Instrument Communication & Networking

Soumyajyoti Sengupta (S0909491)

MSc in Applied Instrumentation & Control (Instrument Communication & Networking)

2010-2011

ASSIGNMENT 1Instrument Communication and Networking Report on development of an Automated test and Measurement System Submitted by SOUMYAJYOTI SENGUPTA Matriculation no: S0909491

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OBJECTIVE:To set up an automated test and measurement system for determination of the cut-off frequency of a low pass filter. And this task has to be completed in two different stages, as described below: Stage 1 Setup an appropriate hardware and design a program in Labview for determination of the cut-off frequency of a low pass filter. Stage 2 Based on the program developed above determine the cut-off frequency of ten different filters (identified by the filter number or a serial number). The results should be written in an automatically generated spreadsheet file by the application of the program in Labview. HARDWARE PROVIDED: PXI with E series data acquisition card GPIB instruments Signal generator or Function Generator Oscilloscope Low pass filters Digital Multimeter SOFTWARE USED: NI Labview

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BASIC CONCEPTS:LOW PASS FILTER: Low pass filter passes a certain low level frequencies and attenuates (reduces the amplitude) the frequencies higher than the cut off frequency. Now cut off frequency is the frequency boundary between pass band and stop band of any filter. The cut-off frequency can be determined from the frequency response curve which is as shown below. Now this it should be noted that the frequency scale is logarithmic rather than linear. The gain (in db) is given by, Gain (db) = 20* log10 Vout Vin

For a low pass filter Vout is always lesser than the Vin. So the logarithmic fraction of Vout and Vin will be negative. The ideal cut-off frequency for a low pass filter is -3db. Therefore the ratio of Vout/Vin is 0.707.Cut off frequency0 - 10 - 20 - 30 Gain (db) - 40 - 50 - 3 db

Pass Band- 60 10 100

Stop band1000

Frequency (Hz)

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TECHNIQUES USED FOR DETERMINATION OF CUT-OFF FREQUENCY: The techniques used to determine the cut-off frequency of a low pass filter are as follows: Successive Approximation, Curve fitting, Interpolation etc. Apart from these methods cut-off frequency can be determined manually by adjusting the amplitude and the frequency of function generator and also by tuning the oscilloscope until the ratio Vout/Vin becomes equal to or less than 0.707. SUCCESSIVE APPROXIMATION: In this technique at the first loop, the initial frequency and peak to peak input voltage are fed into the system set up. Then the input and output voltages are measured and then ratio and gain are calculated. When the ratio reaches the condition , the first loop gets stopped and the signal is jumped to the next successive loop. In this way by the gradual increment of frequency input user can get the desired value of cut-off frequency. INTERPOLATION: In this technique the user uses the known data values to determine the unknown data values. There are different types of interpolation methods: Linear interpolation Polynomial interpolation So this method is a kind of mathematical process used for getting the proper values of cut-off frequency. CURVE FITTING: Here in this technique the curve is constructed to get the best fit with the series of data points and through that way user can get the desired cut-off frequency.

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PART 1Here out of the three basic techniques of finding the cut off frequency the first technique, - successive approximation is going to be used.

HARDWARE SET UP:The pictorial view of the hardware set up for the purpose of determination of cut off frequency of any low pass filter is as shown below. Here in this setting of the hardware it is clearly shown that signal generated by the signal generator passing through the low pass filter and gets entered in the oscilloscope at two different points, input and output named as X and Y. Before starting of the experiment the signal generated should be set at initial frequency and at the peak to peak input voltages. And the signal generator should also be set for high impedance. And PXI has been used for communicating with the signal generator and the PC. From oscilloscope the waveform of both voltages and the point where they get tripped or the cut off frequency point can be achievable. Description of different parts of this hardware set up and the techniques are in the following section.

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Descriptions of two basic instruments used in this technique:y y Signal generator: Agilent 33220A 20MHz Function/Arbitrary Waveform Generator Oscilloscope: Agilent DS03102A Digital Storage Oscilloscope

Brief review of all the components used:PXI: PXI stands for PCI Extensions for Instrumentation. It is a rugged PC-based which is used for measurement and automation applications that require high performances. It provides synchronization between the buses and the software features used in the system. GPIB: The GPIB stands for General Purpose Interface Bus. Its role is to communicate between the computers and the instruments for transferring the data between them. This is also called as IEEE-488. GPIB consists of 16 signal lines and 8 ground lines. Out of those 16 signal lines, there are 8 data lines 3 handshake lines and 5 interface management lines. And out of 8 ground lines there are 5 lines twisted as pairs with the 5 interface management lines. The role of all the lines has been in the following section. The data transfer rate of this bus is up to 1Mbps. TYPES OF GPIB: GPIB can be talkers, listeners and/or controllers. Talker transmits the data onto the interface bus and there can be a single talker at a particular time. Listener receives the information from the interface bus and there can be more than one listener at a time.

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Controller manages the flow of information on the interface bus by sending commands to all devices and there can be only one controller at a time. All GPIB devices communicate between themselves through the way of sending messages. And there are two types of GPIB messages: Device-dependent messages (Data messages): This kind of messages contains device specific information: program instruction, measurement results and data files. Interface messages: This kind of messages manages the bus and usually performs the functions such as, initializing the bus, addressing the devices and setting device modes for the remote or local programming. DESCRIPTION OF ROLE OF DIFFERENT LINES: First of all the schematic diagram of GPIB device is as shown below.

Schematic diagram of GPIB device DATA LINES: The eight data lines carry both data and command messages. The state of the attention line (ATN) determines whether the information is data or commands. All the commands or most of the data use the 7 bit ASCII code set and the eighth bit either be unused or used for parity. HANDSHAKE LINES: Three lines asynchronously control the transfer of message bytes between devices. This process is called 3-wire interlocked handshake. It assures the status of the message bytes that they are sent and received without transmission error. The three lines are:

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y y y

NRFD: It stands for not ready for data because it indicates when a device is ready or not ready to receive a message byte. NDAC: It stands for not data accepted because it indicates when a device has or has not accepted a message byte. DAV: It stands for data valid because it indicates when the signals on the data lines are stable (valid) and can be accepted safely by devices.

INTERFACE MANAGEMENT LINES: Five interface management lines manage the flow of information and those are as follows: y ATN (attention): The controller drives ATN as true when it uses data lines to send commands and drives ATN as false when a talker can send data messages. IFC (interface clear): The system controller drives the IFC line to initialize the bus and become CIC. REN (remote enable): The controller drives the REN line which is used for setting devices at remote or local mode. SRQ (service request): Any device can drive the SRQ line to asynchronously request service from controller. EOI (end or identify): This line has two main purposes: talker uses this line to mark the end of message string and controller uses this line to tell devices to identify their response in a parallel poll.

y y y y

Here in this task the oscilloscope, function generator and the computer are connected through the GPIB cable. The controller is a board installed in the PC. The oscilloscope and the function generator are usually talkers and listeners. All these GPIB instruments are addressed with unique GPIB address. The address string for the used oscilloscope (Agilent 3000/5000 series) is 7and that of function generator (Agilent 33220A) is 7. The low pass filter is connected to the function generator and oscilloscope so that the output and the input voltage can be measured. After the hardware setup the next step is to design the software for performing the task through the writing the program in Labview.

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SOFTWARE DESIGN:Here the program is written in Labview programming for the purpose of determination of cut off frequency of a low pass filter. For writing the program following function palettes are used: GPIB Write:

(timeout ms) specifies the time that the function waits before timing out. (address string) contains the address of the GPIB device with which the function communicates. (data) is the data which is written to the device by the function. (mode) indicates how to terminate the GPIB write. (error in) describes error conditions that occur before the function or VI runs. The default condition is no error. When any error occurred before the VI runs the error in value gets passed to the error out. And if any error occurred during the running of VI it sets its own error status in error out. (status) is true if any error occurred before the VI runs and it is false when no error occurred before the VI runs. Here the default is false. (code) is the error or warning code. The default is zero (0) for this case. If the status is true, code is an error code. (source) is the origin of the error or warning. The default is an empty string. The data given to the function generator are frequency and voltage. The commands are FREQ N HZ which sets the frequency to the value N in Hz and VOLT N VPP which sets the amplitude to the value N peak to peak.

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For the oscilloscope the commands are (:MEAS:VPP? CHAN1) which returns the peak to peak voltage on channel 1 and (:MEAS:VPP? CHAN2) which is used for channel 2. GPIB Read:

(timeout ms) specifies the time (in milliseconds), that the function waits before the timing out. For disabling timeout (timeout ms) should be set to 0. (address string) contains the address of the GPIB device with which the function communicates. (byte count) specifies the number of bytes the function or VI reads from the GPIB device. (mode) specifies the conditions, other than reaching byte count for the termination of read. (error in) describes error conditions that occur before the function or VI runs. The default condition is no error. When any error occurred before the VI runs the error in value gets passed to the error out. And if any error occurred during the running of VI it sets its own error status in error out. (status) is true if any error occurred before the VI runs and it is false when no error occurred before the VI runs. Here the default is false. (code) is the error or warning code. The default is zero (0) for this case. If the status is true, code is an error code. (source) is the origin of the error or warning. The default is an empty string. (data) is the data the function or VI reads. Here the number of byte count given is 1042.11



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Brief description of the labview program: First of all the function generator is initialized to 1 VPP and set to high impedance load using the command (OUTP:LOAD INF). The reason of setting at high impedance is to block the higher frequency signals. The oscilloscope gets initialized to auto scale using the command (:AUTO). The pictorial view of initializing of system is as shown below.

The program consists of two loops. The first loop starts with frequency of 50 Hz and gets incremented with the frequency of 50 Hz and the second loop has the increment of frequency of 1 Hz. The frequency data is given to the function generator using the concatenated string. And before using the concatenated string the number has been converted to decimal string. Between the generator and the oscilloscope time delay function has been used so that the oscilloscope can get time to get adjusted to the commands each time it runs in the loop. The generator and the oscilloscope are also has been addressed properly. The data from the GPIB read has been converted to number to get the output voltage and the input voltage of the filter. For determination of cut off frequency, the Vout/Vin should be less than or equal to 0.707. This condition has been calculated using the formula 20*log(Vout/Vin) = - 3 db. During the running of the loop whenever this condition reaches the first loop gets stopped and the signal jumps to the next loop where the increment of frequency is 1 Hz. Thus in the second loop user can get the proper value of cut off frequency using the same condition (Vout/Vin 0.707).12



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The pictorial views of loop 1 and 2 are as shown below. LOOP 1

LOOP 2

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The pictorial view of the front panel is as given below.

PART 2In this part the same labview program has been used. The differences are the serial number of the filters should be recorded and the program should prompt for the serial number of the filter. And moreover the data for each of the filter should be written in a spreadsheet file. Here in this report the block (prompt user for input) has been used for prompting for the serial number and for the spreadsheet file where the data for each filter should be written which is as shown in the next page.

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Apart from this block an array has been used for writing a series of data in a spreadsheet file. And few number strings have been used for getting different values from where it is required. Here in this part of the task the cut off frequencies of ten different filters have been recorded in a spreadsheet file. The whole block diagram is as shown in the next page.

RESULTS: The values of the cut off frequencies, different values of ratio of Vout/Vin, values of Vout and Vin of ten different filters with their serial numbers are given below in tabular form those which have been written in the file soumyo.xls.

FILTER NO. Vout/Vin Vin Vout 15 0.73 1.06 0.688679 8 0.75 1.08 0.694444 6 0.75 1.08 0.694444 11 0.67 0.97 0.690722 3 0.73 1.05 0.695238 24 0.73 1.06 0.688679 16 0.75 1.08 0.694444 2 0.73 1.05 0.695238 18 0.75 1.08 0.694444 13 0.71 1.01 0.712941

CUT OFF FREQUENCY 1553 232 361 8556 1352 958 97 1472 79 1701

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BLOCK DIAGRAM OF THE PART 2:

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EVALUATION OF PERFORMANCE OF THE SYSTEM: RESOLUTION: The system has a resolution of 1Hz. The program in Labview has been made in such a way that the finest value of the cut off frequency can be measured because as described before in the Labview program after getting the probable range of cut off frequency from the first loop the increment of frequency in the second loop is only one hertz. So the resolution of the system is 1Hz. Here cut off frequency of one filter (filter number 16) has been recorded for seven consecutive times for evaluating the performance of the system properly. The readings are tabulated as shown below. FILTER NUMBER 16 16 16 16 16 16 16 CUT OFF FREQUENCY 97 96 97 101 99 98 95

UNCERTAINTY: Uncertainty is the range of values of the measurement within which the true value of the measurement should lie. Type A: The best estimate ( ) of the cut off frequency is 97.57 Hz. Then the standard deviation of population (n-1)

is 1.998 Hz.

The standard deviation of the measurement is 0.755 Hz.

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TYPE B: Applying the rectangular probability distribution, the standard uncertainty of the cut off frequency can be calculated as SN = (1/2)/ = 0.2887 Hz. So the combined uncertainty will be as follows, SN = (0.7552 + 0.28872) = 0.8083Hz. This value of combined uncertainty (= 0.8083Hz) indicates that the uncertainty is moderate. This uncertainty has been calculated based on the values of cut off frequency of a particular filter. It may be of higher value for other filters. This uncertainty is due to the different factors such as, systemic error, environmental disturbances, noises present in the system or any other external factors.

REPEATABILITY: The cut off frequency of filter number 16 has been recorded for 7 consecutive times. The values are tabulated below: FILTER NUMBER 16 16 16 16 16 16 16 CUT OFF FREQUENCY 97 96 97 101 99 98 95

From the table shown above it is clear that the values of cut off frequency of the filter number 16 are likely different every time. The values are plotted in the following graph from where the repeatability of the value of cut off frequency can be observed clearly. From the graph shown in the next page it is clear that the value of the cut off frequency is not at all constant for any filter. Although it is not varying abruptly but the difference in measured values is quite clear.

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102 f r e q u e n c y 101 100 99 98 97 96 95 94 0 1 2 3 4 5 6 7 8

serial no of measurement

RESPONSE TIME: The response time of the system is moderately fast as the frequency in the first loop gets incremented by 50Hz and then in the second loop it increases by 1Hz in the range of frequency of 50Hz. But the readings of the system are quite efficient because of its resolution of 1Hz.

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CONCLUSION:Here an automated measurement system has been developed based on the Labview programming where the cut off frequencies of 10 different filters can be measured very efficiently. Development of the system can be suggested depending on the following factors: Here after investigating the uncertainty for the particular filter (filter no. 16) it can be suggested that by reducing the systemic error, by making the hardware set up properly and by providing the environmental surrounding as good as possible the uncertainty can be reduced. And at the same time by making these factors (described before) proper repeatability also can be improved. Response time can be improved by employing more loops where values of frequency input can be started from any higher value of frequency such as, 1000Hz with 1000Hz increment and then increment of 100Hz in the following loop and so on. But for getting efficient reading I think the system and program presented in this report is the appropriate one.

REFERENCES:y y y Caledonian University, 2009. Measurement Theory and Devices [Course Module Handbook] Caledonian University, 2010. Instrumentation Communication and Networking[Course Module Handbook] http://www.hit.bme.hu/~papay/edu/GPIB/tutor.htm

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