the circuit
operation and the range of input and output voltages.
Test results for the circuit that verifies the operation of the hardware at over the
range of sinusoidal input. The attached table gives the levels and frequencies at
which to conduct the tests.
A LabVIEW program that displays the measured value of input voltage scaled to
millivolts RMS on a PC. Scaling equations that relate the input voltage to scalar
output and scalar output to millivolts RMS.
What to Present for Evaluation
1.)
2.)
3.)
4.)
5.)
6.)
7.)
Table with lab measurements, signed and dated.
Calculations for the component values used in the design (neatly done)
Schematic of the design.
A block diagram for the system.
A working model of the input voltage measurement system that includes a
LabVIEW program. (schedule with instructor or T.A. for demonstration)
A graph with a plot of the data in Table 3-1 and a plot of the linear approximation
line.
Short discussion (1- 2.5 pages double spaced) of the circuit theory that includes
the formulas used to compute the component values and an explanation of the
circuit operation.
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Task 3 Lab Measurements
Table 3-1 Input Voltage Scaling Test
Test Frequency: 10 kHz
Vin (mV peak ac )
0
Vamp (V peak ac)
VRMS (Vdc)
Vscaler (Vdc)
50
75
100
125
150
175
200
225
250
Table 3-2 Input Voltage Frequency Response
Frequency (Hz)
Vin
(mV peak ac)
20
200
200
200
800
200
2000
200
8000
200
10,000
200
16,000
200
Vamp
(V peak ac)
VRMS
(Vdc)
Vscaler
(Vdc)
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Task 4-Output Voltage Scaling and RMS Conversion
Technical Details and Desired Results
Objective
Design an output voltage measurement circuit for the acquisition of the output voltage
ac signal. Signal conditioning will include the hardware and software necessary to
convert the voltage into a value that can be used in a decibel calculation. Write a
LabVIEW program that displays the output of the circuit scale to show the measured
value correctly. This is similar to Task 3 only with different input/output voltage
requirements.
Design Criteria
Figure 6 shows the signal flow diagram for the output signal conditioning and scaling
circuit. This circuit takes an ac signal that varies in peak value from the test amplifier
and converts it to a proportional dc value. The level depends on the frequency
response characteristic of the test amplifier. The scaling listed in Figure 6 should give
an output of 3 V dc when a 10 V ac peak (7.07 V ac RMS) value appears on the input.
This is 80% of the full-scale value
0-8.8375
Vac RMS
RMS
Converter
Input Scaling
Input
Z Buffer
From
Test Amp
Output
Vin
0- 200 mV
V ac RMS
0-8.8375
Vac RMS
0- -200 mV
Vdc
Vamp
Zin >1 M
Zout <50 
0- 200 mV
Vdc
VRMS
Sign Changer
Av = -1
-5 to +5 Vdc
Vdc
OP AMP
Scaling CKt.
V Scaler
To Data
Acquisiton
Card Analog Input
Figure 6. Output Signal Flow Block Diagram Showing Desired Signal Levels.
The input of the voltage measurement circuit should have a very high input impedance
to minimize the loading effect, so a voltage follower circuit like the one used in Task 3 is
used. The maximum output voltage expected from the test amplifier is 12.5 V ac peak.
This corresponds to a value of 8.8375 V ac RMS. The true RMS converter chip can
only accept voltages up to 200 mV RMS, so the ac level must be reduced.
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A resistive voltage divider is the simplest design that accomplishes this function. When
using a voltage divider, consider the loading effects of the divider resistors on the other
stages. An inverting OP AMP amplifier can also provide the gain reduction necessary.
If an OP AMP is used, the stage should include a variable input resistor to allow
calibration of the overall circuit. An active low-pass filter similar to the one described in
the input circuit can reduce the noise of the circuit and also prove the necessary
attenuation. The 180 degree phase of the inverting amplifier will not affect the
measurements since only a gain plot of the frequency response is desired.
ICs manufactured by Analog Devices performs a true RMS conversion of the ac wave.
These chips are AD737A and AD736. Data sheets for these chips are available in pdf
format. They show typical application circuits and gives the chip's specifications and
limitations. Verify that one will satisfy the design specifications before starting
construction of the application circuits. The chip output is a dc value that is numerically
equal to the RMS value of the input waveform.
The following equation gives the mathematical definition of RMS.
v rms 
Where
1 T
2
0 vt  dt ,
T
vrms = the root mean square value of the voltage,
v(t) = the time varying voltage waveform,
T = the period of the waveform.
To find the true RMS value of an ac wave the waveform function must be squared.
Integrating the resulting function over a single period and dividing by the reciprocal of
the period gives the average value of the squared function. Taking the square root
yields the RMS value of the wave. Using this mathematical definition on any waveform
will always give the correct RMS value. The AD737/736 implements this equation using
integrated analog electronics. Read the circuit operation section in the data sheets and
summarize it for your circuit description.
The output of the AD737A/736 must be scaled to take full advantage of the input range
of the analog input of the data acquisition card. The analog input range is -5 to +5 V
dc. The AD737 has an output range of 0- -200 mV dc for the given range of input. An
OP AMP circuit with a gain of -1 can change this negative voltage to a positive value.
The linear scaling circuits shown in Figure 7 can then implement the proper voltage
scaling. Note that the upper circuit is for the AD736 device and the lower is for the
AD737 converter.
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+12V
R8
R6
R3
R1
R2
R9
D1
R5
U2
U1
+12V
Vin
Vout
R4
R7
R14
Vrms
R11
R15
R10
1
U3
Vout2
R12 3
2
D2
Figure 7. Scaling Circuit for the Output Voltage Signal.
The output of this circuit connects to an analog input of the data acquisition card.
The input voltage circuit should be calibrated and tested before adding it to the system.
Calibration requires performing two operations, zeroing and spanning. To zero the
overall circuit:
a.)
b.)
c.)
d.)
Ground the input voltage, Vin in Figure 6.
Measure the outputs from each OP AMP. Each output should be at zero
except the scalar circuit output, Vscaler in Figure 7, which should read
–5 Vdc. If the Z-buffer or input scaling stages are not zero with a grounded
input, add an offset nulling circuit to the appropriate stage. This circuit is
shown in OP AMP data sheets
Apply the full-scale voltage to Vin and measure the voltages at each stage
in the circuit. If the measured values do not match the theoretical values,
adjust the variable resistors in each stage to eliminate the differences.
Repeat the above steps until the system is values match the theoretical
values to within the measurement tolerances.
The input signal circuits must provide accurate output over a wide range of inputs. The
test amplifier output signal varies from ten's of volts to tenths of volts over the specified
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frequency range. The designed circuit must be tested at different voltage levels in this
range to verify correct operation. The attached tables give the levels and frequencies
for the circuit tests.
Desired Results
1.)
2.)
3.)
4.)
5.)
A working circuit that uses hardware to produce a scaled dc value proportion to
RMS value of the ac output voltage.
Schematic drawing of the hardware used in the circuit with all component values
shown.
A block diagram, similar to Figure 6 that shows an overview of the circuit and the
range of inputs and outputs.
Test results for the circuit that verifies the operation of the hardware at several
voltage and frequency values of sinusoidal input. The attached tables give the
frequencies and input levels for the tests.
A LabVIEW program that displays the measured value of input voltage scaled to
volts RMS on a PC. Scaling equations that relate the input voltage to scalar
output and scalar output to volts RMS.
What to Present for Evaluation
1.)
2.)
3.)
4.)
5.)
6.)
7.)
Table with lab measurements, signed and dated.
Calculations for the component values used in the design. (Neatly done)
Schematic of the design with labeled component values.
A block diagram for the system.
A working model of the output voltage measurement system that includes a
LabVIEW program. (schedule with instructor or T.A. for demonstration)
A graph with a plot of the data in Table 4-1 and a plot of the linear approximation
line.
Short discussion (1- 2.5 pages double spaced) of the circuit theory that includes
the formulas used to compute the component values and an explanation of the
circuit operation.
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Task 4 Lab Measurements
Table 4-1 Output Voltage Scaling Test
Test Frequency: 10 kHz
Vin (V peak ac )
0.5
Vamp (V peak ac)
VRMS (Vdc)
Vscaler (Vdc)
1.0
1.5
2.0
3.0
4.0
6.0
8.0
10.0
12.0
Low level input measurements
Table 4-2 Input Voltage Frequency Response
Frequency (Hz)
Vin
(mV peak ac)
20
200
200
200
800
200
2000
200
8000
200
10,000
200
16,000
200
Vamp
(V peak ac)
VRMS
(Vdc)
Vscaler
(Vdc)
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Intermediate level input measurements
Table 4-3 Input Voltage Frequency Response
Frequency (Hz)
Vin
(V peak ac)
20
2.0
200
2.0
800
2.0
2000
2.0
8000
2.0
10,000
2.0
16,000
2.0
Vamp
(V peak ac)
VRMS
(Vdc)
Vscaler
(Vdc)
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
High level input measurements
Table 4-4 Input Voltage Frequency Response
Frequency (Hz)
Vin
(mV peak ac)
20
10.0
200
10.0
800
10.0
2000
10.0
8000
10.0
10,000
10.0
16,000
10.0
Vamp
(V peak ac)
VRMS
(Vdc)
Vscaler
(Vdc)
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
Spring 2014
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Task 5 – Design of a Sinusoidal Voltage-Controlled Oscillator (VCO)
Technical Details and Desired Results
Objective
Experiment with a commercially available single chip function generator. Design a
voltage-controlled oscillator that will provide a range of sinusoidal frequencies from 20
Hz to 20 kHz. The output should have low harmonic distortion. It should also have a
variable output level that is sufficiently high to drive the test circuit. The wide range of
frequencies requires a design that has three ranges. These ranges will be:0-400 Hz,
0-4000 Hz, 0-40,000 Hz.
Prelab Preparation
Access the Exar website and download the data sheet of the XR2206 precision function
generator. Also, locate and download any application note or design using this device
from the web.
Theoretical Background
Generating a frequency response plot requires that a number of pure sinusoidal signals
at different frequencies be injected into the test circuit. In manual tests, a function
generator produces these signals. The individual performing the test adjusts the
frequency to each required test point. A voltage-controlled oscillator allows an analog
output from a data acquisition board to automatically set the frequency of the signal
source without human intervention. The analog output from the computer data
acquisition board is limited to the range 10 V dc this voltage will drive the VCO
in the test system. Make sure the analog output and the VCO has correct
interface circuits to drive it over the entire range of operation.
Exar produces a single chip precision fucntion generator/voltage controlled oscillator,
the XR-2206; that has high accuracy sine, triangle and square wave outputs. A
frequency range of 2000:1 is possible with an external control voltage. The usable
frequency range is 0.01 Hz to 1 MHz.
Figure 8 shows the functional diagram of the function generator. The XR-2206 has four
function blocks; a voltage controlled oscillator, (VCO), an analog multiplier and sine
shaper; a unity gain amplifier; and two current switches. The VCO produces a
frequency that is proportional to an input current on either pin seven or eight. This
current is set by timing resistors connected from both pin and ground. Two timing
resistor inputs allow the device to generate two discrete frequencies. The FSK input
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Vcc
4
Timing
Capacitor
GND
12
Bias
10
SYNCO
11
TC1
5
VCO
TC2
6
Timing
Resistors
TR1
7
TR2
8
Current
Switches
FSKI
9
Analog Multiplier
And
Sine
Shaper
AMSI
1
WAVEA1
13
+1
STO
2
MO
3
WAVEA2
14
SYMA1
15
SYMA2
16
Figure 8. Functional Diagram of the XR-2206 Function Generator.
switches the current inputs to the VCO and controls its frequency.
External components set the free-running frequency and duty cycle of the function
generator. Figure 9 shows the XR-2206 connected with the minimum number of
external component required to have a functional circuit. The resistors R 1 and R2 set
two free running frequencies according to the following formulas.
f1 
1
R1  C
f2 
1
R2  C
Where f1 and f2 are in Hertz. The values of R1 and R2 are between 4k and 200Kfor
proper operation The recommend values of C are between 1000 pF and 100 F. The
voltage at pin 9 controls which frequency is active. Disconnecting pin 0 activates the
VCO at f1, while grounding the pin activates the VCO at f 2.
The external components between pins 13 and 14 determine the type of wave output.
When switch S! is closed the device produces a sine wave on pin 2. When it is open
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the output is a triangle wave. For this project, connect a 500 ohm potentiometer
between pins 13 and 14 to produce a low distortion sine wave.
.
Vcc
1 F
4
11
5
Square Wave Out
VCO
C
6
R1
R2
7
8
Current
Switches
9
FSK In
THD Adj.
13
RA 500
S1
2
+1
Analog Multiplier
And
Sine
Shaper
Triangle or
Sine
Out
14
15
Sym. Adj.
RB
25k
16
12
10
3
1 F
Output
Adj. R3
25k
Vcc
1 F
5.1k
5.1k
Figure 9. XR2206 Waveform Generator Showing External Connections.
The potentiometers labeled RA and RB in Figure 9 reduce the total harmonic distortion
of the sine wave. The adjust the output for minimum distortion, set RB at midpoint and
adjust RA to minimum distortion. With RA set, adjust RB to further minimize distortion.
Figure 10 shows the external connections for frequency sweep operation. The
frequency of the VCO is proportional to the total current, IT, drawn from either pin 7 or 8.
The timing pins 7 and 8 are internally biased at 3 Vdc with respect to pin 12. The
relationship between current and frequency is linear over a wide range of input current
(1 A to 3 mA) and is given by the following equation. Exceeding 3 mA for I T will
damage the device
f
320  I T (mA )
Hz
C(F)
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Ic
IT
7 or 8
RC
Vc
IB
R
+3 V
12
Figure 10. External Control Voltage Connections
The relationship between control voltage, Vc and the output frequency is
 1   R
f 
 1  
 R  C   Rc
  Vc
  1 
3
 
 
  Hz

The voltage-to-frequency conversion gain, K is given by:
K
 0.32
Hz/V
Rc  C
Note that this gain is negative indicating that the frequency will decrease a control
voltage increases. Also note that in the first equation a value of Vc=3 will cause the
VCO output frequency to equal the value of 1/RC. This is the free running frequency of
the device.
The resistor, R3 controls the output amplitude. The output is approximately 60 mV per
thousand ohms of R3 value. For a uni-polar dc supply, the output will have a dc level of
one half the supply value. Implement a high pass filter with a cut off frequency of 2 Hz
or less to eliminate the dc level
Connecting three different capacitor values between pins 5 and 6 produces three
operating ranges. A selector switch gives manual control of these ranges for circuit
testing. Integrated circuit relays with coils that operate at TTL levels can be used in
place of a selector switch. Digital output from the controlling computer can close the
desired relay contact to select a range.
The XR-2206 can operate from bipolar power supplies but is limited to +-12 Vdc. Refer
to the device data sheet for a description of how to connect this device for bipolar
operation.
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Task 5 Supplement
VCO DAC Scaling and Interface Circuit
This task will enhance the operation of the final project. If time permits, attempt this
part of the VCO design.
The NI6024E and NI 6221data acquisition cards have fixed analog output ranges.
These ranges extend from -10 to +10 Vdc. The VCO frequency changes with the input
control voltage according to Equation 1.
 1   R   Vc  
f 
 1 
(1)
 Hz
 1
3  
 R  C   R c  
Where
Vc= the external control voltage
R = timing resistor connected to either pin 7 or 8.
Rc = series resistor connected between Vc and pin 7 or 8. (See Figure 10)
Pins 7 and 8 are internally biased to 3 Vdc and the value of V c should not fall below this
value. Placing Vc=3 into Equation 1 give the maximum output frequency. As Vc
increases the output frequency falls. The rate of decrease depends on the gain of the
VCO. Setting f=0 and solving the resulting equation gives the maximum control
voltage. Equation 2 shows this relationship
Where
R

Vc,max  3   c  1
R

Vc,max = Value of control voltage that gives zero frequency.
(2)
To achieve the lowest possible VCO frequencies, Vc must be able to reach the value
given above. The highest VCO frequency occurs when V c reaches 3 Vdc. Voltage
input that causes more the 3 mA to flow from either pins 7 or 8 damages the device.
Design Task
Develop an interface circuit that scales the DAC output to meet the input requirements
of the VCO. Figure 2 shows a block diagram of the desired circuit. The circuit design
should produce the desired VCO input span for the DAC span of 20 V dc (+10 to -10 V)
without allowing the VCO input to fall below 3 Vdc. The values of +V cc and –Vcc are
12 V dc. Use Zener diodes to provide stable voltage references for the scaling circuit.
The reference voltages should have potentiometer controls to provide for circuit
adjustment.
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DAC output
-10 to +10 V dc
VCO
DAC Scaler
fmin - fmax Hz
3 – Vc,max Vdc
Figure 11. DAC VCO Interface Circuit Block Diagram.
Figure 3a shows that input/output characteristic of the VCO decreases with increasing
Vc. Figure 3b show the desired characteristic for the overall system.
fmax
fmax
fmin
fmin
3 Vdc
(a)
Vc
Vc,max
(b) -10 V
DAC output
+10 V
Figure 12. Existing and Desired VCO Input/Output Characteristics.
A oscilloscope that has an Fast Fourier Transform function can decompose the
sinusoidal output of the VCO into its frequency components. A pure sine wave will only
have a single frequency component. A distorted sine wave will have a number of
frequency components at integer multiples of the VCO set frequency. The total
harmonic distortion (THD) of the VCO sine wave can be found from the following
equation.
2
2
2
A 
A 
A 
%THD  100   2    3    4   .....
 A1 
 A1 
 A1 
where
A1 = the amplitude of the fundamental VCO
frequency in volts
A2 = the amplitude of the second harmonic of the VCO
frequency in volts
A3 = the amplitude of the third harmonic of the VCO
frequency in volts
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A4 = the amplitude of the fourth harmonic of the VCO
frequency in volts
Use the FFT scope function to measure sine output of VCO. Record the magnitudes in
dB of the first five harmonic components in Table 5-1. Finally, use the equation above
to compute the percent total harmonic distortion and record the values in Table 5-2.
Desired Results
A working prototype voltage control oscillator that has a 20 to 20 kHz frequency range
of sinusoidal output. This range must be divided into three ranges that can be computer
selected. These ranges are: 10-400 Hz, 150-4000 Hz, 2000-40,000 Hz. The output
level must be adjustable to 200 mV peak output. The sine wave should have a low
distortion output. (1% or less). The VCO should operate with power supply values of
15 V dc.
What to Present for Evaluation
1.)
2.)
3.)
4.)
Table with lab measurements signed and dated.
A schematic diagram of the design with all components labeled with values.
A working prototype of the design. (Schedule time with the instructor or the T.A.)
A short discussion (2 – 2.5 pages double spaced) describing the chips operation
and the theory of operation of the circuit. This discussion should include any
design formulas used to find component values.
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Task 5 Lab Measurements
VCO Output
Frequency (Hz)
20
40
80
160
200
400
800
1200
1600
2000
4000
8000
10,000
12,000
16,000
20,000
Table 5-1 Harmonic Distortion Levels
A1 (dBV)
A2(dBV)
A3 (dBV)
A4 (dBV)
A5 (dBV)
Instructor Initials _______________
Date ____________
Designer ______________________________ (Sign here)
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Task 5 Lab Measurements
Table 5-2 Total Harmonic Distortion Calculation
VCO Output
VCO Input
Total Harmonic
Output Voltage
Frequency (Hz)
(Vdc)
Distortion (%)
(ac V peak)
20
200 mV
40
200 mV
80
200 mV
160
200 mV
200
200 mV
400
200 mV
800
200 mV
1200
200 mV
1600
200 mV
2000
200 mV
4000
200 mV
8000
200 mV
10,000
200 mV
12,000
200 mV
16,000
200 mV
20,000
200 mV
Instructor Initials _______________
Date ____________
Designer ______________________________ (Sign here)
Spring 2014
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Task 6-Frequency-to-Voltage Conversion of Input Signal
Technical Details and Desired Results
Objective
Determine how to make frequency measurements using the available inputs and
outputs of the data acquisition hardware. Consider using a linear IC that converts the
frequency to an analog voltage (LM2907 or equivalent). An alternative is to use digital
counters. The data acquisition hardware has counter hardware that can be used to
measure frequency.
Prelab Preparation
Check the Web sites of suppliers such as National, Analog Devices, Burr-Brown, and
Linear Technologies for specifications and pricing on IC’s that perform frequency to
voltage conversion. One such device is an LM2907. This IC produces a linear analog
output for a range of frequency inputs. Check National Instruments website to find
information about measuring frequency using the counters. Investigate the online help
feature of LabVIEW to find what software resources are available to measure
frequency. Locate and copy the pin-out diagrams for the NI data acquisition board
used with your test step-up. The course online project book should have these items.
Technical Specifications and Design Alternatives
There are several ways to measure frequency using a computer based measurement
system. One technique would involve the use of timer circuits built into the data
acquisition hardware. The hardware provided does have this feature. Two design
alternatives exist:
1.) Using external signal conditioning to convert the input test signal's frequency into a
dc value that is proportional to the frequency. This signal can then be input to an
analog channel just like the input and output voltage magnitudes.
2.) Use a timer on the data acquisition board to measure the period of the square wave
output of the VCO. This method uses the least external hardware but requires a
LabVIEW program. Completing the task will require a LabVIEW program to input
the timer measurement and convert it to a frequency. The data acquisition board
(DAQ) digital inputs require TTL compatible input signals. Construction conversion
circuits that will interface the VCO to the (DAQ) with the proper levels.
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The technical specifications below are given for alternative 1- frequency-to-voltage
conversion.
Input frequency: 20 – 20,000 Hz
Range 1: 10-400 Hz
Range 2: 150-4000 Hz
Range 3: 2000-40,000 Hz
Input voltage level: 200 mV peak sinusoidal ac voltage
Output: 0 - 10 V dc: scale to 5V dc for analog input
Output Ripple (max): 50 mV
Power Supply: +15 V dc
Analog Frequency-to-Voltage Conversion
Figure 13. LM2907 Frequency-to-Voltage Converter: 14-pin package.
The LM2907 is an integrated circuit that can convert the frequency of an input signal
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into a dc voltage that varies linearly with the frequency. The device was designed for
speed measurement applications such as hand-held tachometers, cruise control, and
speed governors. This device can also be used to convert the frequency of electronic
signals to an analog voltage if the range of frequencies is low. This device may be
useful in the design of the automated frequency response tester. The device pin out
and a block diagram of the internal circuit are shown in Figure 11.
The device requires only three additional external components to make it operational.
There are three sections integrated into the LM2907: the Schmitt trigger input, the
charge pump, and the output stage. The Schmitt trigger converts the input waveform
into a square wave that is used to drive the charge pump. The Schmitt trigger also
introduces a 15-20 mV hysteresis which prevents false triggering of the converter. With
the internally grounded version of the LM2907 shown above, the input signal must
swing above and below ground for the Schmitt trigger to operate properly.
The charge pump converts the frequency of the input signal into a dc voltage by
charging the capacitor C1 with a constant current (approximately 200 A). The charge
pump applies the constant current to C1 causing it to charge to a voltage equal to V cc/2.
At each zero crossing of the input signal C1 is either charging or discharging to Vcc/2.
So in the time it takes for half a cycle of the input signal, the change on the capacitor is
equal to (Vcc/2) x C1. The average current into C1 then is equal to V cc x fi x C1. The
charge pump mirrors the current in pin 2 to pin 3. The resistor, R1, connected to pin 3
produces voltage impulses that are proportional to the frequency of the input. If an
integrating filter capacitor, C2 is added to pin 3, then a filtered dc voltage that is
proportional to the input frequency develops at this pin. The output to pin 3 is given by
V3 = Vcc x fi x C1 x R1
(1)
Pin 3 is internally connected to the output stage in this version of the LM2907. The
output stage consists of an OP AMP and a output driver transistor. This output stage
can be connected in many different configurations. The simplest connection is to
connect the OP AMP as a voltage follower and allow the voltage on pin 3 to be reflected
in resistor R2 on pin 4. When this connection is used, the final output is given by
Vo = Vcc x fi x C1 x R1
(2)
Several other design formulas are necessary to complete the implementation of a
frequency-to-voltage converter using the LM2907. The maximum frequency that the IC
can register depends on the value of supply voltage and also the size of the capacitor
C1. The maximum input frequency is given by the following equation:
fi(max) 
I2
C1  Vcc
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Where:
fi(max) = maximum input frequency (Hz)
I2 = charge pump constant current (typical value is 150 A)
Vcc = supply voltage
Sizing of the filter capacitor, C2, depends on several factors. Equation (4) gives the
ripple voltage on the output.
Vcc  fi  C1
 V   C1  
 ,
Vr (pp )   cc   
  1 
I2
 2   C2  

Where
(4)
Vr(p-p) = the peak-to-peak ripple voltage.
The maximum ripple voltage will occur at the lowest input frequency. This would be 20
Hz for the frequency response measurements required. Sizing C2 at this frequency will
give the most conservative design.
Setting the size of C2 also effects the response time of the frequency-to-voltage
converter. A large filter capacitor will make for a slow output response when the
frequency of the input changes. The response time of the IC will increase as the size of
C2 increases. Large values of R1 will also slow the response for a given value of C2.
The sizing of C2 will be a design compromise between low ripple voltage and response
speed.
Several factors must be met when sizing R1. Pin 3 is a constant current source so size
the value of R3 with the following formula:
R1 
Where
V3 max
I3 min
(5)
V3max = maximum full-scale output voltage required
I3min = minimum C2 charging current (use 150 A)
This will prevent a reduction of the linearity of the current source due to excessive
loading.
When designing applications using the LM2907, four parameters must be known for the
values of C1, C2, and R1 to be computed. These parameters are:
1) Maximum output voltage
2) Maximum input frequency
3) Maximum allowable output ripple voltage
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4.) Supply voltage
Once these values have been determined, follow these steps to compute the values of
the external components.
1.) Compute the value of R1 using equation 5.
2.) Using the equation below find the value of C1 (should be greater than
100 pF for optimal operation)
V3 max
C1 
R1  Vcc  fi max
3.) Find the value of C2 from the equation below
4.) Select a load resistor for R2. This value should not be so low as to
overload the output transistor. Acceptable values are between 1k
and 10k ohms.
 V  C1  Vc  fi  C1
1 

C2   c 
I2
 2  Vr ( pp ) 

Construct the circuit and check the response time of the circuit. Slow response will
require the adjustment of the values of R1 and C2 changes response time to achieve a
performance balance between low ripple and acceptable response time.
Design Note Concerning Accuracy
Using the LM2907 may be a simple solution, but in this application, there could be
problems concerning the accuracy of the measurements that the IC will give. The
range of frequency measurement covers 3 decades of frequency: 20 Hz to 20,000 Hz.
The voltage span of the analog to digital converter inputs is 10 V, which gives a gain
slope for this measurement of 0.5 mV/Hz approximately. A small offset voltage or
calibration error that may not have significant effects at the upper range of frequency
will cause large errors in the measurements at the lower frequencies.
One solution to this problem is to design the frequency-to-voltage converter to have
three ranges: 10 – 400 Hz, 150 – 4,000 Hz, and 2000 – 40,000 Hz. Figure 12 shows
the analog switch chip, CD4066, controlling three values of the capacitor C1 to give
three ranges of frequency measurement. The designer can substitute integrated relays
with TTL compatible coils for the CD4066 to achieve the same result.
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LabVIEW software can control the range switching of both the VCO and frequency-tovoltage (F-V) converter. A window comparator, implemented in software, will take the
input frequency from the test program and compare it to upper and lower set point
values. This will then write values to the digital I/O port that will activate the appropriate
range of both the VCO and F-V converter.
Figure 14. Frequency Range Controls Using an Analog Switch.
Procedure
1.)
2.)
3.)
Using the method from above, design a converter circuit that will satisfy the
design requirements of the project.
Construct the circuit and test it by applying a 2 V p-p sine signal with the
frequencies shown in the attached data table. Reduce the voltage input to 200
mV peak and see if the circuit performance is affected.
Add a summing amplifier and a sign changing amp to scale the output of the
converter to the range desired ( 5 Vdc) Place these values in the attached data
table
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4.)
Determine the linear equations that relate the output voltage to the input voltage
for all three ranges. Use the best fit linear approximation to determine the
equations parameters. Include these equations in the circuit description.
For Alternative 1: Analog Frequency-to-Voltage Conversion
Desired Results
1.)
2.)
3.)
4.)
5.)
6.)
A working circuit that produces a scaled dc output value that is proportional to
the input frequency.
A LabVIEW program that will activate the three ranges of the circuit and display
the frequency on a PC.
A schematic drawing of the hardware used in the circuit.
Graph of the LM2907 output voltage plotted against the frequency for all three
ranges.
Graph of the scaler output voltage plotted against the frequency for all three
ranges.
Compute the slope and intercepts using a best fit approximation of the plotted
lines and place the values on the graphs in 3 and 4
What to Present for Evaluation
1.)
2.)
3.)
5.)
6.)
7.)
8.)
Table with lab measurements, signed and dated.
Calculations for the component values used in the design (neatly done)
Schematic of the design with component values included.
A working model of the system. (schedule with instructor or T.A. for
demonstration)
Graph of the LM2907 output voltage plotted against the frequency for all three
ranges
Graph of the scaler output voltage plotted against the frequency for all three
ranges. Linear equations for each of the three ranges
Short discussion (1- 2.5 pages double spaced) of the circuit theory that includes
the formulas used to compute the component values and an explanation of
circuit operation.
For Alternative 2: Frequency Measurement Using Digital Counters
Desired Results
1.)
A combination of external connections and software that will accurately measure
the three frequency ranges of the sine signal source.
2.)
Write a LabVIEW program that displays the frequency on a PC.
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3.)
4.)
5.)
A schematic drawing showing the data acquisition board external connections.
A plot of the system’s accuracy compared to the function generator reading for
all three ranges.
Interface circuits for converting the VCO output into a compatible input for
frequency measurement.
This design uses the counters on the NI data acquisition (DAQ) board to measure
frequency. Figure 13 shows a bock diagram of this solution method.
+Vcc
2.5 <V< 5.0 Vdc
0V
-Vcc
0V
VCO
Vin
Voltage Clipping
and Level Clamping
Circuit
Vout
LabVIEW
Program
DAQ Terminal
Board Counter
Connections
MAX Task
Definition
Software
Figure 15. Counter Frequency Measurement Block Diagram
This solution takes the square wave output of the VCO and converts it to a TTL
compatible voltage level. (0-5 Vdc). This TTL signal then connects to the appropriate
terminals on the DAQ interface board. The input from the VCO must be at TTL levels
since damage may occur to the DAQ board if its input exceeds the recommended
levels. The square wave output of the VCO may vary between positive and
negative power supply values or from ground to the positive supply value. This
will depend on the chosen power supplies for the VCO stage. Consult with the
designers of the VCO before designing the clipping/clamping circuit to determine the
power supply utilized.
Use the pin-out diagrams for the appropriate DAQ board to inject the TTL signal
properly into the digital counters used to measure frequency. The measurement and
automation explorer software (MAX) installed on all lab computers gives important
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information regarding the terminal board connections. Define a frequency
measurement task in the MAX and select the measurement range. View the online
tutorials to find out more details on this subject. Once defined, the software will give the
correct connections for the terminal board.
The final stage in this tasks requires the designer to develop a short program to display
the frequency measurements. LabVIEW provides subVI’s for many common tasks
such as frequency measurement. Consult the software help and the online tutorial
videos for more details on where and how to develop this software.
To test this sub-system, Set the output of a function generator to give the waveform
expected from the VCO design. This could be a bipolar or uni-polar square wave
depending on the VCO designer’s power supply choice. Complete the alternative 2
measurement tables using this test signal.
What to Present for Evaluation
1.)
2.)
3.)
4.)
5.)
Table with lab measurements, signed and dated.
Schematic of data acquisition board external connection and interface circuits.
Print out of LabVIEW program used to display the frequency values.
Graph of system accuracy plotted against the frequency for all three ranges.
Short discussion (1- 2.5 pages double spaced) of the circuit theory that includes
the formulas used to compute frequency using counters and an explanation of
circuit operation.
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Task 6 Lab Measurements
Frequency Converter Measurements
Alternative 1
Table 6-1 – Range 1 Measurements
Frequency
(Hz)
Input
Voltage
(ac p-p)
20
2V
40
2V
60
2V
80
2V
120
2V
160
2V
180
2V
200
2V
220
2V
240
2V
300
2V
325
2V
350
2V
400
2V
Converter Output
Voltage (Vdc)
Scaler Output
Voltage (Vdc)
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Task 6 Lab Measurements
Frequency Converter Measurements
Alternative 1
Table 6-2 – Range 2 Measurements
Frequency
(Hz)
Input
Voltage
(ac p-p)
200
2V
400
2V
600
2V
800
2V
1200
2V
1600
2V
1800
2V
2000
2V
2200
2V
2400
2V
3000
2V
3250
2V
3500
2V
4000
2V
Converter Output
Voltage (Vdc)
Scaler Output
Voltage (Vdc)
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Task 6 Lab Measurements
Frequency Converter Measurements
Alternative 1
Table 6-3 – Range 3 Measurements
Frequency
(Hz)
Input
Voltage
(ac p-p)
2000
2V
4000
2V
6000
2V
8000
2V
12000
2V
16000
2V
18000
2V
20000
2V
22000
2V
24000
2V
26000
2V
30000
2V
35000
2V
40000
2V
Converter Output
Voltage (Vdc)
Scaler Output
Voltage (Vdc)
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Task 6 Lab Measurements
Frequency Converter Measurements
Alternative 2: Digital Measurement
Table 6-1 – Range 1 Measurements
Frequency
(Hz)
Measured
Frequency
(DAQ)
Percent Error
20
40
60
80
120
160
180
200
220
240
300
325
350
400
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Task 6 Lab Measurements
Frequency Converter Measurements
Alternative 2: Digital Measurement
Table 6-2 – Range 2 Measurements
Frequency
(Hz)
Measured
Frequency (DAQ)
Percent Error
200
400
600
800
1200
1600
1800
2000
2200
2400
3000
3250
3500
4000
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Task 6 Lab Measurements
Frequency Converter Measurements
Alternative 2: Digital Measurement
Table 6-3 – Range 3 Measurements
Frequency
(Hz)
Measured
Frequency (DAQ)
Percent Error
2000
4000
6000
8000
12000
16000
18000
20000
22000
24000
26000
30000
35000
40000
Instructor initials ______________
Date taken ___________
Designer ____________________________ (sign here)
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Task 7-User Interface and Data Conversion Using LabVIEW
Technical Details and Desired Results
Objective
Develop the program that will collect the data and display the information on the user
interface. The program will also save the results to disk using the data collection
software provided. Build the user interface based on given specifications. This task
must use scaling information from all the Tasks above to be able to display correctly the
information to the user. The designer must develop a working knowledge of the data
collection software to complete this task.
Prelab Preparation
1.) View online videos describing how to construct software interface.
2.) Review the software examples given with the software help files
3.) Read and/or perform exercises from the tutorial materials provided
Procedure
1.)
Construct a control program using the LabVIEW data flow programming
language. The list below gives the required basic functions of the program.
Basic Control Process Steps for Data Acquisition Software
1.)
2.)
3.)
4.)
5.)
6.)
7.)
Read the test frequency from the array of test points.
Determine the range that the test frequency falls. Turn on an indicator lamp on
the display that indicates the range.
Activate the digital I/O line to set the correct ranges on the VCO and the F-V
converter.
Output the appropriate decimal value to an analog output channel on the data
acquisition (DAQ) hardware. Decimal values cause the DAC to produce an
output voltage that is an input to the VCO chip. The values should give test point
frequencies at the VCO output.
An analog input reads the value of input voltage to the EUT.
a.) scale analog value to original units and display the result
An analog input reads the value of output voltage from the EUT
a.) scale analog value to original units and display the result
An analog input reads the value of output voltage from the Frequency-to-voltage
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9.)
10.)
11.)
12.)
13.)
conversion process.
a.)
scale analog voltage value to frequency units and display the result
b.)
If the timer method of frequency measurement is used, read the period
information and convert it to a frequency using a LabVIEW program.
Read the next decimal value that will produce the next test frequency.
Go to step 1. If this is the last array element, go to the next step.
Compute the decibel value of the test frequency from the input and output
voltage measurements.
Plot the results on a semi-log graph.
Save value of decibel and frequency to disk file for further analysis.
Figure 14 is a flowchart that illustrates the steps described above.
Start
Display
graph
and write file
Stop
Calc
dB
YES
Read frequency
value from array
on front panel
Determine
Frequency
Range
Select range
and convert f into
VCO input V
Write range value
to digital
port
NO
Store current Vin,
Vo and f
Wait 0.5-2 s
Light Range
LED
Display
VCO
Frequency
Write VCO to
analog output
Wait 0.2-0.5 s
Last Test
Frequency?
Display
current
values of
Vin, Vo f
Read analog
inputs for Vin,
Vo and f
Scale analog
inputs for Vin,
Vo and f
Figure 16. Test Procedure Flow Chart.
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Figure 17. GUI for Frequency Response Tester.
2.)
Construct the GUI using the LabVIEW components contained in the LabVIEW
software. The user interface is shown in Figure 15. The frequency response
measurements should begin after the on/off switch is pressed with the mouse.
Desired Results
1.)
A working system that graphs the measurements as they are being taken and
shows the digital values of the measurements.
2.)
A block diagram of the software process that includes the parameters of the
blocks used to program the system.
3.)
An operational GUI that allows the user to start and stop the test.
What to Present for Evaluation
1.)
2.)
3.)
A block diagram of the design.
A working model of the system. (Schedule with instructor or T.A. for
demonstration)
A written discussion (2- 3 pages double spaced) of the software operation and
design.
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