c
ICIC International °2009
ISSN 1881-803X
ICIC Express Letters
Volume 3, Number 1, March 2009
pp. 33—40
PC-BASED VIRTUAL BODE ANALYZER DESIGN AND
APPLICATION IN AUTOMATIC MEASUREMENT
OF FILTER MANUFACTURING
Kai-Chao Yao
National Chung-hua University of Education
Department of Industrial Education and Technology
No.2 Shi-Da Road, Changhua City, Taiwan
kcyao@cc.ncue.edu.tw
Received September 2008; accepted December 2008
Abstract. In this research, a PC-based virtual bode analyzer is designed and constructed. It consists of four major parts: (1) Front panel design of software (2) Data
Acquisition devices of hardware (2) Measurement test (4) Application in Automatic Measurement of Filter Manufacturing. This programmable virtual bode analyzer is achieved
by software part, Labview and hardware part, DAQ card and PC. The designed measurement functions include four pallettes: (1) Settings Controls of Plot (2) Run and Log
Buttons (2) Display settings and Cursor Controls (4) Bode Plot Display Window. Every
palette has its own functions. In applying automatic measurement of filter manufacturing, three kinds of often seen filters are demonstrated for measurement tests by designed
virtual bode analyzer to show the capabilities.
Keywords: Virtual, Bode analyzers, Design, Labview, Filter
1. Introduction. Instruments used in building the virtual laboratory are called virtual
instruments (VI). They are designed and built using software. These instruments emulate
the appearance and the function of the real instrument. They are a result of combining a
general purpose computer with a generic data acquisition system in order to emulate several traditional measurement instruments. VI’s are a knowledge area integrating several
others areas. These are the instrumentation system, concurrent programming, graphical
user interface (GUI), real-time system, object oriented program and object oriented technology. 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 LabVIEW7.1 from National Instrument. LabVIEW is a graphical development environment
for testing, measuring and controlling applications. It is an object oriented program in
which different blocks are connected to perform a job instead of writing textual programming language. LabVIEW has built-in capabilities for direct I/O communication through
interfaces including VISA, GPIB, Serial, and Ethernet. LabVIEW delivers extensive
acquisition, analysis, and presentation capabilities in a single environment toseamlessly
develop a complete solution on a selected platform. LabVIEW delivers what engineers
and 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: component characteristic tracer, scalar network analyzer, functional generator, signal analyzer, oscilloscope and frequency meter [3-8]. All have their own special features and
panels. 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 automatic quality measurement of the filter manufacturing process. This kind of virtual
instrument design scheme can be widely applied to the measurement. It is able to integrate the real circuit to form a kind of virtual-real composite circuit design scheme to
make the measurement system even more powerful. For example, [12-14] are about image
or signal processing research, in implementing this research, the proposed technique is a
good method to apply because the programming ability can easily handle those developed
algorithm 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 and
tools are listed below: (1) Computer: Pentine IV (2) Signal Acquisition Device: (DAQ
Card) PCI-6251 M Series (2) Software: Labview 7.1 and NI ELVIS 3.0 (4) Hardware: NI
ELVIS systems shown in Figure 1.
Figure 1. Virtual instrument workstation
In Figure 1, the parts of the marked numbers are explained below: (1). Desk Computer
with Labview installed. (2). Data Acquisition Card. (2). 68 pin Shielded Cable. (4). NI
ELVIS Benchtop Workstation.
3. Main Results. Completing the PC-based virtual body analyzer design and application 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 design
of the front panel can be divided into the following six important parts. The designed
bode analyzer basically has the following functions:
(1) Settings Controls of the Plot
ICIC EXPRESS LETTERS, VOL.3, NO.1, 2009
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(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 the
frequency 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 Signal
Polarity — 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 the
Settings controls. (b) Log Button — Allows one to save the measured data after the
frequency sweep is performed.
(3) Display Settings and Cursor Controls
(a) Y Scale — Selects the scale setting for the Bode Analyzer. One can choose the
Default, Gain, Phase, or Auto setting. (b) Decibel — Selects whether the gain graph is in
dB or linear scale. (c) Maximum — Sets the maximum Y scale value for the graph selected
in the Y Scale control. (d) Minimum — Sets the minimum Y scale value for the graph
selected 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 the
window is the Gain Display. The signal is plotted gain versus frequency. (b) The plot
at the bottom of the window is the Phase Display. The signal is plotted phase versus
frequency.
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-6251
is 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 to
the prototyping board of the workstation, as shown in Figure 1.
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K.-C. YAO
Figure 2. M series PCI-6251 pinout
When connecting signals, because the analog channels are differential, one must establish a ground point somewhere in the signal path. The NI ELVIS Prototyping Board has
six differential AI channels available — ACH<0..5>. These inputs are directly connected
to the DAQ device input channels. The NI ELVIS prototyping board also exposes two
ground reference pins, AI SENSE and AI GND, which are connected to the M Series DAQ
device. Table 1 shows how the NI ELVIS input channels map to the DAQ device input
channels.
Table 1. Analog input signal mapping
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These six channels can be used for external measurement circuit design, such as for
the amplifier circuit and other signal acquisition. These channels make the bode analyzer
more flexible and expansible. Some of the AI channels are used by the internal circuitry
for other instruments in ELVIS, but most of the time one can still use the channel. One
can use ACH<0..2> without interruption. ACH5 is interrupted if any of the impedanceanalyzing capabilities of the DMM, such as the capacitance meter, diode tester, and so
on, are used. If one is using the Oscilloscope, the one must disconnect any signals from
ACH3 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. These
parameters are affected by the external measurement circuit components and the DAQ
device.
Table 3. Specifications of the virtual bode analyzer
3.3. Measurement test. A Bode plot defines in a real graphical format the frequency
characteristics of an AC circuit. Amplitude response is plotted as the circuit gain measured in decibels as a function of log frequency. Phase response is plotted as the phase
difference between the input and output signals on a linear scale as a function of log
frequency. The designed PC-based bode analyzer is used to measure an RC circuit as
shown 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 prototyping
board (c) The bode plot of the R-C circuit
Use the Display options to select the graphing format, and use the cursors to read
points off the frequency characteristic in Figure 3(c). The frequency where the signal
amplitude 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) Construct
the circuit for Bode analysis on the Prototyping Board. (2) Connect the Function Generator FUNC OUT signal to the positive input of the circuit, and connect the reference
point of the circuit to the GROUND signal on the prototyping board. (3) Connect the
Function Generator FUNC OUT signal to the AI signal terminal ACH1+ and connect
ACH1— to the GROUND signal on the prototyping board. (4) Connect the output of the
circuit to the AI signal terminal ACH0+, and connect ACH0− to the GROUND signal
on the prototyping board. (5) Launch the Bode Analyzer software. (6) Select the starting
and ending frequencies from the Start and Stop controls, and click Run.
4. Application in Automatic Measurement of Filter Manufacturing. Apply the
developed PC-based bode analyzer in a practical test of high-pass filters, low-pass filters
and 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 frequency fL can be found from Equation (1)
(1)
2πfL = 1/R1 C1
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 of
the input resistor is equal to the impedance of the input capacitor:
R1 = 1/(2πfL R1 C1 ) = XC
(2)
(a)
(b)
(c)
Figure 4. (a) High-pass filter circuitry (b) The circuit built in the prototyping board (c) The bode plot of the high pass filter
Run the Bode plot software and observe the low frequency response is attenuated while
the high frequency response is similar to the basic Op Amp Bode plot in Figure 4(c). Use
the cursor function to find the low frequency cutoff point, that is, the frequency at which
the 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 the
internal capacitance of the 741 chip being in parallel with the feedback resistor Rf . If one
adds an external capacitor, Cf , in parallel with the feedback resistor Rf , one can reduce
the upper frequency cutoff point to fU . One can predict the new cutoff point from the
equation:
(3)
2πfU = 1/Rf Cf
Short circuit the input capacitor and add the feedback capacitor Cf in parallel with the
100kΩ feedback resistor, as shown in Figure 5(a). The graph in Figure 5(b) shows the
high frequency response is attenuated more than the basic Op Amp response. Use the
cursor function to find the high frequency cutoff point, that is, the frequency at which
the amplitude has fallen by −3dB or the phase change is 45 degrees.
ICIC EXPRESS LETTERS, VOL.3, NO.1, 2009
(a)
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(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 capacitor
in the Op Amp circuit, the response curve has both a low cutoff frequency, fL , and a high
cutoff 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 C1
and run a fourth Bode plot using the same scan parameters. The Figure 6(b) shows the
results of measuring the band-pass filter. By drawing a line at 3dB below the maximum
amplitude region, the frequency range contained by all frequencies above this line defines
the 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 constructed; moreover, the virtual instrument is applied for use in automatically measuring
of filter manufacturing. The completion of this research changes and improves the quality
test process of filter manufacturing that using manual test with traditional instruments.
This virtual bode Analyzer can measures the gain and phase shift versus frequency for
passive or active linear circuits. The frequency measurement points are spaced logarithmically. One can invert the polarity to zero the phase shift for inverting amplifiers. For
the setting considerations, the signal amplitude can be chosen to optimize the system response. It can drive passive circuits with high amplitude and drive high gain circuits with
small amplitude to avoid saturating the output. The circuit should be ground referenced
the circuit under test and use the FUNC OUT signal as its input. The measurement
signal channel is ACH0. The input stimulus is a sine wave signal with a user-specified
amplitude. 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 Science Council, Taiwan, under Grant No.NSC 97-2511-S-018-018-MY2.
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