# A Self-Powered UHF Passive Tag for Biomedical Temperature Monitoring
## 4 Measurements and Results
### Overall Discussions  
This work presents the design of a self-powered RF sensor prototype which allows measuring biomedical temperature signals from 0 \({}^{\circ}\)C to 60 \({}^{\circ}\)C with a sensitivity of 823.29 Hz/\({}^{\circ}\)C, Figure 34. It exhibits a read range of 9.5 m and presents overall dimensions of 8 cm \(\times\) 2 cm, including the silicon die chip and the dipole antenna. The proposed temperature sensor is composed of a High-Frequency section, a Power-Managment stage, and a Low-Frequency section. The HF section is designed for RF energy harvesting and sensor-to-reader communications purposes. It is composed for a dipole antenna, a matching network, a differential rectifier, and a backscatter ASK modulator. The HF section uses a 1 Watt EIRP 860-960 MHz RF-link as an energy source and as a data transmission carrier. The performance of the energy harvester was characterized using the testbench shown in Figure 30. The HF section extracts 13.75 \(\upmu\)W at a separation distance of 9.5 m from the RF generator, showing a overall power conversion efficiency of 10.92% and an input power sensitivity of \(-\)8.99 dBm. In addition, the response of the backscatter ASK modulator was characterized by means of the testbench shown in Figure 31. It can be observed in Figure 33 how lateral sidebands were generated at 910 MHz \(\pm\)\(f_{osc}\), where the frequency of the oscillator is used to map the PTAT \(I_{p}\) current variation into a frequency deviation \(f_{osc}\) as can be seen in Figure 34; this testbench also serves to corroborate the correct operation of the temperature sensor. The PM stage is composed of a shunt regulator, a voltage/current reference and an LDO voltage regulator. Likewise, the LF section is made up of an FVF oscillator and a voltage buffer. In order to decrease the sensitivity of the FVF oscillator to \(V_{DD_{2}}\) power supply variations, a very robust PM stage was designed to provide a very stable \(V_{DD_{2}}\) power supply, a temperature-compensated voltage and current references, \(V_{REF}\) and \(I_{TC}\), and a PTAT \(I_{p}\) current reference. The \(V_{DD_{2}}\) voltage power supply is delivered by the LDO. It can be seen in Figure 17a that the maximum \(V_{DD_{2}}\) voltage deviation is only 900 \(\upmu\)V. This small \(V_{DD_{2}}\) variation occurs when the input voltage of the LDO is varied from 1 V to 1.8 V. However, the input voltage of the LDO will never reach 1.8 V as the shunt regulator limits this value to 1.45 V as it can be appreciated in Figure 19a. Therefore, a \(V_{DD_{2}}\) deviation lower than 900 \(\upmu\)V is expected. The temperature-compensated voltage and current references, \(V_{REF}\) and \(I_{TC}\), were characterized using the testbench shown in Figure 27. The measured \(I_{TC}\) showed a temperature coefficient of 383.33 ppm/\({}^{\circ}\)C, Figure 28a, while the measured \(V_{REF}\) exhibited a 400 ppm/\({}^{\circ}\)C temperature coefficient, Figure 28b. Figure 35 shows how the 13.75 \(\upmu\)W of harvested power are consumed by the main building blocks that compose the proposed temperature sensor prototype. The most power-hungry circuit is the three-stage rectifier, consuming 11.59 \(\upmu\)W, which represents 87% of the total available power. This high power consumption can be explained by means of the Power Conversion Efficiency (PCE) that quantifies the power losses in the three-stage rectifier. The remaining 13% of the total power (1.7875 \(\upmu\)W) are used by the current/voltage references, the shunt regulator, the LDO, and the PTC temperature sensor. Figure 35: Distribution of the power consumption of each of the subcircuits that compose the proposed sensor prototype. On the other hand, to reduce the effect of the Radio Frequency Interference (RFI) on the performance of the proposed temperature sensor prototype, the circuit layout has to be optimized to reduce as much as possible the parasitic coupling of the metal interconnections with the substrate [59].