# An IoT Reader for Wireless Passive Electromagnetic Sensors
## 2 Materials and Methods
### RF Interface
#### 2.3.2 Signal Detector  
The general sketch of the signal detector is shown in Figure 10. The signal detector is composed by a broadband RF amplifier and a Schottky diode detector (SDD). The RF amplifier mitigates the free space losses between the antennas within the wireless link. This is because it is necessary to achieve a signal detectable by the SDD over \(-15\) dBm. The SDD produces an output voltage proportional to the power of the signal received from the sensor. Figure 8: Triangular signal generator configuration (**a**) and its measured triangular signal (**b**). Figure 9: Manufactured signal generator (**a**) and its measured power spectrum (**b**). Figure 11 shows the block diagram of the proposed broadband RF amplifier. It is based on a balanced design [40]. A balanced amplifier typically employs two quadrature couplers and two amplifiers designed in the same way. The quadrature coupler situated at the input of the balanced design produces two signals with equal amplitude and 90\({}^{\circ}\) phase-shift at the two amplifiers inputs. Then, the second quadrature coupler at the output of the balanced scheme combines the amplified signals in phase. In the proposed design, a three-stage Wideband Wilkinson Power Divider [34] with an additional 90\({}^{\circ}\) phase-shifter in one of its outputs has been used instead of the typical quadrature coupler. The broadband amplifier is etched on the commercial dielectric substrate AD1000 (Rogers, Connecticut, CT, USA) with a relative dielectric constant \(\varepsilon_{r}=10.6\), loss tangent \(tan\)\(\delta=0.0023\), substrate thickness \(h=1.27\) mm and a metallization thickness of 17 \(\mu\)m. Figure 12 shows the circuit schematic of the amplifiers where \(C_{1}=1\) nF, \(C_{2}=1.8\) pF, \(C_{3}=3.3\) pF, \(C_{4}=1.8\) pF, \(L_{1}=2.2\) nH, \(R_{1}=150\)\(\Omega\), \(R_{2}=82\)\(\Omega\), \(R_{3}=1\)\(\Omega\) and \(R_{4}=100\)\(\Omega\). The amplifiers are based on the heterostructure field-effect transistor (HFET) model Broadcom ATF-34143 [41]. The characteristic impedance of the high-impedance DC-blocking lines is \(Z_{1}=95\)\(\Omega\) while for the other lines is \(Z_{o}=50\)\(\Omega\). The input and output matching networks are based on a resistive model to achieve a flat response in terms of gain ( \(|S_{21}|\)) and isolation ( \(|S_{12}|\)) within the whole broad bandwidth. Figure 11: Block diagram of the broadband balanced amplifier. Figure 10: General sketch of the signal detector. Figure 13 shows the design of the wideband Wilkinson power divider where \(R_{1}=120\)\(\Omega\), \(R_{2}=220\)\(\Omega\) and \(R_{3}=270\)\(\Omega\) and all the transmission lines have a length of \(\lambda/4\). The characteristic impedances of the microstrip lines are \(Z_{1}=90\)\(\Omega\), \(Z_{2}=70\)\(\Omega\) and \(Z_{3}=55\)\(\Omega\). Figure 14 shows a picture of the manufactured balanced RF amplifier and Figure 15 shows its measured S-Parameters. It can be seen a good performance over the whole required frequency bandwidth (between 1.6 and 1.9 GHz). The reflection coefficient at the input port (\(\mid S_{11}\mid\)) is below \(-17.94\) dB while at the output port (\(\mid S_{22}\mid\)) is below \(-19.54\) dB within the bandwidth of interest. The gain (\(\mid S_{21}\mid\)) is 11.84 dB at 1.6 GHz and decreases up to 10.20 dB at 1.9 GHz. Finally, a good level of isolation is achieved, presenting the \(\mid S_{12}\mid\) parameter a value below \(-25.31\) dB. Figure 14: Manufactured balanced RF amplifier. Figure 13: Schematic of the three-stage wideband Wilkinson power divider. Figure 15: Measured S-Parameters of the manufactured broadband balanced RF amplifier. Figure 16 shows the proposed SDD where \(R_{in}=33\)\(\Omega\), \(C_{out}=1\) nF, \(R_{out}=2200\)\(\Omega\) and the Schottky diode is the model HSMS-2850 [42] by Broadcom. The SDD is etched on the commercial Rogers AD1000 dielectric substrate. The SDD is based on a series configuration, essentially working as a half-wave rectifier.