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A PROJECT REPORT ON BANDSHAPING CIRCUITS FOR THE NEW IF CONVERSION SYSTEM BY ASHOK BOSE B.TECH, 3 rd YEAR INSTITUTE OF RADIO PHYSICS & ELECTRONICS UNDER THE GUIDANCE OF SHRI.B.AJITHKUMAR. RADIO ASTRONOMY AT GMRT. Radio Astronomy: Study of astronomical phenomena using radio waves
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A PROJECT REPORT ONBANDSHAPING CIRCUITS FOR THE NEW IF CONVERSION SYSTEMBYASHOK BOSEB.TECH, 3rd YEARINSTITUTE OF RADIO PHYSICS & ELECTRONICSUNDER THE GUIDANCE OF SHRI.B.AJITHKUMAR
RADIO ASTRONOMYATGMRT • Radio Astronomy: Study of astronomical phenomena using radio waves • GMRT: Project run by NCRA under TIFR • Situated at Khodad,has 30 parabolic antennas of 45m diameter spread over 25km in Y shaped array • Antennas, designed using SMART technology
GMRT RECEIVER SYSTEM • Operating frequency ranges from 150MHz to 1420MHz • RF signals in two polarization down converted to 70MHz & then to 130MHz &175MHz respectively, each of 32MHz b/w • At the CEB the 2 signals converted to 4 baseband of 16MHz b/w which are fed to 4 inputs of ADC
SOFTWARE BACKEND • Software based correlator being developed alternative to existing hardware correlator • The IF of 130MHz &175MHz have to be converted to suitable frequencies & digitized • 30 antennas need 30*2=60 ADC inputs of 32MHz bandwidth with a sampling frequency of 66MSPS
PURPOSE OF THE PROJECT • A sampling rate of 66MSPS require an input signal of bandwidth half the rate • To maintain this criterion, signal must be adequately filtered • This project requires implementation of a desired Bandshaping Filter
BANDPASS SAMPLING • An analog signal of b/w fa must be sampled at a rate of fs > 2fa • If fs < 2fa ,Aliasing will occur & signals outside the Nyquist bandwith corrupt the wanted signal • fs > 2fa holds for integer band position • So signals must be filtered adequately before sampling
PRACTICAL DIFFICULTIES • Very small values of inductances, ~10nH, difficult to make • Large no. of discrete components on a small pcb • Noise due to parasitic cap. & ind. • Initially wrong measurement from Impedance Analyzer
SOME BASIC QUESTIONS • Why do we need to increase the clock frequency of the ADC ? -HIGHER BANDWIDTH • Why do we need a 32 MHz b/w output ? -TO MAINTAIN NYQUIST CRITERION • Why a BPF with -6dB down for 32 MHz b/w ? -LESS DISTORTION AFTER SAMPLING • Why we are designing a 32MHz instead of 33MHz bandwidth BPF ? –BETTER PERFORMANCE
THANKFUL TO • SHRI. B. AJITHKUMAR • SHRI. T.L.VENKATSUBRAMANI • SHRI. YASHWANT GUPTA • SHRI. R.NITYANANDA • ALL GMRT STAFF MEMBERS