Ultra Wide band and High Dynamic range Low Noise Amplifier for Low Frequency applications in Radio Astronomy

In low frequency astronomy « 300 MHz), antennas used to collect radiation from the sky; function like short dipoles (1« )./2) over an appreciable range of wavelengths. Since the short dipoles do not possess appreciable impedance matched bandwidth for 50 [2 systems, coupling of sky radiation to amplifiers connected to the output of antennas will be highly inefficient over a substantial portion of the bandwidth. This in general leads to degradation in the Signal to Noise ratio. This can be compensated by minimizing the noise contribution by the receiver system and ensuring that the sky noise dominates the system temperature despite the inefficiency of coupling. This necessitates the development of a low noise amplifiers at low frequencies. Since the low frequency environment is highly dominated by the strong manmade interference signals, the amplifier should also have a very high dynamic range. In this project an improved design of LNAto achieve low noise, broadband and high dynamic range operation is examined to develop an effective front-end for a new low frequency telescope being designed by Raman Research Institute (RR!), operating in the frequency range of 30-300 MHz. Microwave Integrated Circuit (MIC) approach has been followed in the design of LNAinstead of Monolithic Microwave Integrated circuit (MMIC) technique, since the latter is impractical for implementation in the laboratory and is economically viable for small number of units required as a part of this activity. As a part of this development program we have designed and tested a low noise ii HEMT amplifier (Noise Temperature = 50 K) with wide bandwidth (30-300 MHz) using thermal noise canceling technique. In the second phase we are attempting to increase the dynamic range performance using Post-distortion technique. In the third phase we will develop a differential amplifier with the same characteristics which can be directly connected to the balanced line from a short dipole. LIBRARY RAMAN RESEARCH INSTITU-l t. C.V. RAMAN AVENUE SADASHIVANAGAR BANGALORE-S60 080