ICIC Express Letters ICIC International c°2009 ISSN 1881-803XVolume 3, Number 1, March 2009 pp. 33—40

PC-BASED VIRTUAL BODE ANALYZER DESIGN ANDAPPLICATION IN AUTOMATIC MEASUREMENT

OF FILTER MANUFACTURING

Kai-Chao Yao

National Chung-hua University of EducationDepartment of Industrial Education and Technology

No.2 Shi-Da Road, Changhua City, Taiwankcyao@cc.ncue.edu.tw

Received September 2008; accepted December 2008

Abstract. In this research, a PC-based virtual bode analyzer is designed and con-structed. It consists of four major parts: (1) Front panel design of software (2) DataAcquisition devices of hardware (2) Measurement test (4) Application in Automatic Mea-surement of Filter Manufacturing. This programmable virtual bode analyzer is achievedby software part, Labview and hardware part, DAQ card and PC. The designed measure-ment functions include four pallettes: (1) Settings Controls of Plot (2) Run and LogButtons (2) Display settings and Cursor Controls (4) Bode Plot Display Window. Everypalette has its own functions. In applying automatic measurement of filter manufactur-ing, three kinds of often seen filters are demonstrated for measurement tests by designedvirtual bode analyzer to show the capabilities.Keywords: Virtual, Bode analyzers, Design, Labview, Filter

1. Introduction. Instruments used in building the virtual laboratory are called virtualinstruments (VI). They are designed and built using software. These instruments emulatethe appearance and the function of the real instrument. They are a result of combining ageneral purpose computer with a generic data acquisition system in order to emulate sev-eral traditional measurement instruments. VI’s are a knowledge area integrating severalothers areas. These are the instrumentation system, concurrent programming, graphicaluser interface (GUI), real-time system, object oriented program and object oriented tech-nology. There are several systems for developing the virtual instrument such as LabVIEW,Look-Out, BridgeVIEW and LabWindows/CVI [1].The computer package used in this study to build the virtual bode analyzer is Lab-

VIEW7.1 from National Instrument. LabVIEW is a graphical development environmentfor testing, measuring and controlling applications. It is an object oriented program inwhich different blocks are connected to perform a job instead of writing textual program-ming language. LabVIEW has built-in capabilities for direct I/O communication throughinterfaces including VISA, GPIB, Serial, and Ethernet. LabVIEW delivers extensiveacquisition, analysis, and presentation capabilities in a single environment toseamlesslydevelop a complete solution on a selected platform. LabVIEW delivers what engineersand scientists need to build test and measurement, data acquisition, embedded control,scientific research, and process monitoring systems [2].There are various virtual instruments that have been designed and implemented: com-

ponent characteristic tracer, scalar network analyzer, functional generator, signal ana-lyzer, oscilloscope and frequency meter [3-8]. All have their own special features andpanels. They are applicable to laboratory work as well as industrial applications as well[9-11].

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This paper designs a PC-based virtual bode analyzer and applied it to practical au-tomatic quality measurement of the filter manufacturing process. This kind of virtualinstrument design scheme can be widely applied to the measurement. It is able to in-tegrate the real circuit to form a kind of virtual-real composite circuit design scheme tomake the measurement system even more powerful. For example, [12-14] are about imageor signal processing research, in implementing this research, the proposed technique is agood method to apply because the programming ability can easily handle those developedalgorithm instead of building circuits. Moreover, the graphical programming is flexible,reusable and user friendly [15].

2. Systems. The experimental system is shown in Figure 1 and the required devices andtools are listed below: (1) Computer: Pentine IV (2) Signal Acquisition Device: (DAQCard) PCI-6251 M Series (2) Software: Labview 7.1 and NI ELVIS 3.0 (4) Hardware: NIELVIS systems shown in Figure 1.

Figure 1. Virtual instrument workstation

In Figure 1, the parts of the marked numbers are explained below: (1). Desk Computerwith Labview installed. (2). Data Acquisition Card. (2). 68 pin Shielded Cable. (4). NIELVIS Benchtop Workstation.

3. Main Results. Completing the PC-based virtual body analyzer design and applica-tion for automatically measuring of filter manufacturing consists of four major works.

3.1. Front panel design of software part. The PC-based virtual bode analyzer designof the front panel can be divided into the following six important parts. The designedbode analyzer basically has the following functions:(1) Settings Controls of the Plot

ICIC EXPRESS LETTERS, VOL.3, NO.1, 2009 35

(a) Start — Specifies the frequency at which to start the Bode plot sweep. (b) Steps —Specifies the number of evenly spaced frequency points per decade. (c) Stop — Specifies thefrequency at which to stop the Bode plot sweep. (d) FGEN FUNC OUT Peak Amplitude— Sets the peak amplitude of the Function Generator output signal. (e) Op-Amp SignalPolarity — Inverts the measured values of the input signal during Bode analysis.(2) Run and Log Buttons

(a) Run Button — Starts the frequency sweep with the parameters specified by theSettings controls. (b) Log Button — Allows one to save the measured data after thefrequency sweep is performed.(3) Display Settings and Cursor Controls

(a) Y Scale — Selects the scale setting for the Bode Analyzer. One can choose theDefault, Gain, Phase, or Auto setting. (b) Decibel — Selects whether the gain graph is indB or linear scale. (c) Maximum — Sets the maximum Y scale value for the graph selectedin the Y Scale control. (d) Minimum — Sets the minimum Y scale value for the graphselected in the Y Scale control. (e) Cursors — Off: Turns the graph cursors on or off.(4) Bode Plot Display Window

The Display Window includes the following two plots: (a) The one at the top of thewindow is the Gain Display. The signal is plotted gain versus frequency. (b) The plotat the bottom of the window is the Phase Display. The signal is plotted phase versusfrequency.The Display Window has the following indicators, which are located at the bottom of

the window: (a) Frequency (b) Phase (c) Gain (d) Gain (dB).

3.2. Data acquisition devices of hard ware part. In this research, M Series PCI-6251is used as data acquisition interface. Figure 2 shows the shape and pinout of PCI-6251.In the device, one side is connected to the computer and the other side is connected tothe prototyping board of the workstation, as shown in Figure 1.

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Figure 2. M series PCI-6251 pinout

When connecting signals, because the analog channels are differential, one must estab-lish a ground point somewhere in the signal path. The NI ELVIS Prototyping Board hassix differential AI channels available — ACH<0..5>. These inputs are directly connectedto the DAQ device input channels. The NI ELVIS prototyping board also exposes twoground reference pins, AI SENSE and AI GND, which are connected to the M Series DAQdevice. Table 1 shows how the NI ELVIS input channels map to the DAQ device inputchannels.

Table 1. Analog input signal mapping

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These six channels can be used for external measurement circuit design, such as forthe amplifier circuit and other signal acquisition. These channels make the bode analyzermore flexible and expansible. Some of the AI channels are used by the internal circuitryfor other instruments in ELVIS, but most of the time one can still use the channel. Onecan use ACH<0..2> without interruption. ACH5 is interrupted if any of the impedance-analyzing capabilities of the DMM, such as the capacitance meter, diode tester, and soon, are used. If one is using the Oscilloscope, the one must disconnect any signals fromACH3 and ACH4 to avoid double-driving the channels.

Table 2. AI channel resource conflicts in ELVIS

Table 3 shows the result of specification tests for every measurement function. Theseparameters are affected by the external measurement circuit components and the DAQdevice.

Table 3. Specifications of the virtual bode analyzer

3.3. Measurement test. A Bode plot defines in a real graphical format the frequencycharacteristics of an AC circuit. Amplitude response is plotted as the circuit gain mea-sured in decibels as a function of log frequency. Phase response is plotted as the phasedifference between the input and output signals on a linear scale as a function of logfrequency. The designed PC-based bode analyzer is used to measure an RC circuit asshown in Figure 3(a), and measure the gain and phase as Figure 3(c).

(a) (b) (c)

Figure 3. (a) The R-C circuitry (b) The circuit built in the prototypingboard (c) The bode plot of the R-C circuit

Use the Display options to select the graphing format, and use the cursors to readpoints off the frequency characteristic in Figure 3(c). The frequency where the signalamplitude has fallen to −3dB is the same frequency where the phase is 45 degrees.

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The following steps show the measuring signals with this Bode Analyzer: (1) Constructthe circuit for Bode analysis on the Prototyping Board. (2) Connect the Function Gen-erator FUNC OUT signal to the positive input of the circuit, and connect the referencepoint of the circuit to the GROUND signal on the prototyping board. (3) Connect theFunction Generator FUNC OUT signal to the AI signal terminal ACH1+ and connectACH1— to the GROUND signal on the prototyping board. (4) Connect the output of thecircuit to the AI signal terminal ACH0+, and connect ACH0− to the GROUND signalon the prototyping board. (5) Launch the Bode Analyzer software. (6) Select the startingand ending frequencies from the Start and Stop controls, and click Run.

4. Application in Automatic Measurement of Filter Manufacturing. Apply thedeveloped PC-based bode analyzer in a practical test of high-pass filters, low-pass filtersand band-pass filters to show the capability and feasibility of this virtual instrument.

4.1. High-pass filters. Figure 4(a) shows a high-pass filter circuitry. The cut-off fre-quency fL can be found from Equation (1)

2πfL = 1/R1C1 (1)

where fL is measured in Hertz. This is the frequency where the Gain (dB) has fallen by−3dB. This point (−3dB) occurs when the impedance of the capacitor equals the resistor.The high-pass Op Amp filter equation is similar. At the −3dB point, the impedance ofthe input resistor is equal to the impedance of the input capacitor:

R1 = 1/(2πfLR1C1) = XC (2)

(a) (b) (c)

Figure 4. (a) High-pass filter circuitry (b) The circuit built in the proto-typing board (c) The bode plot of the high pass filter

Run the Bode plot software and observe the low frequency response is attenuated whilethe high frequency response is similar to the basic Op Amp Bode plot in Figure 4(c). Usethe cursor function to find the low frequency cutoff point, that is, the frequency at whichthe amplitude has fallen by −3dB or the phase change is 45 degrees.4.2. Low-pass filters. The high-frequency rolloff in the Op Amp circuit is due to theinternal capacitance of the 741 chip being in parallel with the feedback resistor Rf . If oneadds an external capacitor, Cf , in parallel with the feedback resistor Rf , one can reducethe upper frequency cutoff point to fU . One can predict the new cutoff point from theequation:

2πfU = 1/RfCf (3)

Short circuit the input capacitor and add the feedback capacitor Cf in parallel with the100kΩ feedback resistor, as shown in Figure 5(a). The graph in Figure 5(b) shows thehigh frequency response is attenuated more than the basic Op Amp response. Use thecursor function to find the high frequency cutoff point, that is, the frequency at whichthe amplitude has fallen by −3dB or the phase change is 45 degrees.

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(a) (b)

Figure 5. (a) Low-pass filter circuitry (b) The bode plot of the low-pass filter

4.3. Band-pass filter. If one allows both an input capacitor and a feedback capacitorin the Op Amp circuit, the response curve has both a low cutoff frequency, fL, and a highcutoff frequency, fU . The frequency range (fU−fL) is called the bandwidth. For example,a good stereo amplifier would have a bandwidth of at least 20,000Hz. The Figure 6(a)shows a bandpass circuit built on the NI ELVIS protoboard. Remove the short on C1and run a fourth Bode plot using the same scan parameters. The Figure 6(b) shows theresults of measuring the band-pass filter. By drawing a line at 3dB below the maximumamplitude region, the frequency range contained by all frequencies above this line definesthe band-pass.

(a) (b)

Figure 6. (a) The band-pass filter circuit built in the prototyping board(b) The bode plot of the band-pass filter

5. Conclusions. In this research, a PC-based virtual bode analyzer is designed and con-structed; moreover, the virtual instrument is applied for use in automatically measuringof filter manufacturing. The completion of this research changes and improves the qualitytest process of filter manufacturing that using manual test with traditional instruments.This virtual bode Analyzer can measures the gain and phase shift versus frequency forpassive or active linear circuits. The frequency measurement points are spaced logarith-mically. One can invert the polarity to zero the phase shift for inverting amplifiers. Forthe setting considerations, the signal amplitude can be chosen to optimize the system re-sponse. It can drive passive circuits with high amplitude and drive high gain circuits withsmall amplitude to avoid saturating the output. The circuit should be ground referencedthe circuit under test and use the FUNC OUT signal as its input. The measurementsignal channel is ACH0. The input stimulus is a sine wave signal with a user-specifiedamplitude. In terms of efficiency, function renewal, cost and expansibility of instruments,the developed PC-based virtual bode analyzer is superior to the traditional one.

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6. Acknowledgment. This study was funded by a grant provided by the National Sci-ence Council, Taiwan, under Grant No.NSC 97-2511-S-018-018-MY2.

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