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Radiometer Aids Fire Detection, Fig. 2

The radiometer’s amplified bandpass filter (BP1) consists of a band-limited amplifier (AMP2) cascaded with a bandpass filter (BP2); both are centered at 80 Hz, the receiver switching frequency (fs). The amplifier [Fig. 2(a)] has a 160-Hz 3-dB bandwidth centered at fs. Active bandpass filter BP2 is a second-order Chebyshev circuit [Fig. 2(b)], realized by cascading two multiple-feedback bandpass cells. The filter is designed to provide 0.5-dB pass band ripple, 21.5-Hz equiripple bandwidth, and 60-dB peak gain at the center frequency. Figure 3 shows the measured transfer function of this amplified BPF, with experimental measured results compared with the filter’s computer-simulated response. The measured amplitude gain is 97 dB at the 80-Hz center frequency.

Radiometer Aids Fire Detection, Fig. 3Radiometer Aids Fire Detection, Fig. 4

The radiometer’s synchronous AC/DC demodulator is obtained by cascading a unity-gain inverting amplifier to a quadruple single-pole single-throw (SPST) switch (Fig. 4). The four switch ports are connected and controlled in such a way that the device output voltage for one-half period of the control signal is equal to input voltage VIN and, for the following one-half period of the control signal, is equal to the output of the inverting amplifier. The switches are controlled by two complementary transistor-transistor-logic (TTL) signals at 80 Hz. 

Radiometer Aids Fire Detection, Fig. 5

Lowpass filter LP1 is an active Butterworth third-order filter with 10-Hz cutoff frequency. It employs one second-order Sallen & Key cell cascaded with a first-order passive cell (Fig. 5). The second-order cell has equal R and C. Thus, the resulting filter presents nonunitary DC gain—2 V/V, in the current design. Figure 6 shows the measured and simulated filter amplitude gain; the filter exhibits roughly 6-dB amplitude gain and 10-Hz cutoff frequency.

Radiometer Aids Fire Detection, Fig. 6

The complete radiometer prototype (Fig. 7) comprises all RF and video devices already described. Additional components complete the receiver: an ADC with sampling frequency of 50 Hz connected at the output of the LPF, as well as the circuitry for the generation of modulating square-wave LO1 that controls both the input RF switch and the synchronous demodulator. A matched load is used as reference source.

Radiometer Aids Fire Detection, Fig. 7

As a first step, the sensor was characterized in terms of transfer function (linear relation between the input noise power/ antenna noise temperature and the output voltage mean value) and the associated resolution (standard deviation of the radiometer output voltage—i.e., standard deviation of the measured antenna noise temperature or radiometric resolution). This was done by connecting the antenna input port to a calibrated noise source while the radiometer output voltage, VOUT has been acquired with the ADC. Several acquisitions were performed for different values of input power (PRF) in the range from -102 to -82 dBm (in this context, the input power is calculated as the product of the input power noise density times the noise bandwidth). The lowpass filter output is acquired by an ADC (with total dimensions of 50 × 30 cm).

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