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RADIO ASTRONOMY AT GMRT

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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RADIO ASTRONOMY AT GMRT

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  1. 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

  2. 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

  3. 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

  4. CURRENT RECEIVER SYSTEM

  5. 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

  6. 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

  7. CONVERSION OF NEW IF SIGNALS

  8. THE NEW SYSTEM

  9. 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

  10. EFFECTS OF SAMPLING

  11. INTEGER BAND POSITIONING

  12. NEED FOR BANDSHAPING FILTER

  13. BANDPASS FILTER DESIGN

  14. BANDPASS FILTER CONFIGURATION

  15. 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

  16. LP-HP CIRCUIT (CENTRE FREQ. 130MHz)

  17. LOWPASS –HIGHPASS COMBINATION

  18. TEST SETUP

  19. EXPERIMENTAL RESPONSE

  20. 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

  21. THANKFUL TO • SHRI. B. AJITHKUMAR • SHRI. T.L.VENKATSUBRAMANI • SHRI. YASHWANT GUPTA • SHRI. R.NITYANANDA • ALL GMRT STAFF MEMBERS

  22. THANK YOU

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