Abstract
This paper presents a simple total power radiometer to noninvasively measure the temperature of the human body. The proposed 3-GHz radiometer consists of an antenna collecting the noise power generated by a target, a low-noise and high-gain receiver amplifying the noise power, and a detector converting the noise power to voltage. A single-pole-triple-throw (SP3T) switch is placed between the antenna and the receiver, while a personal computer is used to control the SP3T switch, collect and process the data such as detector output voltages and physical temperatures of the reference noise sources and the target. The fabricated radiometer shows a good performance agreement with a thermometer in the temperature measurement of water from 25.0 to 43.1 °C. For the accurate prediction of the target temperature, the radiometer is calibrated adaptively to the environment and radiometer variations. For this purpose, two reference noise sources (hot and cold) are proposed using matched and mismatched resistors at room temperature. These resistor-based noise sources offer a reliable performance without complex temperature control systems. Furthermore, they can be easily calibrated in real time by periodically measuring the physical temperatures of the resistors. In addition, the logarithmic detector with wide dynamic range is adopted and logarithmically-fitted based on the measurement results instead of linear approximation, which reduces the error caused by the limited dynamic range of resistor-based noise sources. In order to further increase the accuracy, the performance imbalances between ports in the SP3T switch are also taken into account by employing offsets in the radiometer output voltages.
Highlights
Microwave radiometers can be used for non-invasive thermal measurement of the human body such as core body temperature monitoring [1] and thermal imaging of breast cancer [2]
Microwave radiometers experience a variation of performance, such as gain and noise figure, caused by variations in the bias supply and ambient temperature, which leads to the increase of measurement errors [3]
It is a total power radiometer consisting of a low-noise amplifier (LNA) and a band-pass filter (BPF)
Summary
Microwave radiometers can be used for non-invasive thermal measurement of the human body such as core body temperature monitoring [1] and thermal imaging of breast cancer [2]. A microwave radiometer is proposed for medical applications using the matched and mismatched resistors at room temperature as hot and cold noise sources. High-sensitivity logarithmic detectors, approximated as linear for easy calibration, can be used in the radiometers [15] This can increase errors when the input temperature deviates from those of the reference noise sources. Performance imbalances among ports in the single-pole-triple-throw (SP3T) switch used to select a port (among reference loads and antenna) can cause an error in the temperature prediction [9] Such errors are minimized in this work by employing offset voltages of the radiometer output voltages.
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