Lecture5_12052016.pdf
- 1. SYSTEM NOISE TEMPERATURE ,C/N AND G/T RATIO • Thermal noise in its pre amplifier • PN=KTsB • SYSTEM NOISE TEMPERATURE IS ALSO CALLED EFFECTIVE INPUT NOISE TEMPERATURE OF THE RECEIVER. • IT IS DEFINED AS THE NOISE TEMPERATURE OF A NOISE SOURCE LOCATED AT THE INPUT OF A NOISELESS RECEIVER WHICH WILL PRODUCE THE SAME CONTRIBUTION TO THE RECIEVER OUT PUT NOISE AS THE INTERNAL NOISE OF THE ACTUAL SYSTEM ITSELF
- 2. SYSTEM NOISE TEMPERATURE ,C/N AND G/T RATIO • Ts is located at the input to the receiver. • RF amplifier • IF amplifier • Demodulator • Over all gain at the receiver G • Narrowest bandwidth is B • Noise power at the demodulator input is
- 4. Noise temp contt--- ) ( ) ( in RF Rf m m If If Rf M If n in RF RF m If m m If If If n s r S r T T KB G B KT G G B KT G G G P T T KB G G G B KT G G B KT G P B KT P BG KT G P N C Pr is the signal power at the input of the RF section of the receiver signal power at the demodulator input will be PrG
- 6. Noise temp cont--- • G/T ratio is 40.7 db k-1 at 4 GHz and 50 elevation • Gr varies with frequency f^2 • Ts depends upon the sky noise temperature
- 7. LogK LogL T G d Log G P N C B N N BL KT d G G P N C A S R T T dBHz A S R T T 10 10 log 10 4 20 log 10 4 0 2 EIRP Gr/Ts -- ratio is called figure of merit Noise temp cont---
- 8. Atmospheric and ionospheric effect on link design • Absorption • refraction • Diffusion(diffraction) • Rotation of polarization of plane depend on path length more pronounced at small elevation angles Absorption and diffusion--- lower layers ---- increase in noise power at receiving antenna