A channelized 2nd IF/LO downconverter for the EOS Microwave Limb Sounder

The Earth Observing System (EOS) Microwave Limb Sounder (MLS) is scheduled for launch in 2004 on the EOS Aura spacecraft. The design, assembly and test of the flight 2^nd Intermediate Frequency/Local Oscillator (2^nd IF/LO) subsystem for this instrument has been completed and is presented here. The 2^nd IF/LO subsystem consists of 5 separate microwave assemblies, 1 for each of the 5 millimeter wave radiometer front ends, providing a total of 33 separate IF channels. Some key requirements of the subsystem are as follows: provide frequency multiplexing of overlapping or closely spaced 1^st IF channels while maintaining low ripple in the passbands; generate 18 different 2^nd LO frequencies, in the range of 4-20 GHz, with low phase noise and a placement resolution of 400 KHz; downconvert the 1^st IF´s to a common 2^nd IF frequency centered at 900 MHz; minimize cost and schedule by using common designs for the 5 different assemblies wherever possible.