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1.Basic Principle of Microwave

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1.Basic Principle of Microwave ST-48 DESCRIPTION SHORT : 2 DAYS MICROWAVE AND UHF (DIGITAL) CONTENTS 1. Basic Principle of Microwave 2. Need to digital Microwave and advantages of microwave 3. Pulse code modulation. 4. Modulation techniques 5. Radio equipment – Block diagram explanation (NEC-Make ) 6. Primary and higher order Mux 7 Fading , noise and jitter 8 Space and frequency diversity 9 Microwave Tower 10 Microwave Earthing - importance and measurement 11 Power supply arrangements 12 Periodical arrangements 13 End to end alignment 14 Digital UHF equipment – functioning 15 18 GHZ microwave system used for block working 2 1. BASIC PRINCIPLE OF MICROWAVE Microwave essentially means very short wave . The microwave frequency spectrum is usually taken to extended form 1 GHZ to 30 GHZ.The main reason why we have to go in for microwave frequency for communication is that lower frequency band are congested and demand for point to point communication Continue to increase. The propagation of the microwave takes place in spacewave in view of the high gain and directivity in the form of a beam and is similar to that of light . 2. NEED TO DIGITAL MICROWAVE The modern communication is towards digital transmission to digitize the analog signals PCM techniques are used . Digital techniques are widely becoming popular for application in switching and multiplexing , thus necessecitating the use of a new transmission means on radio for the medium and high capacities both for long haul application and junction working of interexchanges, in urban areas . Thus an extremely rapid transition from analog to digital radio system at present used in Indian Rly . ADVANTAGE OF DIGITAL MICROWAVE Radio links for direct transmission of PCM signals at standard bit rates of 2,8,34 and 140 MB/ Sec.Facilities found by using digital microwave are mentioned below :- a) Radio repeaters being of re-generative type will give an error free output and there is no accumulation of noise from hop to hop. b) Better circuit quality upto threshold,level of the radio receiver . c) Total system economy is better . d) Since transmission system is digital , more operational advantages are found , like storage , re- transmission etc. of information can be easily achived . e) For data transmission , digital radio is more efficient . f) In digital multiplexing , the number of channels loaded does not affect the performance. 3. PULSE CODE MODULATION Pulse code modulation is adopted for digital multiplexing of channels . In this , each analog signal is converted into a digital signal such outputs of a number of channels are multiplexed by time division multiplexing to form a digital base band . The various stages involved in the pulse code Modulation are : 1) Filtering , 2 ) sampling . 3) Quantising , 4) Encoding , 5) Linecoding 1) FILTERING :-- The incoming analog VF signal frequency is band limited to 0`3 to 3`4 KHZ,by a low pass filter. 2) SAMPLING :-- The band limited signal is sliced at regular intervals of time to generate pulses of the continuous analog wave . The Sampling theorem stipulates a minimum number of samples. The theorem says that the sampling rate should be equal to or higher than double the highest signal frequency present in the band limited analog signal . 3 To allow a margin for the fiter, the band limited frequency is taken as 0 – 4KHZ and the sampling frequency is decided as 8 KHZ .Hence the time period of sampling would be equal to 1/ 8000 Sec =125 us. 3)QUANTISATION:-- The process of sampling generates PAM signals into PCM signal. The discrete amplitude levels of the samples are first quantified with reference to a standard amplitude scale .This process is called Quantisation . This gives 28= 256 levels for quantising ( 128 for the positive swing and 128 for the negative swing). This type of quantisation is called Linear Quantisation. 4) ENCODING:-- Conversion of analog samples into a binary signal is call Encoding. The practical PCM system use 8 bit code . 5) LINE CODING:-- The binary signal with its DC component is unsuitable for its transmission on transformer and AC coupled transmission lines.Hence the binary code is modified to suit the characteristics of the line. One popular code called HDB3 ( High Density Bipolar 3 ) is employed. Bit Rate: -- Total No. of bits/ frame= 32x 8 = 256 No. of frame/ Sec =8000 Total no. of bits / Sec= 8000x 256 = 2,048000 bits =2048 K Bit =2.048 M Bits 4. Modulation Technique Digital information modifies the amplitude , the phase and the frequency of the carrier in discrete steps . In most of the Digital Radio System , the frequency of the carrier is fixed ;So we need to consider only the phase and amplitude . The phase and magnitude can be represented in polar or vector co-ordinates as a discrete point in the socalled I-Q plane .When the carrier phase is shifted between to values to represent binary 0 and 1 it is called phase shift keying (PSK) .In this system , when four phase states ( 45˚,135˚,225˚,&315˚) are represented , it is called Quardrative phase shift keying (QPSK) . The modulation phase state can be generated by adding together appropriate amplitudes of in-phase and quadrature carrier . Because by having four discrete states , we can transmit more information per state in this case . 4 The above figure shows the simplified block diagram of a QPSK(4_PSK) modulator . In QPSK , the incoming bit-stream is divided into two parallel streams so that each time one bit is fed simultaneously to both I and Q balanced modulation to construct the 2 bit symbols the carrier output from the modulator is switched under the control of the digital bit stream and by adding together the I & Q outputs the phase . State diagram is generated . In this case , the band limiting filter is a bandpass filter at IF . 5. RADIO EQUIPMENT – BLOCK DIAGRAM EXPLANATION 7GHZ DIGITAL MICROWAVE RADIO EQUPT (NEC-MAKE) The Radio equipment is compact and fully solid state . Microwave Transmitter receiver employing microwave Integrated circuits (MIC) for the RF circuit and Field Effect Transistors ( FET) for the low noise microwave amplifier . The Digital Transmitter Receiver with switch over units , provide highly efficient transmission services .The 7GHz radio frequencies band (7125 to 7725 MHz) which have a transmission capacity of 340 Mb/s digital signal alongwith 2Mb/s way side digital signals . Performance & General Specifications: 1.Operation Frequency Band : 7125 to 7725MHz 2. Repeating Type : Regenerative Repeating 3. Transmission Capacity : 34.368Mb/s x 1stream (Equivalent 480cl/s) 4.Modulation :4-phase PSK 5.Demodulation :Coherent Detection 6. Service channel :Analogue:3CH Digital : 4CH 5 7. Type of switch : Relay switching (RF) Hitless switching 8. Wayside Signal : 2.048 Mb/s x 1(optional) 9.Power sourse : 48V DC 10.Power consumption : Approx. 144 W 11.Ambient temperative : 0˚c to + 50˚ c Transmitter -- Receiver 1 .TX Out –Put Power : +30 dbm (+ 1.0 db to -1.5 db) 2. TX frequency stability : Withen +_20 p pm 3. RX Noise figure : Less than 4.0 db 4.RX IF frequency : 70 MHZ 5.RX ACC range : More than 50 db 6.RX Level to the bit error rateof 1x1o : Lower than – 79.5dbm 7.RX Level to the bit error rate of 1x1o : Lower than –83.5dbm 8.TX –RX frequency Difference : 151.614 MHZ 9. Adjacent Channel frequency Difference : 34MHZ POWER –Supply ( PS) : The PS comprises the DC—DC CONVC.modules. The DC—DC Conv,produces a regulated +5 v Dc , + 9v Dc , and –10 v Dc from an input power of 48 v Dc. The regulated +5V DC and –10V DC thus obtained operate modules in the transmitter –Receiver . When an excessive current flows or an abnormal output voltage goes in to modules , the corresponding power switch (circuit breaker type) of DC-DC converter trips to interrupt the input power. Auxiliary Unit (AUX) : The AUX comprises the following two modules. (i) Switch Over SWO) ii) Switch Over Control (SWO Cont.). The SWO consists of transmitter (Tx) section and receiver (Rx) section. Tx Section :The HDB-3 signal (34.368Mbps) is applied to attenuator (ATT) and its level to attenuated approx 3dB. Then the signal is delivered to the hybrid and divided into two routes respectively to be supplied to REG and PROT system . The signal is attenuated approx 3dB . Rx Section : The 34.368Mbs HDB-3 input signal is applied to data switchover circuit , where one of REG and PROT is selected by the control signal . When the data switch over operates , the SW OPR indicator lights .The SWO CONT consists of ASc circuit , alarm and control circuits . Tx DPU – The Tx DPU consists of line equalizers, bipolar to unipolar convertor , serial - parallel convertor,Multiplexer , Scrambler . 34Mb/s unipolar signal is divided into two 17Mb/s unipolar signals in the serial - parallel convertor . Multiplexer : This circuit multiplexes the digital service channel, way side signal, switch over control , frame pulse and parity check bits to the corresponding time slot in data signal produced by the buffer memory. Scambler : This scramble pattern is a 432 bits random pattern produced at the timing pulse generator. Transmitter—The Tx circuit consists of phase modulator and Transmitter RF circuit . 6 Phase Modulator (PH MOD) : The PH .MOD contains a Tx LOGIC circuit, a Quad MOD circuit and Sc AMP circuit . A 70 MHz IF carrier is generated at the 70MHz oscillator and splits into two carrier (LO1,and LO2)for each 0—π modulator . The Phase difference between the two 70 MHz carriers has π/2 radians. In each o— π modulator , the phase of the 70MHz carrier is modulated to a o—phase or π —phase by input pulse streams. The output of the two 0- π modulator are then combined for four phase assignment onto a 70 MHz IF carrier.
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