# A Self-Powered UHF Passive Tag for Biomedical Temperature Monitoring
## 4 Measurements and Results
### Read Range Characterization  
The read range characterization of the proposed self-powered temperature sensor prototype was done using the setup depicted in Figure 31. The E4425B ESG-AP Series 3 GHz RF generator along with five RF adapters, a RG142B/U coaxial cable, and a 2000-1035-R Anritsu antenna were used to provide the RF link. The temperature sensor was separated from the antenna at a distance \(r=2.54\) m. This separation corresponds to the maximum separation distance in which the sensor would turn on. To read the backscatter ASK modulated signal generated by the sensor, a DSAV048A Infiniium V-Series 8 GHz oscilloscope, two RF adapters, and a RG142B/U coaxial cable were used. To quantify the radiated power \(P_{Tx}\) the Equation (38) must be evaluated where \(P_{G_{Tx}}\) is the E4425B RF generator output power (20 dBm), \(I_{L_{RF-A}}\) is the insertion loss (0.1 dB) of the  
Figure 31: Testbench used for read range and temperature characterization. Figure 30: RF power harvester characterization. (**a**) Testbench. (**b**) Transient measurement response. \(n\) RF adapters (5), and \(I_{LCABLE}\) is the insertion loss of the RG142B/U coaxial cable (1.329 dB). The radiated power is 18.171 dBm or 65.62 mW. \[P_{Tx}=P_{G_{Tx}}-n\cdot I_{LRF-A}-I_{LCABLE} \tag{38}\]  
The separation distance between the self-powered sensor and the RF transmitter can be computed by Equation (39), where \(\lambda\) is the 910 MHz wavelength, \(P_{T_{x}}=18.171\) dBm according to Equation (38), \(G_{T_{x}}=4.6\) dBi is the 2000-1035-R Anritsu transmitter antenna gain, \(G_{R_{x}}=1.543\) dBi is the self-powered sensor antenna gain, \(P_{C}=5.5\) uW is the self-powered sensor power consumption, \(\Gamma\) is the reflection coefficient between the temperature sensor antenna and the temperature sensor rectifier, and \(PCE\) is the Power Conversion Efficiency of the rectifier. \[r=\frac{\lambda}{4\pi}\sqrt{\frac{P_{Tx}G_{Tx}G_{Rx}}{2P_{C}}(1-|\Gamma|) \cdot PCE} \tag{39}\]  
Figure 32a presents the read range calculation based on the previous parameters where \(\Gamma\) and \(PCE\) were swept from 0 to 1; the read range is depicted in colored contour lines. As the measured read range distance is \(r=2.54\) m, it can be inferred that the reflection coefficient and the power conversion efficiency are in the ranges of \(0<\Gamma<0.6\) and a \(0.4<PCE<1\). A power consumption of 13.75 uW and a power efficiency of 10.92% were also found. This increment in the chip power consumption is due to errors in the antenna-chip bonding wire welding process. The setup was performed using an irradiated power of 65.62 mW. Assuming a \(P_{Tx}=1\)\(W\), Equation (39) cab be re-evaluated obtaining Figure 32b. By setting a range of \(0<\Gamma<0.6\) and a \(0.4<PCE<1\), it can be extrapolated that the worst read range of the proposed self-powered temperature sensor is 9.5 m.  
Figure 32: Proposed self-powered sensor read range. (**a**) Measured read range for 65.62 mW irradiated power. (**b**) Extrapolated read range for 1 W irradiated power. As stated at the beginning of this section, the ASK backscatter modulated signal generated by the sensor was measured by a DSAV048A Infiniium V-Series 8 GHz oscilloscope with the aim of a 2000-1200-R Anritsu antenna. The sensor prototype was exposed in a free environment to an external temperature source. Figure 33 presents how the temperature sensor generates lateral sidebands around a 910 MHz RF carrier, 910 MHz \(\pm\)\(f_{osc}\). The location of these lateral sidebands corresponds with the temperature applied into the surface of the self-powered sensor.