A 1.4-GHz GaAs MMIC Radiometer for Noninvasive Internal Body Thermometry

In this article, we present an on-chip Dicke radiometer for noninvasive internal body temperature measurements. The switch and low-noise high-gain amplifier are implemented in a 0.1- $\mu \text{m}$ enhancement-mode pseudomorphic high-electron-mobility transistor (pHEMT) GaAs process. The single-pole double-throw (SPDT) switch in a series-shunt topology with a resonating inductor shows an insertion loss of 0.29 dB and an isolation of 30.8 dB at 1.4 GHz. The low-noise amplifier (LNA) has a measured stable gain of 45.2 dB and a noise figure of 0.52 dB. Following the gain path, two bandpass filters with a 64-MHz power-equivalent bandwidth centered at 1.4 GHz help reduce electromagnetic interference. The monolithic microwave integrated circuit (MMIC) of the radiometer shows 45 dB of gain and 0.88-dB noise figure at 1.4 GHz. When connected to a near-field antenna probe, measurements are obtained on a skin–muscle phantom to track the muscle temperature. The near-field antenna is designed to receive blackbody radiation from a 15-mm-thick muscle phantom under a 2-mm-thick skin phantom. To retrieve the subcutaneous temperature of the muscle, the calibrated radiometer measures total noise power with an integration time of 2 s. Compared to a thermocouple measurement performed 30 s before the radiometric measurement, an average error of 0.77 K is obtained. In vivo thermometry of water inside a human cheek is performed for validation, and the impact of varying skin properties on temperature estimation is studied in simulations.