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Safety applications, such as fire detection, are increasingly making use of RF/microwave technology. Microwave radiometers serving as remote temperature sensors have been proposed for surveillance use (e.g., forest-fire prevention in ground-based systems1) and for temperature monitoring in industrial applications (for example, in food industries2 or cement kilns3). The use of microwave radiometers in these applications can overcome the limitations of other technologies, such as infrared (IR) and optical sensors.

Microwave radiometers can operate effectively in dusty, smoky, and steamy environments, making the radiometric approach an appealing solution for temperature sensing in harsh environments. In addition, the RF/microwave approach for such sensors offers reduced costs compared to other technologies. To demonstrate the effectiveness of RF/microwave technology for fire detection, a low-cost Ku-band radiometer prototype was designed and tested. The results confirm the validity of the approach and represent a first step toward the realization of a low-cost radiometer suitable for temperature sensing. When used as fire sensor, the receiver has been capable of detecting a fire spot of 0.09 m2 area at a distance of 3 m with a 20-dBi horn antenna.

Figure 1 presents a schematic diagram of the proposed radiometer. An input single-pole, double-throw (SPDT) switch (SW1) alternatively connects the receiver input to the antenna and to the reference load. The RF section also is comprised of an isolator, a low-noise amplifier (AMP1), a bandpass filter (BP1), and a square-law detector (DET1). Additional low-frequency devices include an amplifier (AMP2) cascaded with the active bandpass filter (BP2), a synchronous AC/DC demodulator, and a lowpass filter (LP1). A modulating square-wave local-oscillator (LO1) source controls both the RF input switch and the synchronous demodulator at a switching rate of fs. An analog-to-digital converter (ADC) at the output of the receiver digitizes processed signals.

Radiometer Aids Fire Detection, Fig. 1

The receiver is basically a Dicke radiometer, a configuration preferred to a total power radiometer architecture for its improved performance in terms of enhanced achievable resolution.4 Additional features have also been added to the basic Dicke configuration, such as an amplified detector. Compared to a superheterodyne receiver (or standard Dicke architecture), this direct-detection receiver does not require a downconversion mixer or local oscillator (LO) for signal translation. As a result, the receiver design exhibits lower noise temperature and power consumption, simpler structure, and reduced costs compared to a superheterodyne receiver.

Radiometer Aids Fire Detection, TableMoreover, with respect to a Dicke radiometer architecture, an RF isolator is connected between the output of RF switch SW1 and the input port of low-noise-amplifier (LNA) AMP1. This minimizes variations in the noise figure of amplifier AMP1 due to the effects of the input load (the receiver connected to the antenna or to a reference load), for improved radiometer performance in terms of measurement accuracy. Active filters are used in the video section of the receiver, allowing for low-cost amplification of received modulated input signals. Based on this architecture, a Ku-band radiometer prototype operating at a center frequency of 13 GHz was been designed and assembled. The switching rate, fs, is set to 80 Hz.

The radiometer was assembled entirely with standard commercial RF/microwave components operating at Ku-band frequencies, except for bandpass filter BP1. The main features of these components are listed in the table. All of the standard components have been selected based on a previous performance analysis of the radiometer receiver as a whole. Of primary importance are the low-loss input switch and the low-noise amplifier (LNA). Moreover, the amplifier gain has been selected to provide the required radiometer resolution with a feasible radiometer integration time.

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