a set of labview-based virtual tools for communication

A SET OF LABVIEW-BASED VIRTUAL TOOLS FOR COMMUNICATION LABS IN ECE DEPARTMENTS Levent Sevgi, Çağatay Uluışık Doğuş University, Electronics and Communication Engineering Department, Zeamet Sok. No. 21, Acıbadem / Kadıköy, 34722 Istanbul - Turkey

Abstract A set of virtual tools for analog and digital communication labs is designed and introduced. The National Instrument’s NI-ELVIS set and the LabVIEW student edition package are used for this purpose. The designed double-side band, single-side band, carrier-suppressed double-side band amplitude modulation and demodulation tools may be used in basic undergraduate communication lectures. Moreover, these virtual tools may be used in distance education over internet via remote panels which basically relies on the simulation of engineering courses to illustrate the physical phenomena, like Java applets, PSpice, Matlab, etc.

I. INTRODUCTION Virtual tools and labs in engineering education have increasingly gained attention parallel to the developments in the electronics and computer technologies as well as wide spread use of internet facilities. This usage also lowers the costs of the establishment/modification of engineering labs substantially. On the other hand, the untouchables of the engineering education --mathematics, physics, experience and practice − should be kept in mind at all stages. As a result, the challenge is to establish an intelligent balance between virtual and real labs, so as to optimize cost problems, while graduating sophisticated engineers with enough practice [1-9]. Recently, National Instruments (NI) has introduced a LabVIEW-based design and prototype environment, the NI Educational Laboratory Virtual Instrumentation Suite (NI ELVIS), especially for the engineering labs. NI ELVIS combines virtual instrumentation, data acquisition, and a prototype station so that students can create, measure, and analyze "real" circuits. It reduces the cost of lab equipment while reinforcing theory through hands-on learning. Designed for the open-platform, NIELVIS combines the PC, industry standard NI LabVIEW software, and NI data acquisition (DAQ) devices to allow prototyping, design, testing, measurement, and data analysis with multi-instrument functionality, such as an oscilloscope, digital multimeter, function generator, power supply, bode analyzer, etc. NI-ELVIS certainly enhances engineering and science education worldwide by providing educators and students with innovative software and hardware to connect the curriculum with the real world. NI-ELVIS has already been used worldwide from physics departments, to bioelectromagnetics. A quick interest search may show that [10], for example, the electrical and computer engineering (ECE) department of Georgia Institute of Technology in USA uses NI-ELVIS in lectures such as ECE 3041: Instrumentation and Circuits, and ECE 3042: Microelectronic Circuits, the physics department of Hanyang University in Korea in PHY 323: Physical Measurement, PHY 222: Electrical Circuits, and PHY 316: Optical Experiments, the mechanical engineering department at Kettering University in USA in MECH 231: Signal Analysis for Mechanical Systems, the ECE department of Texas A&M University in USA in EE 214: Electrical Circuit Theory and in ECE 325: Electronics, the biomedical engineering department of University of Texas at Austin in BME 311: Network Theory and in BME 221: Measurement and Instrumentation Laboratory, etc.

II. COMMUNICATION LABS AT DOĞUŞ The ECE Department of Doğuş University in Istanbul has started to use NI-ELVIS in Analog and Digital Communication lectures, ECE 311 and ECE 312, respectively since 2004. The Analog Communication Lab (in ECE 311) contains the experiments and VTs for: ƒ ƒ ƒ ƒ ƒ ƒ ƒ

The Fourier Transform Double SideBand (DSB) AM Signals Double Sideband (DSB) Amplitude Modulators DSB Suppressed Carrier (DSBSC) AM Signals Single SideBand (SSB) AM Signals Envelopes and Envelope Recovery (Demod. of DSB) The Angle Modulation (FM & PM).

Dogus_FFT.vi DSB_AM_Soft.vi DSB_AM.vi DSBSC_AM.vi SSB_AM.vi Demod_DSB.vi FM-PM.vi

Similarly, experiments and VTs of the Digital Communication Lab (in ECE 312) are: ƒ ƒ ƒ ƒ ƒ ƒ ƒ

Sampling Time Division Multiplexing – TDM Pulse Code Modulation – PCM Amplitude Shift Keying – ASK Frequency Shift Keying – FSK Binary Phase Shift Keying – BPSK Delta Modulation – DM.

Sampling.vi TDM.vi PCM.vi ASK.vi FSK.vi BPSK.vi Delta.vi

Some of the analog comm. virtual tools are presented in this study. A typical example is the amplitude modulation (AM) tool. Modulation is a process that modifies one or more parameters of a signal with another signal. The modulated (modified) signal is called carrier signal and principally its frequency is much higher than the modulating signal in order to protect the original message signal (modulating signal) from the channel distortions. In AM, the amplitude of the carrier is modulated by the message signal. The double side-band (DSB) amplitude modulated (AM) signal is defined as:

x DSB (t ) = (1 + m x(t ) )xc (t ) = (1 + m Am cos( wm t ) )Ac cos( wc t )

(1)

where m is the modulation index, x(t) is the message signal, xc(t) is the carrier signal and xDSB(t) is the modulated signal. In (1), ω m and ω c are angular frequencies in rad/s, where ω m /(2π), is the message frequency and ω c /(2π) is the radio, or carrier frequency. Notice that the modulating signal contains both a DC component and an AC component. It is the DC component which gives rise to the term at ω c frequency- the carrier- in the DSB signal. In (1), m is a constant which defines the depth of modulation, and can be calculated as the ratio of the amplitude of the AC part to the amplitude of the DC part. The magnitude of m can also be measured directly from the AM display itself. Thus, (2) P−Q

m=

P+Q

where P and Q are defined in Fig. 1. It should be noted that: • For all values of m ≤ 1, the envelope of the DSB signal has the same shape as that of the message. • For all values of m > 1, the envelope is not a copy of the message shape.

In case of m > 1 (over-modulation) it is not possible to recover the original message signal from the envelope of the DSB modulated signal. On the other hand, simple envelope detection methods can be used for message recovery (demodulation) purposes when m ≤ 1. The functional block diagram of the DSB modulator is shown in Figure 2. As shown, the message signal is multiplied by the carrier signal and the resultant signal is then added to the carrier.

III. LABVIEW-BASED VIRTUAL TOOLS FOR ANALOG COMM. A general view of the front panel of VTs that are designed for the communication labs is as shown in Fig. 3. This front panel belongs to the double-side-band AM VT DSB_AM_SOFT. This example belongs to a 50 Hz single tone message signal modulating a 500 Hz carrier signal with the modulation depth of m=0.8. The window on top presents the graph of message signal vs. time, while the one at bottom belongs to the AM signal (i.e. the signal in eq. (1)) vs. time plot. All the parameters of these two signals can be specified and altered with the knobs and buttons reserved at the left. A sinusoidal message signal modulates the amplitude of a carrier signal. The frequency and the amplitude of the message signal and of the carrier signal can be chosen as desired using numerical controls and knobs. The message signal may be sinusoidal, rectangular, or triangular. Noise can also be added to the message signal. Finally, the user can also display the signals in frequency domain by changing the position of a toggle switch as shown in the figure. A photo of a complete student lab set is shown in Fig. 4 with the LabView installed PC, NI-ELVIS set, and an oscilloscope. The message and the carrier signals are generated by using the “Simulate Signal” Express VI tool as shown in Fig. 5a and 5b. The block diagram of the soft version VT, which uses only the LabView software and a PC, is shown in Fig. 5a . The block diagram of the software-hardware combination VT, which uses the NI-ELVIS set, is given in Fig. 5b. The user can display the signals on the PC screen or on an oscilloscope by using a slide- switch. If the user changes the status of the SCREEN/SCOPE slide-switch to SCOPE, the signal is sent via the DAC to the NI-ELVIS board and then forms the analog output which feeds the oscilloscope. Which signal (the message or the modulated signal) is to be sent to the scope is determined using another slide switch CH1/CH2. Fig. 6 shows the message and AM signals of Fig. 3 in the frequency domains. The 50 Hz message signal, the 500 Hz carrier signal and the two side bands at 450 Hz and 550 Hz are clearly observed in the figure. The noise-added message and AM signals of the same example are plotted in Fig. 7. Here, the signal and noise amplitudes are 1.0 V and 0.5 V, respectively. Since power is proportional with the square of the voltage amplitudes, these values correspond to a signal-to-noise-ratio (SNR) of 4.0 (or 6.0 dB). The frequency domain graphs of this example are shown in Fig. 8. Another example is given in Fig. 9 for the noise added rectangular message signal with the SNR of approximately 10 dB. Variety of different versions of AM modulation VTs listed in Sec. II , are also designed and used in analog communication labs; double side band suppressed carrier (DSBSC), single side band (SSB), etc. An example is given in Fig. 10, where frequency characteristics of multi-tone modulated message signal (top) and DSBSC AM signal are presented. In this front panel the user selects the frequency of the message signal (fm) and the VT automatically generates replicas at 0.5fm, 0.7fm, 1.3fm, 1.5fm. The 5tone message signal is modulated with the user specified carrier frequency and its frequency spectrum is shown (bottom). The modulation type (DSB, DSBSC, SSB, etc.) of the VT is selected from the pulldown menu at bottom left. The second pull-down menu is reserved for different filter types. The effects of, for example, inverse Chebychev filter for the LSSB AM signal is given in Fig. 11. Practical filters are not ideal therefore some message signals would be present at the upper band after filtration. An example for the software-hardware hybrid usage of the NI-ELVIS set is given in Figs. 12 and 13. A photo of the set is shown in Fig. 12. Here, the PC generated (soft) AM signal is fed into a series combination of a diode and a resistor. The output is obtained from the terminals of the resistor. The

output of this resistor on the NI-ELVIS board is then fed back into the PC as well as given to the oscilloscope. The block diagram of this procedure is as given in Fig. 5b. The PC screen in Fig. 12 is zoomed out and plotted alone in Fig. 13. The diode is a non-linear element and reproduces integer multiples of all present frequencies. In this example, the frequencies of the message and carrier signals are 100 Hz and 1 kHz, respectively. The harmonics of the message signal at 200 Hz, 300 Hz, etc., are clearly observed in Fig. 13 at the top figure. Also DSB AM signal at 1 kHz, at 2 kHz, etc. can be distinguished. Once, the output of the resistor is (soft) filtered the characteristic DSB AM signal (i.e., 1 kHz carrier signal, lower side signal at 900 Hz and upper side signal at 1100 Hz) is obtained as shown at the bottom figure. The demodulation of the AM signal is nothing but re-multiplication by the carrier frequency and low pass filtration as shown in the block diagrams in Fig. 14. The top and bottom block diagrams in this figure correspond to unsuppressed and suppressed carrier AM signal demodulations, respectively. A typical example of the demodulation of the DSB signal is given in Fig. 15. Modulated and demodulated 50 Hz message signals are shown at top and bottom, respectively.

IV. CONCLUSIONS AND DISCUSSIONS Novel virtual tools (VT) are designed for the ECE communication labs at Doğuş University of Istanbul. The design steps, block diagrams as well as characteristic examples are presented. These VTs certainly lower the cost of the lab establishment while improving teaching/learning capability. Since engineering should be based on practice, the hybrid usage of hardware and software via NI ELVIS product bundle is an optimum solution. It should be noted that these approaches will also form the fundamentals of distance education.

REFERENCES [1] L. SEVGI, Complex Electromagnetic Problems and Numerical Simulation Approaches, IEEE Press – John Wiley and Sons, June 2003 [2] L. Sevgi, Ç. Uluışık, "A Matlab-based Visualization Package for Planar Arrays of Isotropic Radiators", IEEE Antennas and Propagation Magazine, Vol. 47, No. 1, pp. 156-163, Feb 2005 [3] L. Sevgi, Ç. Uluışık, F. Akleman, "A Matlab-based Two-dimensional Parabolic Equation Radiowave Propagation Package", IEEE Antennas and Propagation Magazine, (to appear) 2005 [4] L. Sevgi, "A Ray Sooting Visualization Matlab Package for 2D Ground Wave Propagation Simulations", IEEE Antennas and Propagation Magazine, Vol. 46, No 4, pp.140-145, Aug 2004 [5] L. Sevgi, Ç. Uluışık, "A Matlab-based Transmission Line Virtual Tool: Finite-Difference time-Domain Reflectometer", IEEE Antennas and Propagation Magazine, (in review) July 2005 [6] L. B. Felsen, L. Sevgi, "Electromagnetic Engineering in the 21st Century: Challenges and Perspectives”, (introductory paper) Special issue of ELEKTRIK, Turkish J. of Electrical Engineering and Computer Sciences, Vol. 10, No.2, pp.131-145, Feb 2002 [7] L. Sevgi, "EMC and BEM Engineering Education: Physics based Modeling, Hands-on Training and Challenges", IEEE Antennas and Propagation Magazine, Vol. 45, No.2, pp.114-119, April 2003 [8] L. Sevgi, İ. C. Göknar, "An Intelligent Balance in Engineering Education", IEEE Potentials, Vol. 23, No.4, pp. 40-41, Oct/Nov 2004 [9] L. Sevgi, Ç. Uluışık, "A LabView-based Virtual Instrument for Engineering Education: A Numerical Fourier Transform Tool", Special issue of ELEKTRIK, Turkish J. of Electrical Engineering and Computer Sciences, (to appear) Jan 2006 [10] Visit http://www.ni.com

FIGURE CAPTIONS Figure 1: The AM modulated signal vs. time. Figure 2: A simple block diagram of the AM. Figure 3: The front panel of the DSB_AM_Soft VT. A sinusoidal message signal modulates the amplitude of a carrier signal (fM=50 Hz, fc=500 Hz, Modulation Type: Double Side Band -Amplitude Modulation DSB AM) Figure 4: The NI-ELVIS set in Doğuş ECE Communication Lab. Figure 5: (a) Main block diagram of the DSB_AM_Soft VT, (b) Main block diagram of the DSB_AM soft-hard VT. Figure 6: The front panel of the DSB_AM_Soft VT in the frequency domain ((fM = 50 Hz, fc=500 Hz, Modulation Type: Double Side Band AM). Figure 7: The effect of additive noise in time domain ((fM = 50 Hz, fc=500 Hz, SNR= 6 dB, Modulation Type: Double Side Band AM). Figure 8: The effect of additive noise in the frequency domain (fM = 50 Hz, fc=500 Hz, SNR = 6 dB, Modulation Type: Double Side Band AM). Figure 9: The front panel of the DSB_AM_Soft VT. A rectangular pulse train modulates the amplitude of the carrier signal (fM = 50 Hz, fcr=500 Hz, Modulation Type: Double Side Band AM). Figure 10: The front panel of the SSB_AM VT in the frequency domain with a multi-tone message signal (fc=500 Hz, Modulation Type: Double Side Band Suppressed Carrier AM). Figure 11: The front panel of the SSB_AM VT in the frequency domain with a multi-tone message and after the filtration (fc=500 Hz, Modulation Type: Lower Single Side Band AM, Filter Type: Inverse Chebychev). Figure 12: A photo of the NI-ELVIS set at Doğuş ECE Lab: DSB AM Experiment using NI-ELVIS, DAC, LabView software, oscilloscope, and some basic electrical components such as diodes and resistors. Figure 13: DSB AM Experiment using NI-ELVIS set: The frequencies of the message and carrier signals are 100 Hz and 1 kHz, respectively. Observe the harmonics of the message signal at 200 Hz, 300 Hz, etc., and AM signal at 1 kHz, at 2 kHz, etc. The output of the resistor is also soft-filtered and shown. Figure 14: The block diagrams of the AM demodulation experiments; (top) for suppressed carrier AM signal, (bottom) for the DSB AM signal. Figure 15: An example for the front panel of the Demod_DSB VT. The message signal is obtained from the modulated signal (fM = 50 Hz, fcr=500 Hz, Modulation Type: Double Side Band AM).

s(t) P

Q

Fig. 1

mx(t)

×

∼

+ x c (t)

Ac cos(wc t)

Fig. 2

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Fig. 5a

Fig. 5b

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Fig. 6

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Fig. 8

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Fig. 10

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Fig. 12

Fig. 13

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Modulated signal Message signal

Modulated signal Message signal Carrier signal

Fig. 14

Fig. 15

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