Low power silicon germanium electronics for microwave radiometers

Space-based radiometric observations of key hydrological parameters (e.g., soil moisture) at the spatial and temporal scales required in the post-2002 era face significant technological challenges. These measurements are based on relatively low frequency thermal microwave emission (at 1.4 GHz for soil moisture and salinity, 10 GHz and up for precipitation, and 19 and 37 GHz for snow). The long wavelengths at these frequencies coupled with the high spatial and radiometric resolutions required by the various global hydrology communities necessitate the use of very large apertures (e.g., >20 m at 1.4 GHz) and highly integrated stable RF electronics on orbit. Radio-interferometric techniques such as Synthetic Thinned Array Radiometry (STAR), using silicon germanium (SiGe) low power radio frequency integrated circuits (RFIC), is one of the most promising technologies to enable very large non-rotating apertures in space. STAR instruments are composed of arrays of small antenna/receiving elements that are arranged so that the collecting area is smaller than an equivalent real aperture system, allowing very high packing densities for launch. A 20-meter aperture at L-band, for example, will require >1000 of these receiving elements. SiGe RFIC's reduce power consumption enough to make an array like this possible in the power-limited environment of space flight. An overview of the state-of-the-art will be given, and current work in the area of SiGe radiometer development for soil moisture remote sensing will be discussed.


precipitation, and 19 and 37 GHz for snow).
The long wavelengths at these frequencies coupled with the high spatial and radiometric resolutions required by the various global hydrology communities necessitate the use of very large apertures (e.g., >20 m at 1.4 GHz) and highly integrated stable RF electronics on orbit.

Radio-interferometric techniques such as Synthetic Thinned Array Radiometry
(STAR), using silicon germanium (SiGe) low power radio frequency integrated circuits (RFIC), is the one of the most promising technologies to enable very large nonrotating apertures in space. STAR instruments are composed of arrays of small antenna/receiving elements that are arranged so that the collecting area is smaller than an equivalent real aperture system, allowing very high packing densities for launch.
A 20-meter aperture at L-band, for example, will require >1000 of these receiving elements.
SiGe RFIC's reduce power consumption enough to make an array like this possible in the power-limited environment of space flight. An overview of the state-of-the art will be given, and current work in the area of SiGe radiometer development for soil moisture remote sensing will be discussed.

I. INTRODUCTION
The  power consumption and center frequency requirements listed above the L-band receiver should have a bandwidth of 25 MHz and a system noise temperature of less than 250 K. Figure  1 shows a block diagram of the SiGe L-band radiometer.
It is largely based on a series of parts from Maxim Integrated Products, Inc. of Sunnyvale, CA. These products provide all of the building blocks for low noise receivers that cover DC to 6 GHz. These building blocks have built in biasing and some internal matching to 50 Ohms. Since this family of parts was designed to be attractive to the portable consumer electronics markets they were designed to be very low power consumption. Table 2 shows the performance of the various parts of the block diagram. The total power consumption is estimated to be 265 roW. Layouts have been designed for the mixer, the LNA's and the IF amplifiers. Figure 2 is a photograph of the L-band mixer in the standard microwave test fixture.
These layouts are done on Rogers Duroid 6002 0.01-inch thick substrate using microstrip circuit techniques.
IV. TESTRESULTS Figures 3 shows the results of testing that was performed on the LNA vs. the simulation data. The mixer has also been designed and tested. It has a conversion gain of 6 dB, RF return loss of 7 dB and IF return loss of better than 10 dB over 32 MHz bandwidth.
These numbers and the measured power consumption all match well with the published data from the manufacturer.
The designs of the IF amplifier and the local oscillator are not complete and the noise figure of the LNA has not yet been tested.