In order to reduce the effect of gain and noise instabilities in the RF chain of a microwave radiometer, a Dicke radiometer topology is often used, as in the case of the proposed surface water and ocean topography (SWOT) radiometer instrument. For this topology, a single-pole double-throw (SPDT) microwave switch is needed, which must have low insertion loss at the radiometer channel frequencies to minimize the overall receiver noise figure. Total power radiometers are limited in accuracy due to the continuous variation in gain of the receiver. Currently, there are no switches in the market that can provide these characteristics at 92, 130, and 166 GHz as needed for the proposed SWOT radiometer instrument.
High-frequency SPDT switches were developed in the form of monolithic microwave integrated circuits (MMICs) using 75-ìm indium phosphide (InP) PIN-diode technology. These switches can be easily integrated into Dicke switched radiometers that utilize microstrip technology. The MMIC switches operate from 80 to 105 GHz, 90 to 135 GHz, and 160 to 185 GHz. The 80- to 105-GHz switches have been tested and have achieved 15-dB return loss (>18 dB for the asymmetric design), and >15-dB isolation. The isolation can be tuned to achieve >20-dB isolation from 85 to 103 GHz. The 90- to 135-GHz SPDT switch has achieved 15- dB return loss, and 8- to 12-dB isolation. However, it has been shown that the isolation of this switch can also be improved. Although the 160- to 185- GHz switch has been fabricated, it has not yet been measured at the time of this reporting. Simulation results predict this switch will have 20-dB return loss, and >20-dB isolation. /p>
The switches can be used for a radiometer such as the one proposed for the SWOT Satellite Mission whose three channels at 92, 130, and 166 GHz would allow for wet-tropospheric path delay correction near coastal zones and over land. This feat is not possible with the current Jason-class radiometers due to their lower frequency signal measurement and thus lower resolution.
The design work was done by Oliver Montes, Douglas E. Dawson, and Pekka P. Kangaslahti of Caltech for NASA’s Jet Propulsion Laboratory. The processing of the InP MMIC circuits was done by Kwok Loi and Augusto Gutierrez from NGST. NPO-48083
This Brief includes a Technical Support Package (TSP).
Single-Pole Double-Throw MMIC Switches for a Microwave Radiometer
(reference NPO-48083) is currently available for download from the TSP library.
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