4-channel digital commu based on fsk

Project On DIGITAL COMMUNICATION SYSTEM BASED ON FSK : 

Project On DIGITAL COMMUNICATION SYSTEM BASED ON FSK GUIDED BY :- PROFF. DAS CHOUDHARY PRESENTED BY :- RAJAN KUMAR (0501212311) RAGHUNATH PRASAD (0501212322) PRAVIN KUMAR (0501212327) PRAGYAN PARMITA SOREN (0501212199)

INTRODUCTION TO THE PROJECT : 

INTRODUCTION TO THE PROJECT PROJECT DEFINITION The project is of making a communication system by designing a transmitter & a receiver for four digital channels using FSK modulation.  The transmitter is designed in such a way so as to transmit the signal at variable frequency with the help of a varactor diode and the receiver can detect the entire transmission band. The type of modulation being used is FSK(frequency shift keying). An ADC used prior to an FM transmitter, yields the purpose of attaining FSK.We will be multiplexing the inputs for having 4 channels of the digita data and finally get the demultiplexed data at the output.

INTRODUCTION OF COMMUNICATION SYSTEM : 

INTRODUCTION OF COMMUNICATION SYSTEM BLOCK DIAGRAM OF A COMMUNICATION SYSTEM ENCODING DECODING MODULATION DISTORTION MODULATION Information source Transmitter Channel Receiver Destination Noise source

INTRODUCTION OF COMMUNICATION SYSTEM : 

INTRODUCTION OF COMMUNICATION SYSTEM INFORMATION SOURCE: The communication system exists to communicate a message. This message comes from the information source. These may be words, group of words, code symbols or any other prearranged units. The amount of information contained in any given message is measured in bits.

TRANSMITTER : 

TRANSMITTER The message that comes from the information source is unsuitable for immediate sending because it is not electrical in nature. Incoming sound signals are converted into electrical variations before modulation. In the transmitter, the information modulated the carrier. The modulation methods (Analog: AM, PM, FM & Digital: ASK, PSK, FSK) vary from one system to another system depending upon the application

CHANNEL NOISE : 

CHANNEL NOISE A signal will deteriorate during the process of transmission and reception as a result of some distortion in the system or because of introduction of noise, which is unwanted energy usually of random character present in a transmission system due to any cause. Noise will limit the transmission system. When the noise is severe, it will mask a given signal so much that the signal becomes unintelligibly and sourseless. Noise will have its greatest effect on the weakest signal.

RECEIVER : 

RECEIVER The main function of receiver is demodulation. The receiver depends upon Modulation system Operating frequency Frequency range Type of display The output of receiver may be fed to a loud speaker, video display unit, teletypewriter, various radar display, CRO, television picture tube or computer.

APPLICATIONS : 

APPLICATIONS A transceiver(transmitter-receiver) is advantageous because of its low cost, less component requirement (due to sharing), portability, less energy consumption and ease of operation. Generally, transceivers are used in mobile phones, ham radio, cordless phones, ‘walkie-talkie’, automatic telegraphy etc. In mobile phones about 650 MHz frequency transceivers are used while in satellites, frequency is more than 1 GHz.

MODULATION : 

MODULATION To transmit a low frequency signal faithfully to a longer distance we use it to modulate the high frequency carrier signal. There are various types of Modulation used: Analog and Digital Modulation Depending on the type of the data transmitted. Our data being digital we will use the digital modulation technique. There are mainly three types of digital modulation: 1. ASK (Amplitude Shift Keying) 2. PSK (Phase Shift Keying) 3. FSK (Frequency Shift Keying)

AMPLITUDE SHIFT KEYING : 

AMPLITUDE SHIFT KEYING With Amplitude Modulation the digital signal is used to switch the carrier between amplitude levels, hence it is referred to as ASK. In this particular case the carrier is switched ON and OFF. While using ASK with pulses, ON corresponds to mark(digital 1) and OFF to space(digital 0), hence the method is known as ON-OFF keying

PHASE SHIFT KEYING : 

PHASE SHIFT KEYING In this case the phase of the carrier gets inverted when input signal varies from 1 to 0 or vice versa. Thus the information resides in the phase of the pulse. FREQUENCY SHIFT KEYING Here the frequency of the carrier is varied according to the amplitude of the input digital data. In the case of binary signal two carrier frequencies are used, one corresponding to the binary 0 and the other to a binary 1. Thus FSK may be thought upon as a digital FM system i.e. an FM system in which the carrier frequency is midway between the mark (1) and space (0) frequencies, and modulation is by a square wave(the bit-pattern).

WAVE FORM OF PSK & FSK : 

WAVE FORM OF PSK & FSK

ADVANRTAGE OF FSK : 

ADVANRTAGE OF FSK While using amplitude modulation with pulses, ON corresponds to mark and OFF to space. Such a system has inherent disadvantage that there is no real indication for the space. In addition such a system will suffer from the usual ailments of amplitude modulation. An amplitude limiter when used in the receiver, can take the full advantage of noise immunity of FSK & PSK but the receiver design of PSK is complex & costly.

ADVANRTAGE OF FSK (CONTD..) : 

ADVANRTAGE OF FSK (CONTD..) Hence we will use the technique of Frequency shift Keying (FSK) for modulating the signal. Here the carrier frequency is shifted in steps or levels corresponding to the levels for the digital modulating signal. So in case of a binary signal, two carrier frequencies are used, one corresponding to binary 1 and other to binary 0. Thus, a 0 is transmitted by a pulse of frequency f0, and 1 is transmitted by a pulse of frequency f1

FUNDAMENTAL METHODS TO REALIZE FSK : 

FUNDAMENTAL METHODS TO REALIZE FSK Separate oscillator method:- In this method two carriers are generated from separate oscillators, independent of each other and this is indicated by the separate subscripts for the amplitudes and fixed phase angles. The combined signal therefore has discontinuities in amplitude and phase, which are undesirable.

The modulating signal, in general, is given byBinary 0: e (t) =Acos( 2pft + a)Binary 1: e (t) =Acos( 2pft + a) : 

The modulating signal, in general, is given byBinary 0: e (t) =Acos( 2pft + a)Binary 1: e (t) =Acos( 2pft + a)

SINGLE OSCILLATOR METHOD (CPFSK): : 

SINGLE OSCILLATOR METHOD (CPFSK): Alternatively, the modulation can be achieved by frequency modulating a common carrier, which prevents discontinuities from occurring. Where a single oscillator is frequency modulated by the digital signal, the method is referred to as continuous phase frequency shift keying(CPFSK)

SINGLE OSCILLATOR METHOD (CPFSK) : 

SINGLE OSCILLATOR METHOD (CPFSK)

TRANSMITTER : 

TRANSMITTER BASIC BLOCK DIAGRAM MODULATING SIGNAL GENERATOR MIXER ( FOR SUPERPOSITION) OSCILLATOR (CARRIER GENERATOR)

WORKING PRINCIPLE OF TRANSMITTER : 

WORKING PRINCIPLE OF TRANSMITTER FSK transmitter can be implemented by using an FM transmitter and having an ADC in its input signal section. The colpitt oscillator works as the carrier wave generator. A colpitt oscillator will have transistor as the active element, while inductance and capacitance act as passive elements. By using a varactor diode the capacitance of the oscillator can be varied based on the bias applied to the diode.

WORKING PRINCIPLE OF TRANSMITTER(CONTD..) : 

WORKING PRINCIPLE OF TRANSMITTER(CONTD..) The input signal from the microphone is directly fed to the varactordiode. Thus the capacitance of the varactor diode is varied with the audio signal and this, in effect, varies the oscillation frequency. The centre frequency of the oscillation is determined by the DC voltage fed to the varactor diode. This voltage is varied using a potentiometer. So it can be used to set the transmitting frequency.

CIRCUIT DIAGRAM OF TRANSMITTER : 

CIRCUIT DIAGRAM OF TRANSMITTER

TRANSMITTER COMPONENTS : 

TRANSMITTER COMPONENTS COLPITTS OSCILLATOR shows the basic Colpitts circuit, where two capacitor and one inductor determine the frequency of oscillation. The feedback needed for oscillation is taken from a voltage divider made by the two capacitors .Using the Colpitts oscillator for a variable frequency oscillator VFO is best done by using a variable inductance for tuning, instead of tuning one of the two capacitors.

TRANSMITTER COMPONENTS : 

TRANSMITTER COMPONENTS VARACTER The name ‘varactor’ is derived from ‘variablereactor The variable reactance is obtained through the voltage-capacitance variation of a p-n junction. Applying a reverse bias Vr widens the depletion region and hence increases the exposed charge. This is similar to the action of a capacitor.

TRANSMITTER COMPONENTS : 

TRANSMITTER COMPONENTS ZENER DIODE Zener breakdown takes place in a very thin junction i.e. when both sides of the junction are very heavily doped and consequently the depletion layer isnarrow

RECIEVER : 

RECIEVER BLOCK DIAGRAM RF TUNED AMP MIXER DE-EMPHASIS IF TUNED LIMITINGAMP IF TUNED AMP DISC REMINITOR VOLUME AND TONE CONTROLLER VARACTOR TRIMETER LOCAL OSCI DELAYED AGC DETECTOR AUDIO AMP AFC

BASIC REQUIRMENTS OF RECEIVER : 

BASIC REQUIRMENTS OF RECEIVER RF Amplifer Mixer Local Oscillator Multistage IF Amplifer Detector LPF Audio Amplifer Audio Power Amplifer

WORKING PRINCIPLE : 

WORKING PRINCIPLE The first stage is the RF amplifier, using variable tuned circuits that track each other and the local oscillator. This stage boosts the weak signal level from the antenna above the noise level to provide some signal selectivity . The output signal from the RF amplifier is fed to one input of the Mixer circuit at the local oscillator signal to the other. The functions of mixer and oscillator are frequently combined in the same circuit. The oscillator is variably tuned so as to track the incoming signal frequency.

WORKING PRINCIPLE(contd……) : 

WORKING PRINCIPLE(contd……) The mixer output is fed to two cascaded IF amplifiers which are fixed tune and provided with sufficient selectivity to reject adjacent channel signals. The output of the IF amplifier chain is fed to the detector circuit where the audio signal is extracted from the IF carrier i.e., demodulated. The detector also provides the signals for AGC and AFC in FM receivers. The audio signal from the detector is passed through a low pass filter to remove unwanted high frequency components and then through a volume control to an audio amplifier. The audio amplifier is usually one level audio stage followed by a poweramplifier and a speaker.

CIRCUIT DIAGRAM OF RECIEVER : 

CIRCUIT DIAGRAM OF RECIEVER