Southern Illinois University - Carbondale



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 4

Organization, Management, and Evaluation of the Project 4

Task List 5

Technical Details of the Project 6

Graphical User Interface (GUI) and Displayed Variables: 7

Data Acquisition Board General Specifications 8

Task 1 - Function Generator Voltage-to-Frequency Characteristic 9

Objective 9

Procedure 9

Linear Least-Squares Curve Fitting 10

Desired Results 10

Table 1-1 Range 1 Data 12

Table 1-2 Range 2 Data 13

Table 1-3 Range 3 Data 14

Task 2 -Construction of Test Amplifier 15

Objective 15

Procedure 15

Task 2 Amplifier Measurements 17

Task 3-Input Voltage RMS Conversion and Scaling 18

Objective 18

Design Criteria 18

Task 3 Lab Measurements 22

Table 3-1 Input Voltage Scaling Test 22

Table 3-2 Input Voltage Frequency Response 22

Task 4-Output Voltage Scaling and RMS Conversion 23

Objective 23

Design Criteria 23

Desired Results 26

What to Present for Evaluation 26

Task 4 Lab Measurements 27

Table 4-1 Output Voltage Scaling Test 27

Table 4-2 Input Voltage Frequency Response 27

Table 4-3 Input Voltage Frequency Response 28

Table 4-4 Input Voltage Frequency Response 28

Task 5 – Design of a Sinusoidal Voltage-Controlled Oscillator (VCO) 29

Technical Details and Desired Results 29

Task 5 Supplement 33

Task 5 Lab Measurements 36

Table 5-1 Harmonic Distortion Levels 36

Table 5-2 Total Harmonic Distortion Calculation 37

Task 6-Frequency-to-Voltage Conversion of Input Signal 38

Objective 38

Prelab Preparation 38

Technical Specifications and Design Alternatives 38

Analog Frequency-to-Voltage Conversion 39

Design Note Concerning Accuracy 42

For Alternative 1: Analog Frequency-to-Voltage Conversion 44

Desired Results 44

What to Present for Evaluation 44

For Alternative 2: Frequency Measurement Using Digital Counters 44

Desired Results 44

Task 6 Lab Measurements 47

Table 6-1 – Range 1 Measurements 47

Table 6-2 – Range 2 Measurements 48

Table 6-3 – Range 3 Measurements 49

Task 6 Lab Measurements 50

Table 6-1 – Range 1 Measurements 50

Task 6 Lab Measurements 51

Table 6-2 – Range 2 Measurements 51

Table 6-3 – Range 3 Measurements 52

Task 7-User Interface and Data Conversion Using LabVIEW 53

Objective 53

Prelab Preparation 53

Basic Control Process Steps for Data Acquisition Software 53

Desired Results 55

What to Present for Evaluation 55

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.

[pic]

Figure 1. Block Diagram of the Frequency Response Testing System.

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 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.

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.

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.

[pic]

Figure 2. GUI for Frequency Response Tester.

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)

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. 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.

2. 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

3. Connect a multimeter to the wiper arm of the potentiometer so that the input voltage can be measured.

4. 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.

5. 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.

6. 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.

7. 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.

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:

[pic]

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.

[pic]

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 y-axis 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.

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.) Data Tables 1-1, 1-2, and 1-3 with lab measurements, signed and dated.

2.) Graphs of frequency vs. increasing and decreasing VCO voltage for each range.

3.) Calculation of maximum hysteresis for each range

4.) Graph of average VCO voltage vs. frequency for each range

5.) Best fit equations for all three ranges

6.) LabVIEW program that controls VCO frequency and changes ranges

Table 1-1 Range 1 Data

|Desired | Measured Frequency (Hz) |VCO voltage (Vdc) |VCO voltage (Vdc) |

|Frequency (Hz) | |(decreasing f) |(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)

Table 1-2 Range 2 Data

|Desired | Measured Frequency (Hz) |VCO voltage (Vdc) |VCO voltage (Vdc) |

|Frequency (Hz) | |(decreasing f) |(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)

Table 1-3 Range 3 Data

|Desired | Measured Frequency (Hz) |VCO voltage (Vdc) |VCO voltage (Vdc) |

|Frequency (Hz) | |(decreasing f) |(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)

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:

[pic]

Figure 3. Two-Stage Amplifier: Automated Test System Test Circuit.

C1 = 0.1 (F

C2 = 470 pF

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.

2.) 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.

3. 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 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.

4. 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

5.) 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.) Table with lab measurements, signed and dated.

2.) Calculations for the values of resistors computed in procedure (neatly done).

3.) Schematic of the design.

4.) Sample calculation for the decibel calculation.

5.) Graph of the circuit frequency response.

6.) 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.

Task 2 Amplifier Measurements

| | Input Voltage |Output Voltage |Circuit Gain |

|Frequency (Hz) |(ac peak) |(ac peak) |(db) |

|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 | | |

Instructor initials ______________

Date taken ___________

Designer ____________________________ (sign here)

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.

[pic]

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.

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.

[pic],

where 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.

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.

[pic]

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. Ground the input voltage, Vin in Figure 4.

b. 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, add an offset null circuit to the appropriate stage. The OP AMP data sheets give the connections for adding offset null to the IC.

c. 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.

d. Repeat the above steps until the system is values match the theoretical values to within the measurement tolerances.

Desired Results

1.) A working circuit that uses hardware to produce a scaled dc value proportional to the RMS value of the ac input voltage.

2.) Schematic drawing of the hardware used in the circuit with all component values shown.

3.) A block diagram, similar to Figure 4 that shows an overview of the circuit operation and the range of input and output voltages.

5. 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.

6. 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.) Table with lab measurements, signed and dated.

2.) Calculations for the component values used in the design (neatly done)

3.) Schematic of the design.

4.) A block diagram for the system.

5. A working model of the input voltage measurement system that includes a LabVIEW program. (schedule with instructor or T.A. for demonstration)

6. A graph with a plot of the data in Table 3-1 and a plot of the linear approximation line.

7. 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.

Task 3 Lab Measurements

Table 3-1 Input Voltage Scaling Test

Test Frequency: 10 kHz

| | | | |

|Vin (mV peak ac ) |Vamp (V peak ac) |VRMS (Vdc) |Vscaler (Vdc) |

|0 | | | |

|50 | | | |

|75 | | | |

|100 | | | |

|125 | | | |

|150 | | | |

|175 | | | |

|200 | | | |

|225 | | | |

|250 | | | |

Table 3-2 Input Voltage Frequency Response

| | Vin |Vamp |VRMS |Vscaler |

|Frequency (Hz) |(mV peak ac) |(V peak ac) |(Vdc) |(Vdc) |

|20 |200 | | | |

|200 |200 | | | |

|800 |200 | | | |

|2000 |200 | | | |

|8000 |200 | | | |

|10,000 |200 | | | |

|16,000 |200 | | | |

Instructor initials ______________

Date taken ___________

Designer ____________________________ (sign here)

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

[pic]

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.

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.

[pic],

Where 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.

[pic]

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. Ground the input voltage, Vin in Figure 6.

b. 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

c. 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.

d. 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 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.) A working circuit that uses hardware to produce a scaled dc value proportion to RMS value of the ac output voltage.

2.) Schematic drawing of the hardware used in the circuit with all component values shown.

3.) A block diagram, similar to Figure 6 that shows an overview of the circuit and the range of inputs and outputs.

4.) 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.

5. 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.) Table with lab measurements, signed and dated.

2.) Calculations for the component values used in the design. (Neatly done)

3.) Schematic of the design with labeled component values.

4.) A block diagram for the system.

5. A working model of the output voltage measurement system that includes a LabVIEW program. (schedule with instructor or T.A. for demonstration)

6. A graph with a plot of the data in Table 4-1 and a plot of the linear approximation line.

7.) 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.

Task 4 Lab Measurements

Table 4-1 Output Voltage Scaling Test

Test Frequency: 10 kHz

| | | | |

|Vin (V peak ac ) |Vamp (V peak ac) |VRMS (Vdc) |Vscaler (Vdc) |

|0.5 | | | |

|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

| | Vin |Vamp |VRMS |Vscaler |

|Frequency (Hz) |(mV peak ac) |(V peak ac) |(Vdc) |(Vdc) |

|20 |200 | | | |

|200 |200 | | | |

|800 |200 | | | |

|2000 |200 | | | |

|8000 |200 | | | |

|10,000 |200 | | | |

|16,000 |200 | | | |

Instructor initials ______________

Date taken ___________

Designer ____________________________ (sign here)

Intermediate level input measurements

Table 4-3 Input Voltage Frequency Response

| | Vin |Vamp |VRMS |Vscaler |

|Frequency (Hz) |(V peak ac) |(V peak ac) |(Vdc) |(Vdc) |

|20 |2.0 | | | |

|200 |2.0 | | | |

|800 |2.0 | | | |

|2000 |2.0 | | | |

|8000 |2.0 | | | |

|10,000 |2.0 | | | |

|16,000 |2.0 | | | |

Instructor initials ______________

Date taken ___________

Designer ____________________________ (sign here)

High level input measurements

Table 4-4 Input Voltage Frequency Response

| | Vin |Vamp |VRMS |Vscaler |

|Frequency (Hz) |(mV peak ac) |(V peak ac) |(Vdc) |(Vdc) |

|20 |10.0 | | | |

|200 |10.0 | | | |

|800 |10.0 | | | |

|2000 |10.0 | | | |

|8000 |10.0 | | | |

|10,000 |10.0 | | | |

|16,000 |10.0 | | | |

Instructor initials ______________

Date taken ___________

Designer ____________________________ (sign here)

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

[pic]

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 R1 and R2 set two free running frequencies according to the following formulas.

[pic] [pic]

Where f1 and f2 are in Hertz. The values of R1 and R2 are between 4kΩ and 200KΩ for proper operation The recommend values of C are between 1000 pF and 100 μF. 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 f2.

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 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.

.

[pic]

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 μA to 3 mA) and is given by the following equation. Exceeding 3 mA for IT will damage the device

[pic] Hz

[pic]

Figure 10. External Control Voltage Connections

The relationship between control voltage, Vc and the output frequency is

[pic] Hz

The voltage-to-frequency conversion gain, K is given by:

[pic] Hz/V

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.

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.

[pic] Hz (1)

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 Vc 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

[pic] (2)

Where Vc,max = Value of control voltage that gives zero frequency.

To achieve the lowest possible VCO frequencies, Vc must be able to reach the value given above. The highest VCO frequency occurs when Vc 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 +Vcc 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.

[pic]

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.

[pic]

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.

[pic]

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

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. Table with lab measurements signed and dated.

2. A schematic diagram of the design with all components labeled with values.

3. A working prototype of the design. (Schedule time with the instructor or the T.A.)

4. 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.

Task 5 Lab Measurements

Table 5-1 Harmonic Distortion Levels

|VCO Output Frequency (Hz) |A1 (dBV) |A2(dBV) |A3 (dBV) |A4 (dBV) |A5 (dBV) |

|20 | | | | | |

|40 | | | | | |

|80 | | | | | |

|160 | | | | | |

|200 | | | | | |

|400 | | | | | |

|800 | | | | | |

|1200 | | | | | |

|1600 | | | | | |

|2000 | | | | | |

|4000 | | | | | |

|8000 | | | | | |

|10,000 | | | | | |

|12,000 | | | | | |

|16,000 | | | | | |

|20,000 | | | | | |

Instructor Initials _______________

Date ____________

Designer ______________________________ (Sign here)

Task 5 Lab Measurements

Table 5-2 Total Harmonic Distortion Calculation

|VCO Output Frequency (Hz) |VCO Input (Vdc) |Total Harmonic Distortion (%) |Output Voltage |

| | | |(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)

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.

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

[pic]

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 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 Vcc/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 Vcc 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:

[pic] (3)

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.

[pic], (4)

Where 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:

[pic] (5)

Where 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

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)

[pic]

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.

[pic]

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.

LabVIEW software can control the range switching of both the VCO and frequency-to-voltage (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.

[pic]

Figure 14. Frequency Range Controls Using an Analog Switch.

Procedure

1.) Using the method from above, design a converter circuit that will satisfy the design requirements of the project.

2.) 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.

3. 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

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. A working circuit that produces a scaled dc output value that is proportional to the input frequency.

2. A LabVIEW program that will activate the three ranges of the circuit and display the frequency on a PC.

3.) A schematic drawing of the hardware used in the circuit.

4.) Graph of the LM2907 output voltage plotted against the frequency for all three ranges.

5.) Graph of the scaler output voltage plotted against the frequency for all three ranges.

6.) 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.) Table with lab measurements, signed and dated.

2.) Calculations for the component values used in the design (neatly done)

3.) Schematic of the design with component values included.

5. A working model of the system. (schedule with instructor or T.A. for demonstration)

6. Graph of the LM2907 output voltage plotted against the frequency for all three ranges

7. Graph of the scaler output voltage plotted against the frequency for all three ranges. Linear equations for each of the three ranges

8. 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.

3.) A schematic drawing showing the data acquisition board external connections.

4.) A plot of the system’s accuracy compared to the function generator reading for all three ranges.

5.) 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.

[pic]

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 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.) Table with lab measurements, signed and dated.

2.) Schematic of data acquisition board external connection and interface circuits.

3.) Print out of LabVIEW program used to display the frequency values.

4.) Graph of system accuracy plotted against the frequency for all three ranges.

5.) 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.

Task 6 Lab Measurements

Frequency Converter Measurements

Alternative 1

Table 6-1 – Range 1 Measurements

|Frequency (Hz) |Input Voltage (ac |Converter Output Voltage (Vdc) |Scaler Output Voltage (Vdc) |

| |p-p) | | |

|20 |2 V | | |

|40 |2 V | | |

|60 |2 V | | |

|80 |2 V | | |

|120 |2 V | | |

|160 |2 V | | |

|180 |2 V | | |

|200 |2 V | | |

|220 |2 V | | |

|240 |2 V | | |

|300 |2 V | | |

|325 |2 V | | |

|350 |2 V | | |

|400 |2 V | | |

Instructor initials ______________

Date taken ___________

Designer ____________________________ (sign here)

Task 6 Lab Measurements

Frequency Converter Measurements

Alternative 1

Table 6-2 – Range 2 Measurements

|Frequency (Hz) |Input Voltage (ac |Converter Output Voltage (Vdc) |Scaler Output Voltage (Vdc) |

| |p-p) | | |

|200 |2 V | | |

|400 |2 V | | |

|600 |2 V | | |

|800 |2 V | | |

|1200 |2 V | | |

|1600 |2 V | | |

|1800 |2 V | | |

|2000 |2 V | | |

|2200 |2 V | | |

|2400 |2 V | | |

|3000 |2 V | | |

|3250 |2 V | | |

|3500 |2 V | | |

|4000 |2 V | | |

Instructor initials ______________

Date taken ___________

Designer ____________________________ (sign here)

Task 6 Lab Measurements

Frequency Converter Measurements

Alternative 1

Table 6-3 – Range 3 Measurements

|Frequency (Hz) |Input Voltage (ac |Converter Output Voltage (Vdc) |Scaler Output Voltage (Vdc) |

| |p-p) | | |

|2000 |2 V | | |

|4000 |2 V | | |

|6000 |2 V | | |

|8000 |2 V | | |

|12000 |2 V | | |

|16000 |2 V | | |

|18000 |2 V | | |

|20000 |2 V | | |

|22000 |2 V | | |

|24000 |2 V | | |

|26000 |2 V | | |

|30000 |2 V | | |

|35000 |2 V | | |

|40000 |2 V | | |

Instructor initials ______________

Date taken ___________

Designer ____________________________ (sign here)

Task 6 Lab Measurements

Frequency Converter Measurements

Alternative 2: Digital Measurement

Table 6-1 – Range 1 Measurements

|Frequency (Hz) |Measured |Percent Error |

| |Frequency (DAQ) | |

|20 | | |

|40 | | |

|60 | | |

|80 | | |

|120 | | |

|160 | | |

|180 | | |

|200 | | |

|220 | | |

|240 | | |

|300 | | |

|325 | | |

|350 | | |

|400 | | |

Instructor initials ______________

Date taken ___________

Designer ____________________________ (sign here)

Task 6 Lab Measurements

Frequency Converter Measurements

Alternative 2: Digital Measurement

Table 6-2 – Range 2 Measurements

|Frequency (Hz) |Measured |Percent Error |

| |Frequency (DAQ) | |

|200 | | |

|400 | | |

|600 | | |

|800 | | |

|1200 | | |

|1600 | | |

|1800 | | |

|2000 | | |

|2200 | | |

|2400 | | |

|3000 | | |

|3250 | | |

|3500 | | |

|4000 | | |

Instructor initials ______________

Date taken ___________

Designer ____________________________ (sign here)

Task 6 Lab Measurements

Frequency Converter Measurements

Alternative 2: Digital Measurement

Table 6-3 – Range 3 Measurements

|Frequency (Hz) |Measured |Percent Error |

| |Frequency (DAQ) | |

|2000 | | |

|4000 | | |

|6000 | | |

|8000 | | |

|12000 | | |

|16000 | | |

|18000 | | |

|20000 | | |

|22000 | | |

|24000 | | |

|26000 | | |

|30000 | | |

|35000 | | |

|40000 | | |

Instructor initials ______________

Date taken ___________

Designer ____________________________ (sign here)

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. Read the test frequency from the array of test points.

2. Determine the range that the test frequency falls. Turn on an indicator lamp on the display that indicates the range.

3. Activate the digital I/O line to set the correct ranges on the VCO and the F-V converter.

4. 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.

5.) An analog input reads the value of input voltage to the EUT.

a.) scale analog value to original units and display the result

6.) An analog input reads the value of output voltage from the EUT

a.) scale analog value to original units and display the result

7.) An analog input reads the value of output voltage from the Frequency-to-voltage 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.

9. Read the next decimal value that will produce the next test frequency.

10.) Go to step 1. If this is the last array element, go to the next step.

11.) Compute the decibel value of the test frequency from the input and output voltage measurements.

12.) Plot the results on a semi-log graph.

13. Save value of decibel and frequency to disk file for further analysis.

Figure 14 is a flowchart that illustrates the steps described above.

[pic]

Figure 16. Test Procedure Flow Chart.

[pic]

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.) A block diagram of the design.

2.) A working model of the system. (Schedule with instructor or T.A. for demonstration)

3.) A written discussion (2- 3 pages double spaced) of the software operation and design.

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