Microwaves have the inherent capability to penetrate layers of insulating or low-conductivity materials which are non-transparent at optical or infrared frequencies. This property has been exploited to detect fire in forest environments, optically masked to an external observer by the vegetation canopy, and to develop imager for the identification of non-metallic concealed weapons. To suit these emerging applications, the design of a microwave radiometric sensors must address a number of new issues. Firstly they should be manufacturable at low industrial costs. Secondly they should feature a good radiometric resolution, typically below 1K, with integration time ranging from 10ms to 1s, in accordance with the specific application. Thirdly, the reduction of size, weight and power consumption is very important in both imager or sensors network scenarios. The main solution to satisfy all the aforementioned constraints is the miniaturization of the radiometric sensor, which can be scaled down to a single-chip. In this way cost, size and weight can be directly reduced. Moreover the thermal stabilization system (needed to reduce the receiver drifts and thus functional to the achievement of high absolute accuracy) is easily implemented at chip-level.

   Particularly, we have focused on its applications to the early detection of the forest fires and the civil safeguard. The microwave radiometric sensor, which is operating in the 12.75−13.25 GHz ISM (industrial, scientific and medical) band, has been designed according to a ‘total power’ architecture, since this allows us to obtain both circuit simplicity and excellent sensitivity. This circuit is based on a homodyne receiver since its building blocks can be realized on-chip without requiring any external components. A system analysis has been carried out in order to define the specifications of all the building blocks, according to the characteristic performance reachable with their implementation on a standard CMOS 90 nm process provided by ST-Microelectronics.

   Theoretical and feasibility study by CAD system analysis is carried out. This analysis has allowed us to confirm the feasibility of this kind of sensor on a single die of silicon and provides us the system and the building block specifications.

   The design of the main building blocks of the integrated radiometric receiver at 13 GHz, in 90-nm CMOS technology, have been designed (by means of the CMOS 90nm technology by ST-Microelectronics) and measured.

   In detail, the Low Noise Amplifier (LNA), an active inductance called Boot-Strapped Inductor (BSI), that is the key element for the implementation of the low insertion-loss antenna switch and the low-phase-noise VCO, and the Voltage Controlled Oscillator (VCO) are shown in the following figures.

Chips (desiged by means of the CMOS 90nm process by ST-Microelectronics)