Study of modulation using graphical programming and virtual instruments.
Technologies change fast. As these changes occur, industries need trained technologists and scientists to meet their new workforce requirements. Advanced electronics and computerization are revolutionizing today's industries and the engineering technology and science programs are under pressure to modernize their programs to meet the challenges of this changing technology or to maintain the accreditation of the programs. This requires upgrading laboratories with modern equipment and calls for increased funding and resources. But in recent years there is an increase in enrollment and decrease in resource allocation making it increasingly difficult to modernize the laboratories to provide adequate levels of laboratory and course work (Korrapati & Swain, 2000). This calls for alternative cost effective methods such as Computer Based Virtual Engineering Laboratory (CBVEL), which can be accomplished by using the following:
A Graphical Programming software (LabVIEW) and hardware (DAQ boards, GPIB interface etc.) from National Instrument.
B Object Oriented Programming Software (Visual Basic, Java).
We strongly believe that by modifying the existing laboratories through the addition of computers with appropriate DAQ and SCXI instrumentation, software (LabVIEW, Visual Basic, & Java) we can better educate and train our graduates to serve the needs of the technological and engineering community. Students will be skilled with hardware and software that is used throughout industry, at other undergraduate institutions, and graduate schools. Because of the flexibility of Visual Basic, LabVIEW and the associated interface, the system will be customized to suit the instructional and research needs of various departments. Virtual Instruments (VIs) relating to different courses will be integral part of this CBVEL. Examples of some of these VIs are Digital Electronics, Analog and Communications, Digital Signal processing, Digital Filters. These modules will be used to better train the engineering technology, sciences, and information technology graduates (Swain, Anderson, & Korrapati, 2000). This paper is arranged as follows:
BLOCK DIAGRAM OF CBVEL
The block diagram of CBVEL is presented below (Swain, Anderson & Korrapati, 1999):
The various component of this CBVEL are personal computers, LabVIEW software, programming languages like C++, Java, Visual Basic, DAQ cards, GPIB and other interfaces. These components can be purchased from National Instruments. The LabVIEW graphical programming language is extremely versatile, and can be used to design and develop Virtual Instruments for various courses. The following are some of the application areas of LabVIEW:
Simulation--simulates physical processes
Data Acquisition--data acquisition from outside source
Data Processing--built in analysis library that includes signal generation, measurement, filters, windows, curve fitting, probability and statistics, linear algebra, and numerical methods,
Instrument and Control--Virtual Instruments (vi)
Program Development--Object oriented/graphical programming
Fuzzy Logic--fuzzy logic tool box
ANALOG COMMUNICATION VIs
This section describes in brief the underlying principles behind Amplitude Modulation, Frequency Modulation and Demodulation (Miller, 1996).
The frequency of human voice ranges from about 20 Hz to 3000 Hz. Transmission of such low frequencies as radio waves is impractical and useless because of a) interference problem and b) the largeness of antenna. The solution to this problem is Modulation. Modulation is a process of mixing a low frequency information and high frequency carrier through a nonlinear device. The transmission takes place at the high frequency (the carrier) which is modified to carry the low frequency information. Assume that the high frequency carrier is given by the following equation:
v = [V.sub.P] sin (wt+F) (1)
v = instantaneous value,
[V.sub.P] = peak value,
w = angular velocity = 2Of,
F = phase angle
Any one of these three terms could be varied to produce a modulated signal that contains intelligence. If the amplitude term VP is varied, the resulting modulation is called as Amplitude Modulation (AM), if f term is varied, the resulting modulation is called as Frequency Modulation (FM), and if F is varied, the resulting modulation is called as Phase Modulation (PM). In reality, most of the transmission is carried out as a combination of all these three modulation techniques. In this paper we will concentrate on the Amplitude Modulation VI, Frequency Modulation VI and Demodulation VI. The IT graduates encounter the modulation and demodulation concept (MoDem) in their networking and telecommunication classes. The modulation/demodulation VIs will help them to understand the subject matter better.
The amplitude for the AM wave form can be written as a sum of carrier amplitude [E.sub.c] and intelligence amplitude [e.sub.i].
E = [E.sub.c] + ei (2)
But [e.sub.i] = [E.sub.i] sin [w.sub.i]t (3)
E = [E.sub.c] + [E.sub.i] sin [w.sub.i]t (4)
Let m be a measure of the extent to which carrier voltage is varied by intelligence, and m is defined as
m = modulation index = [E.sub.i] / [E.sub.c] (5)
Equation 5 gives [E.sub.i] = m [E.sub.c], and substituting [E.sub.i] in equation 4 gives
E = [E.sub.c] + mEc sin [w.sub.i]t = [E.sub.c] (1+ m sin [w.sub.i]t) (6)
The instantaneous value of the AM wave is
e = [E.sub.c] (1+ m sin [w.sub.i]t) sin [w.sub.c]t (7)
Equation 7 can be written as
e = [E.sub.c] sin [w.sub.c]t + (m [E.sub.c]/2) cos ([w.sub.c] - [w.sub.i])t - (m [E.sub.c] /2) cos ([w.sub.c] + [w.sub.i])t (8)
Equation 8 shows that the AM waveform contains three terms:
1. the upper side frequency [f.sub.c] + [f.sub.i]
2. the lower side frequency [f.sub.c] - [f.sub.i]
3. the carrier [f.sub.c]
In an AM, the carrier amplitude and frequency is always constant whereas the sidebands are usually changing in amplitude and frequency. The carrier contains most power since its amplitude is always approximately twice of the sideband's amplitude. The total power in an AM is given by
[P.sub.t] = [P.sub.c] (1 + [m.sup.2]/2) (9)
Where [P.sub.t] = total transmitted power (watts),
[P.sub.c] = carrier power (watts),
m = modulation index
Both frequency modulation and phase modulation fall under the general category of angle modulation. Frequency modulation is a special case of angle modulation where the instantaneous frequency of a carrier is varied by an amount proportional to the modulating signal amplitude. Noise and bandwidth are two basic limitations on the performance of a communication system. The performance of an amplitude modulated communication system degrades with the magnitude of the external noise, and they have limited bandwidth. FM was developed to address these limitations of FM. The mathematical analysis of angle modulation is complex, and requires the use of high-level mathematics. In this paper we will use a formula to design our VI. The equivalent formula for FM is given by:
e = A sin ([w.sub.c]t + [m.sub.f] sin [w.sub.i]t) (10)
e = instantaneous voltage,
A = peak value of original carrier wave,
[w.sub.c] = carrier angular velocity,
[m.sub.f] = modulation index for FM, and
[w.sub.i] = modulating signal angular velocity.
[m.sub.f] = modulation index for FM = d/[f.sub.i] (11)
d = maximum frequency shift caused by the intelligence signal (deviation)
[f.sub.I] = frequency of intelligence signal.
The VI for amplitude modulation is designed using equation 7 and is presented in Figure 1. Figure 2 presents a VI for modulation index and power. The VI for frequency modulation is designed using equation 10 and is presented in Figure 3. Figure 4 presents the VI for demodulation.
It uses the AM VI of Figure1 and a Butterworth Filter to filter out the carrier. The students can use these VIs to study the AM, FM, and Demodulation concept. The interested reader may refer to Sokoloff (1998) to learn how to program in LabVIEW (Anderson, Korrapati, & Swain, 2000; Swain, Anderson, & Korrapati, 2000).
CONCLUSION AND DISCUSSION
The AM VI presented in Figure 1 is simulated with 50% modulation index, 150 Hz carrier frequency and 10 Hz signal frequency. The user can simulate this VI with different values for these three parameters and observe over modulation, under modulation, and 100% modulation. The Modulation Index and Power VI in Figure 2 is simulated with modulation index values of 0 to 1.5 with 0.25 increments in modulation index. The sideband power and total power is calculated and presented for each value of modulation index. This VI is helpful to decide the amount of modulation. Usually, in an AM most AM transmitters attempt to maintain between 90% to 95% modulation as a compromise between efficiency and the chance of drifting into overmodulation. Figure 2 represents the FM VI. This VI is simulated with signal frequency of 10 Hz and carrier frequency of 100 Hz. The lower sidebands are at frequencies less than carrier frequency (100 Hz) and the upper sidebands are at frequencies greater than carrier frequency. The demodulation VI is presented in Figure 4. This VI uses the modulation VI of Figure 1. The output of the modulation VI is passed through a Butterworth Filter VI to remove the carrier and recover the original signal.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Anderson, J. A., Korrapati. R. B., & Swain. N. K. (2000). Digital signal processing using virtual instrumentation. Proceedings of SPIE Vol. 4052.
Korrapati, R. B. & Swain. N. K. (2000). Study of Modulation using Virtual Instruments. Proceedings of Academy of Information and Management Sciences, 4(1), National Conference on Allied Academies, Spring 2000.
Miller G. M. (1996). Modern Electronic Communication. New Jersey: Prentice Hall Publishing.
Sokoloff, L. (1998). Basic Concepts of LabVIEW 4. New Jersey: Prentice Hall Publishing.
Swain, N. K., Anderson, J.A, & Korrapati, R. B. (1999). Application of Graphical Programming, Object Oriented Programming, And Virtual Instruments in Education. Proceedings of 2nd International Conference On Trends in Information Technology, CIT 99, pp 257-299.
Swain, N. K., Anderson, J. A., & Korrapati. R. B. (2000). Computer based virtual engineering Laboratory (CBVEL) and Engineering Technology Education. 2000 Annual ASEE Conference Proceedings.
Raghu B. Korrapati, Webster University
Nikunja K. Swain, South Carolina State University
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|Author:||Korrapati, Raghu B.; Swain, Nikunja K.|
|Publication:||Academy of Information and Management Sciences Journal|
|Date:||Jul 1, 2000|
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