# A Self-Powered UHF Passive Tag for Biomedical Temperature Monitoring
## 4 Measurements and Results
### Low-Frequency Section Measurements  
To perform the characterization of the LF section, the prototype chip was attached into a Printed Circuit Board (PCB) shown in Figure 26. As the LF section includes the temperature sensor, the welded elements on the PCB and the PCB itself must withstand temperature variations in the range of 0 \({}^{\circ}\)C to 60 \({}^{\circ}\)C without modifying their electrical parameters. For this reason, the PCB was manufactured using a FR4 substrate that has an operating temperature range of \(-\)65 \({}^{\circ}\)C to 160 \({}^{\circ}\)C. Moreover, all the welded connectors are Tyco Electronics 1-1337445-0 BCNs, whose operating temperature range is \(-\)65 \({}^{\circ}\)C to 160 \({}^{\circ}\)C. In the same way, several RG142B coaxial cables were used, which support an operating temperature range of \(-\)70 \({}^{\circ}\)C to 200 \({}^{\circ}\)C.  
Figure 25: Fabricated prototype of the proposed self-powered temperature sensor.  
Figure 26: Implemented PCB for low-frequency measurements.  
Figure 27 presents the test-bench used to characterize the temperature dependence of the current \(I_{TC}\), the voltage reference \(V_{REF}\), and the oscillation frequency \(f_{osc}\). The BK Precision 1672 power supply is used to energize the LF section by applying a 1.2 V voltage at the \(V_{DD_{2}}\) input, generating \(V_{REF}\) and \(f_{osc}\). The power supply is also used to bias at \(V_{TC}=0.2\) V an on-chip 10x current mirror that sinks a temperature-compensated current \(I_{TC}\). An 8808A-Fluke precision multimeter was used to measure the temperature-compensated current \(I_{TC}\), while a TBS-1022 Tektronix oscilloscope was employed to monitor the voltage reference \(V_{REF}\) and the oscillation frequency \(f_{osc}\). As shown in Figure 27, the PCB was placed inside a Russell's G-Series temperature chamber where the temperature was swept from 0 \({}^{\circ}\)C to 65 \({}^{\circ}\)C in 1 \({}^{\circ}\)C steps.  
Figure 28 shows similar results between the simulations and measurements of the temperature-compensated current source \(I_{TC}\), the voltage reference \(V_{REF}\), and the oscillation frequency \(f_{osc}\). Furthermore, Monte Carlo simulations for 1000 samples are included. Figure 28a shows a measured \(I_{TC}\) temperature coefficient of 383.33 ppm/\({}^{\circ}\)C while the simulation presented a temperature coefficient of 350 ppm/\({}^{\circ}\)C. Figure 28b depicts a \(V_{REF}\) measured 400 ppm/\({}^{\circ}\)C temperature coefficient versus a 341.66 ppm/\({}^{\circ}\)C simulated temperature coefficient. The simulated frequency response shows a sensitivity of 823.29 Hz/\({}^{\circ}\)C, while the measurement exhibits a 831.53 Hz/\({}^{\circ}\)C sensitivity as illustrated in Figure 28c. Figure 28c presents the maximum and minimum values obtained from a 1000 samples Monte Carlo simulation for 10 \({}^{\circ}\)C, 20 \({}^{\circ}\)C, 30 \({}^{\circ}\)C, 40 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C temperatures. The Monte Carlo histograms for 10 \({}^{\circ}\)C, 30 \({}^{\circ}\)C, and 50 \({}^{\circ}\)C are also included.  
Figure 27: Test-bench of the LF section.