Design of Low Cost Modem Using Tele Typewriter Signals - Ijcns.com

International Journal of Computer Communication and Information System ( IJCCIS)– Vol2. No1. ISSN: 0976–1349 July – Dec 2010Design of Low Cost Modem Using Tele Typewriter SignalsD. Muthu Kumaran 1 , C. Charithartha Reddy 2 , C. Annadurai 31,2Final Year Students, 3 Assistant Professor, Department of ECE,SSN College of Engineering, Kalavakkam, Tamil Nadumuthukumaran.parktown@gmail.comAbstract - A Low cost Wireless Data Modem usingTele Typewriter Signals and Frequency Shift Keying(FSK) Modulation Technique is proposed here.Basically a Modem sends and receives Digital datasignals to and from various Computer Networks andperipheral devices. The transmission media used inmost of the Modem is Fiber Optic cable. The mainreason for using Fiber Optic cable is to avoidElectro Magnetic Interference (EMI). But the costinvolved in using this Fiber Optic cable is high. Soour aim is to design a modem whose transmissionmedium can be laid with low cost as well as themodem should avoid Electro Magnetic Interference(EMI). The Wireless Modem designed here consistsof 5 stages namely a) Modulation stage, b)Transmission stage, c) Reception stage, d) Filteringstage and e) Demodulation stage. The Modulationstage consists of IC 555 Timer working in AstableMode and generating Frequency Shift Keying (FSK)signals. Transmission is Wireless and is achievedusing IR LED. The Transmitted signal is receivedusing a spectrally matched Phototransistor. TheFiltering stage consists of a second order Low PassFilter (LPF). Demodulation is achieved using IC565(Phase Locked Loop) which recovers the inputmessage signal. The modem designed here makesuse of Tele Typewriter signals which neglectsconduction and radiation interference from externalenvironment. The Bandwidth of such TeleTypewriter signals is 200Hz. For signals with suchlower bandwidth Frequency Shift Keying (FSK) isthe best Modulation Technique for transmittingdigital information. The Probability of Error for thisWireless Data Modem is better with minimal BitError Rate (BER). The wireless transmissionmedium is achieved using the combination of IRLED and Phototransistor. So the cost involved inthis transmission medium is very low compared toFiber Optic cable. Hence the whole setup is Stable,cheap and accurate device for transmitting digitaldata.137Index terms - Frequency Shift Keying (FSK), TeleTypewriter signals, Low Pass Filter (LPF), ElectroMagnetic Interference (EMI), Phase Locked Loop(PLL), Probability of Error, Bit Error Rate (BER),Fiber Optic cable, cost.I. INTRODUCTIONWireless Data Modem plays a very vital role in thefield of digital data communication. In this scientificera, data transfer is very crucial and most of the datatransfer between various computer networks andperipheral devices is done using Modem. Thetransmission medium used in Modem is Fiber Opticcable which is cable of avoiding Electro MagneticInterference (EMI). But the cost involved in usingFiber Optic cable as the transmission medium is veryhigh. So the alternate choice is to use a wirelessMedium which is cheap. But wireless medium is notimmune to Electro Magnetic Interference (EMI). Sofor improving the immunity of the Modem againstElectro Magnetic Interference (EMI) we use Tele TypeWriter signals. For the transfer of digital data usingTele Type writer signals whose frequency range is from1070Hz to 1270Hz, a Wireless Data modem isproposed. The Tele Typewriter includes a Start Signal,Data information defined by signal transitions between1070Hz to 1270Hz and a stop signal which are operatedupon so that they are converted to signals adopted foruse in digital logic circuits in a unique manner toneglect Electro Magnetic Interference (EMI) of both theradiation and conduction type. So for transmittingdigital information using such signals, the basicmodulation and demodulation technique used isFrequency Shift Keying (FSK). It is one of theFrequency Modulation technique in which Digital datato be transmitted by means of a carrier signal whosefrequency is shifted between two preset frequenciesnamely f H and f L, where f H corresponds to logic 1 and f Lcorresponds to logic 0 of the input binary message datato be transmitted. The process of FSK modulation canbe best understood by means of Figure 1. The messagesignal to be transmitted is in the binary form of 0’s and1’s. The carrier signal is a continuous analog signal ofdesired frequency as shown in Figure 1. So when themessage signal is of logic 1, the resultant modulatedsignal’s frequency will be f H. When the message signalis of logic 0, the resultant modulated signal’s frequencywill be f L. Thus the carrier signal frequency is shiftedbetween two preset frequencies. Thus the resultant FSKmodulated signal is shown in the Figure 1.

International Journal of Computer Communication and Information System ( IJCCIS)– Vol2. No1. ISSN: 0976–1349 July – Dec 2010Figure 1: Frequency Shift Keying ModulationSo for transmitting digital data using Tele TypewriterSignals, we use f H=1070Hz and f L=1270Hz. The majorreason for choosing Frequency Shift Keyingmodulation technique is due to the small bandwidthrequirement of the Tele Typewriter signals. TheBandwidth is 200Hz which is very low. The usageother modulation techniques like amplitude modulationor other frequency modulation techniques like ASK(Amplitude Shift Keying), PSK (Phase Shift Keying),the input binary message signal is not effectivelyreproduced. The obtained message signal is distortedand does not resemble the input message signal. Theaccuracy of FSK technique is better than othermodulation techniques. The Probability of Error ofFSK technique is also good. So because of thesereasons Frequency Shift keying has been selected as themodulation technique in our modem.II. BLOCK DIAGRAM DESCRIPTIONFigure 2: Block DiagramThe basic block diagram of the Wireless DataModem is shown in Figure 2. It consists of five stagesnamely a) Modulator Stage, b) Transmitter Stage, c)Receiver Stage, d) Filter stage and e) DemodulatorStage. The aim is to transmit the Input digital dataeffectively from one network to another network orfrom one network to other peripheral devices. The firststep is to modulate the input digital data with the carriersignal using Frequency Shift Keying ModulationTechnique. This is achieved using IC 555 Timer, whichproduces the FSK modulated signal whose f H=1070Hzand f L=1270Hz, which means that when the inputmessage signal is of logic 1 the frequency of themodulated signal is 1070Hz and when the inputmessage signal is of logic 0 the frequency of themodulated signal is 1270Hz. The modulated signal isthen transmitted using IR LED. Thus Wireless Datatransfer is achieved by the using IR LED. Thetransmitted signal is received at the Receiver side bymeans of a spectrally matched Phototransistor whichthen sends the obtained modulated signal to the FilterStage. The Filter stage consists of a Second order LowPass Filter (LPF). The final stage is the Demodulatorstage which consists of Phase Locked Loop (PLL) anda RC ladder Filter. The Phase Locked Loop isimplemented using IC 565. Thus the demodulatorStage separates the message signal from the carriersignal and hence the digital data signal is obtainedsuccessfully. The Baud rate which denotes the rate oftransfer of data for this modem is given asBaud rate (1)Here =( 1+ 2)=1.722ms, 1=0.935ms,2=0.787ms, where 1 is the time during which bit 1lasts and 2 is the time during which bit 0 lasts andhence the Baud Rate is 580 bps (Bits per Second).III. MODULATIONModulation is the process of altering thecharacteristics of the carrier signal in accordance to themessage signal. Here Frequency Shift KeyingModulated signal is obtained using IC 555 Timer. Thecircuit diagram for FSK Signal generation using IC555 Timer is shown in Figure 3. It consists of a IC 555Timer which works in Astable Mode. The resistors R a,R b and capacitor C determines the Frequency of theFSK modulated signal. The standard digital data inputfrequency is usually 150Hz. When the input is HIGHi.e. when the input binary data is of logic 1, the PNPtransistor Q is off and IC 555 Timer works in thenormal astable mode of operation. The frequency ofthe output FSK modulated signal is given by theequationf 0= 1.45/(R a+2R b)C (2)Figure 3: FSK Generation using IC 555 timer138

International Journal of Computer Communication and Information System ( IJCCIS)– Vol2. No1. ISSN: 0976–1349 July – Dec 2010The Resistors R a, R b and capacitor C are selected insuch a way that the value of f 0 is 1070Hz. The circuitdiagram when the input is HIGH is shown inthe Figure 4.Figure 6: FSK modulated signalIV. TRANSMISSIONFigure 4: When the Input is HIGHHere the values of R a= 50K ohm, R b= 47K ohmand C=10nF. When the input is LOW i.e. when theinput binary data is of logic 0, the PNP transistor Q ison and its connects the resistance R c across theresistance R a. Thus now the frequency of the outputFSK modulated signal is given by the equationf 0= 1.45/((R a||R c)+2R b)C (3)The Resistors Rc is selected in such a way that thevalue of f 0 is 1270Hz. The circuit diagram when theinput is LOW is shown in the Figure 5. Herethe value of R c=50K ohm. The capacitor C1 is used tobypass noise and ripples from the supply. The resultantFSK modulated signal along with the digital binaryinput data at 150Hz is shown in the Figure 6. From theFigure 6 it is evident that during logic 1 of the binaryinput data f H=1070Hz and during logic 0 of the binaryinput data f L=1270Hz. Thus the Frequency ShiftKeying (FSK) modulated signal is obtained from themodulator stage.Figure 7: IR LED emitterAn infrared emitter is an LED made from GalliumArsenide, which emits near-infrared energy at about880nm. The Frequency Shift Keying signal from theModulator is now wirelessly transmitted to the receiverside using IR LED. The IR LED used as a transmitteris shown in figure 7. The circuit diagram for thisprocess is shown in figure 8. The value of the limitingResistor R is calculatedFigure 8: Transmitter using IR LEDusing the formula(4)Here Supply voltage V s is 5V, LED Voltage drop V f is3.3V and LED current rating I f is 20mA. Hence thevalue of R is 85 ohms.Figure 5: When the Input is LOW139

International Journal of Computer Communication and Information System ( IJCCIS)– Vol2. No1. ISSN: 0976–1349 July – Dec 2010V. RECEPTIONFigure 9: Phototransistor DetectorThe infrared Phototransistor acts as a transistorwith the base voltage determined by the amount of lighthitting the transistor. Hence it acts as a variable currentsource. Greater amount of IR light cause greatercurrent to flow through the collector-emitter leads. Thesignal from the Transmitter side should be received atthe Receiver side by a spectrally matchedPhototransistor. The Phototransistor used as a Receiveris shown in the figure 9. The circuit diagram for thisprocess is shown in figure 10. Thus the whole processof wireless transfer of data is achieved using thecombination of IR LED and a spectrally matchedPhototransistor. This wireless transmission channel ischeaper in comparison to the transmission mediumobtained using Fiber Optic cable. The importantparameter for a Modem, which is the Probability ofError is good for this combination of IR LED andphototransistor.Figure 10: Receiver using PhototransistorVI. FILTERThe Low Pass Filter (LPF) is used to eliminate theunnecessary high frequency components from any lowfrequency signal and hence helps in improving theProbability of Error of a System. Here a second orderLPF as shown in the figure 11 is used to remove thehigh frequency Noise components introduced by theWireless Channel. The timing resistors are calculatedusing the following formulaef L=1/(2*pi*R 1*C 1) (5)f H=1/(2*pi*R 2*C 2) (6)So for f L=1070Hz and f H=1270Hz, we get R 1=745 ohm,R 2=1.25K ohm, C 1=200nF, C 2=100nF.Figure 11: Low Pass FilterVII. DEMODULATIONThe process of eliminating the Carrier signal andrecovering the base band message signal from themodulated signal is termed as Demodulation. Here theprocess of demodulation is achieved using PhaseLocked Loop (PLL) IC 565. The circuit diagram forachieving the process of demodulation using PLL IC565 is shown in figure 12. In the 565 PLL thefrequency shift is usually accomplished by driving aVoltage Controlled Oscillator (VCO) with the digitaldata signal so that the two resulting frequencies correspondto the logic 0 and logic 1 states of the digital datasignal. The frequencies corresponding to logic 1 andlogic 0 states are commonly called the mark and spacefrequencies. The demodulator receives a signal at oneof the two distinct carrier frequencies 1,270 Hz or1,070 Hz representing logic levels of mark (- 5 V) orspace (+ 14 V), respectively. Capacitance coupling isused at the input to remove a dc level. As the signalappears at the input of 565 PLL, the PLL locks to theinput frequency and tracks it between the two possiblefrequencies with a corresponding dc shift at the output.Resistor R 1 and capacitor C 1 determine the free-runningfrequency of the VCO. Capacitor C 2 is a loop filtercapacitor that establishes the dynamic characteristics ofthe demodulator. Capacitor C 2 is chosen smaller thanusual one to eliminate overshoot on the output pulse. Athree-stage RC ladder filter is employed for removingthe sum frequency component from the output. TheVCO frequency is adjusted with R 1 so that the dcvoltage level at the output (pin 7) is the same as that atpin 6. An input at frequency 1,070 Hz drives thedemodulator output voltage to a more positive voltagelevel, driving the digital output to the high level (spaceor + 14 V). An input at 1,270 Hz correspondinglydrives the 565 dc output less positive with the digitaloutput, which then drops to the low level (mark or – 5V). The comparator prevents the amplitude changesoccurring at the output stage. Thus the demodulatorstage effectively reproduces the input digital data at150Hz. The resultant digital data signal obtained fromthe FSK signal by the Demodulation process is shownin figure 13.140

International Journal of Computer Communication and Information System ( IJCCIS)– Vol2. No1. ISSN: 0976–1349 July – Dec 2010Figure 12: Demodulator using PLL 565Figure 13: Input Digital data from FSK signalVIII. PROBABILITY OF ERRORThe most important parameter that defines theefficiency of a Modem is the Probability of Error andBit Error Rate (BER). BER means the amount of errorintroduced in the transmitted signal by the channel’snoise. Usually the noise considered in a wirelessmedium is Additive White Gaussian Noise with zeromean and power Spectral density as N o/2. Theexpression for Probability of Error for this wireless datamodem which uses Non Coherent FSK demodulation isgiven asp e=1/2*exp(-E b/(2*N o)) (7)where E b is the transmitted signals energy per bit. Theplot of Probability of Error and E b/N o is shown inFigure 14. From the Figure 14 it is very clear that theBit Error Rate (BER) for this wireless data Modem isvery low.Figure 14: Bit Error Rate (BER)IX. CONCLUSIONThus in this paper a Low cost Wireless DataModem using Tele Typewriter signals and FrequencyShift Keying (FSK) Modulation and Demodulationtechnique is designed and its importance to transmitdigital data wirelessly with minimal Bit Error Rate(BER) without getting affected by Electro MagneticInterference (EMI) of conduction and radiation typefrom the external environment is analyzed.X. REFERENCE[1]. Tom McDermott, Wireless Digital Communications: Designand Theory, Tucson Amateur Packet Radio Corporation,Tucson, Arizona, 1996.[2]. Peyton Z. Peebles, Digital Communication Systems, Prentice-Hall, Englewood Cliffs, New Jersey, 1987.[3]. Smon Haykins, Communication Systems, John Wiley, 4 thEdition, 2001.[4]. Jhn G. Proakis, Digital Communication, McGraw Hill, 3 rdEdition, 1995.[5]. Tub & Schilling, Principles of Digital Communication, TataMcGraw-Hill, 28 th reprint, 2003.141