An RF Rectifier-Based Readout Circuit for a 12.5 GHz Atopic Dermatitis Sensor

This thesis describes a read-out circuit for the detection of the severity degree of Atopic Dermatitis, a pathological skin condition damaging the stratum corneum, which is the most superficial layer of the skin. Three skin samples (healthy, 7μm lesion, and 15μm lesion) are evaluated through the excitation of a complementary split ring resonator (CSRR), a planar microwave device exhibiting maximum sensitivity to dielectric properties at resonance. A 4-port hybrid coupler is designed in microstrip technology, analysing the standalone CSRR input port impedance Z11, that is approximately equal to 100 Ω. A reference value, equal to 50 Ω is chosen for the line computation, because it guarantees high reflection from the resonator into the coupler. An RF excitation signal is connected to port 1, while at port 2 is connected the touchstone file, containing the CSRR response. Port 3 is terminated with an adjustable load, while at port 4 is connected a rectifying sub-circuit, that is first designed, to achieve the matching condition, as a standalone. The overall system is designed by means of Harmonic balance technique with the objective of providing output DC voltages at the rectifying branch output, for the three reference skin conditions, distinct at least by 50 mV. A comparison in term of input impedances, scattering parameters, port 4 input power, and DC output voltages, is provided by three different configurations. The baseline is an extension of the standalone rectifier matching procedure, and it operates at 10.5 dBm. On the other hand, the Port 3 tuned differing from the previous only in the impedance value at port 3, and the enhanced one, having also a different rectifying design, work at 0 dBm, a desirable value for wearable applications, ensuring long battery life and low emitted radiation. Finally, a comparative sensitivity analysis is provided, among the three configurations, to evaluate the voltage discrimination for the skin conditions under exam.