# An IoT Reader for Wireless Passive Electromagnetic Sensors
## 2 Materials and Methods
### RF Interface
#### 2.3.1 Signal Generator  
The general sketch of the signal generator is shown in Figure 7. It is mainly based on a voltage controlled oscillator (VCO) whose oscillation frequency is controlled by the input voltage (\(V_{TUNE}\)). The input voltage determines the instantaneous oscillation frequency of the VCO. The input voltage of the VCO is generated by a monolithic function generator integrated circuit. The function generator produces a low-frequency periodic triangular signal which forces the VCO to perform the required frequency sweep. In particular, the VCO generates an increasing sweep and a decreasing sweep of the whole sensor operation band per period. This means that the sensor will response with a periodic double notch (one during the increasing sweep and another during the decreasing one). These notches are associated to the resonant frequency of the sensor immersed in a particular LUT. This will be used to determine the desired sensing magnitude in Section 3.  
The chosen VCO is the model JTOS-2000+ manufactured by the company Mini-Circuits (New York, NY, USA). The datasheet [37] specifies that the VCO can be operated between 1.37 and 2 GHz for an input voltage between 6 and 17 V. The output power level of the VCO is 12 dBm approximately. The triangular function generator is based on the Exar Corporation XR-2206 monolithic function generator integrated circuit [38]. The signal generator is configured to generate a triangular signal of period \(T=10\) ms approximately. The used configuration is shown in Figure 8a where \(V_{CC}\) = 24 V, \(R_{1}\) = 10 k\(\Omega\), \(R_{2}\) = 5.1 k\(\Omega\), \(R_{3}\) = 5.1 k\(\Omega\), \(R_{4}\) = 5.1 k\(\Omega\), \(R_{5}\) = 20 k\(\Omega\), \(C_{1}\) = 1 uF and \(C_{2}\) = 1 uF. The frequency of the triangular signal is determined by \(f=1/(R_{1}\cdot C_{1})\) (\(\approx\)100 Hz). The DC level at the output (Pin 2, \(V_{OUT}\)) is approximately the same as the DC bias at Pin 3. In this case, the Pin 3 is biased through a voltage divider to give an output DC level of \(V_{CC}/2\). The amplitude of the output triangular signal is controlled  
\begin{table}
\begin{tabular}{c c c} \hline
**Material** & **Relative Permittivity (\(\epsilon_{r}\))** & **Resonant Frequency (\(f_{r}\) [GHz])** \\ \hline Free Space & 1 & 1.804 \\ Paraffin Oil & 2.45 & 1.748 \\ Almond Oil & 2.96 & 1.739 \\ Lemon Oil & 3.03 & 1.737 \\ CHCl\({}_{3}\) & 5.77 & 1.649 \\ \hline \end{tabular}
\end{table}
Table 1: Measured resonant frequency of the radiating layer transmission coefficient ( \(|S_{21 rad}|^{2}\)) for different LUTs.  
Figure 7: General sketch of the signal generator.  
with \(R_{5}\). \(R_{4}\) provides a fine adjustment for the waveform symmetry. The measured triangular signal with the proposed configuration is shown in Figure 8b. It has been measured with the DSO-X 3034A digital oscilloscope (Agilent, Santa Rosa, CA, USA) [39]. It can be seen that the voltage varies between 8.25 and 15.50 V while the oscillation frequency is 92.20 Hz. This input voltage signal, \(V_{\textit{TUNE}}\), is appropriate to generate the required frequency sweep between 1.6 and 1.9 GHz with the VCO.  
Figure 9a shows the picture of the manufactured signal generator, which contains the previous triangular signal generator connected to the VCO. Figure 9b shows the measured power spectrum of the generated signal. The manufactured circuit covers the desired frequency range to interrogate the sensors with an output power over 10 dBm.