Receiver Chain for the STEAMR Instrument
Paper i proceeding, 2009
The STEAMR instrument is based on a linear array of 14 heterodyne receivers. In the instrument, a linear fixed array is used instead of 1-2 single channels being scanned in altitude, thus providing an order of magnitude improvement in sensitivity and simplification in parts of the optics. Omnisys Instruments AB has currently a contract for the development of the radiometer core, including frontends, back-ends, control and power interface electronics. The optics will not be covered in this presentation.
The Front-Ends consist of subharmonic Schottky mixers with integrated low noise amplifiers and local oscillator chains. The instrument will operate with an effective local oscillator frequency of 337.5 GHz and with an IF frequency coverage of 6-18 GHz, and both sidebands, 12 GHz wide, will be used. The Back-End spectrometers consist of broadband IQ-mixer based duplex filter stages with consequent wideband autocorrelator chips. The subharmonic mixers use VDI diodes and both DSB and SSB versions have been developed. A large effort has been focused on realizing an integrated Front-End unit with low noise amplifier and local oscillator chain. Operation will be in either sideband separating mode or dual sideband mode, or with a mix depending on altitude covered.
Three different types of local oscillator chains are being prototyped. The first use an active chain up to 84 GHz followed by a schottky doubler, the second an active chain followed by a HBV tripler and the third an all active chain with buffer amplifiers at 167.5 GHz. Test results will be presented for the mixers, the first and third LO chain alternatives and possibly for the HBV alternative as well. Both alternatives have shown compliance to our requirements and the all active chain is currently the baseline. Omnisys has developed a single chip autocorrelation spectrometer capable of processing more than 6 GHz of bandwidth with up to 1024 channels of resolution. For the STEAMR instrument, two of these will be operated in parallel to cover 12 GHz with each configured to process 256 channels, i.e. 512 channels will be used over the 6-18 GHz bandwidth. In terms of performance, nothing comparable has ever been provided in either ground or space radiometers.