| Popis: |
A development goal defined by ASAC is: “Larger bandwidths and better receiver sensitivity: enabling gains in speed”. This broad goal requires careful scrutiny, as for a given SIS mixer extending the IF frequency range from 4-8 GHz (for Band #6 from 5-10 GHz) to 4-12 GHz, 4-16 GHz and ultimately to 4-20 GHz will always result in higher, on the average, receiver noise, reducing the implied benefits. This is because the noise of HEMTs increases proportionally with frequency. A perfect noise match is only possible at discrete frequencies within IF band resulting in larger average noise penalties for wider IF bandwidth. Therefore, a theoretical and experimental study has been undertaken to assess the expected noise penalties for the current design of Band #6 SIS mixer working with 4-12 GHz, 4-16 GHz and 4-20 GHz IF amplifiers. New designs of broadband IF amplifiers covering 4-12 GHz, 4-16 GHz and 4-20 GHz using commercially available devices from Diramics have been developed. The Diramics’ devices deliver state-of-the-art minimum noise temperatures at 15 K ambient that is 2.5 K at 10 GHz and 10 K at 40 GHz. These noise temperatures are also very close to the natural noise temperature limits of InP cryogenic HEMTs. The early experiments indicate that optimal from the noise point of view direct integration of 4-12 GHz amplifier with Band 6 SIS mixer is indeed possible (the current Band 6 SIS mixer is optimally integrated with HEMT amplifier over 5-10 GHz IF bandwidth). For larger IF bandwidths a direct integration of IF amplifier with SIS mixer results in relatively large variation of noise temperature across IF band is which favors the use of cryogenic isolators. The average noise penalty across the total IF bandwidth is strongly dependent on SIS mixer conversions loss. For the current SIS mixer design and assumed 7 dB conversion loss the average noise penalty across the IF band for 4-16 GHz and 4-20 GHz as compared with 4-12 GHz version, is estimated at 15 K and 30 K, respectively. |
