## 4 Investigation Results  
### Overall System  
In this section, the two proposed designs of the two prototypes are implemented and built on two-layer PCBs. The used BJT and TIA boards are shown in Figure 14, respectively. Their dimensions are about \(40\times 50\,\mathrm{mm}\). The main characteristics and power parameters of used components are summarized in Table 2.  
Both WuRx architectures are equipped with ANT-868-CW antenna and the B39871B bandpass filter to pass the signal in the 868 MHz frequency band. The envelope detector consists of two SMS7630 Schottky diodes from Skyworks Solution Inc. to demodulate the received signal. As explained in Section 3.4, the chosen diodes reach the lowest detectable RF power of \(-74\,\mathrm{dBm}\).  
Figure 13: Manchester coded wake-up packet. (**a**) Pattern with carrier burst, preamble, and address; (**b**) OOK signal on kilohertz band.  
The demodulated signal is amplified by the MIC861 OA at the TIA implementation or by BFP405 RF transistor at the BJT implementation. Microchip's MIC861 OA was selected because of its GBWP of 400 kHz at a low supply current of 4.6 \(\upmu\)A [47]. With \(G=20\), the resulting upper corner frequency is 20 kHz, sufficient to amplify the 18.7 kHz LF signal. LTSpice simulation shows that general-purpose transistors are not suitable due to a corner frequency lower than 18.7 kHz. This is coursed by high parasitic capacitance and the high \(R_{\text{C}}\). The BFP405 is a high-frequency transistor, which was utilized due to its low parasitic capacitance, resulting in a corner frequency of more than 100 kHz. From Equation (12), with \(V_{\text{V+}}=3.0\,V\), \(V_{\text{T}}=26\,\text{mV}\), \(R_{\text{C}}=2.4\,\text{M}\Omega\), and the AS3933 load resistance, \(R_{\text{L}}=2\,\text{M}\Omega\)[44] will result in \(G\approx-58\). The voltage gain is not strongly dependent on the transistor's current gain \(\beta\).  
For the investigation, the WuPt transmission is generated by the SPSGRFC-868 radio module from STMicroelectronics. The MCU utilized on the boards is a 16-bit MSP430G2553  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Components** & **Parameters** & **Values** \\ \hline \multirow{3}{*}{Antenna ANT-868-CW} & Center Frequency & 868 MHz \\  & Impedance & 50 \(\Omega\) \\  & Peak Gain & \(-2.3\,\text{dBi}\) \\  & Bandwidth & 30 MHz \\ \hline SPIRIT1 SPSGRFC-868 & Supply Current TX Mode & 21 mA \\  & Supply Current RX Mode & 9 mA \\ \hline \multirow{3}{*}{SAW Filter B39871B} & Center Frequency & 869 MHz \\  & Maximum Insertion & 2.5 dB \\ \cline{1-1}  & Attenuation & \\ \cline{1-1}  & Usable Pass band & 2 MHz \\ \hline Schottky Diodes & Voltage Sensitivity & 40 mV/\(\upmu\)W \\ \cline{1-1}  & Video Resistance & 5 k\(\Omega\) \\ \hline \multirow{3}{*}{BFP405 NPN RF Bipolar Transistor} & Maximum Gain & 23 dB \\ \cline{1-1}  & Minimum Noise Figure & 1.25 dB \\ \cline{1-1}  & Collector Current & 1.3 \(\upmu\)A \\ \hline MIC861 & Supply Current & 4.2 \(\upmu\)A \\ \cline{1-1} Operational Amplifier & Gain Bandwidth Product & 350 kHz \\ \hline \multirow{3}{*}{AS3933 LF Wake-Up-Chip} & Supply Current & 3.1 \(\upmu\)A \\ \cline{1-1}  & Preamble Detection & 12 \(\upmu\)A \\ \hline \multirow{3}{*}{MSP430 Microcontroller} & Supply Current Standby & 860 nA \\ \cline{1-1}  & Supply Current Active & 2 mA \\ \cline{1-1}  & System Clock & 8 MHz \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of component values for the wake-up receiver.  
Figure 14: PCBs of the proposed architectures. 1—bandpass filter, 2—envelope detector, 3—LF WuRx IC, 4—TIA amplifier, 5—BJT amplifier.  
manufactured by Texas Instruments running at 8 MHz. The MSP430 can enter multiple low-power modes. In order to keep a clock crystal for timing active, the LPM3 is used. When in low-power mode 3, it consumes \(P_{\text{MSP\_LPMS}}=2.55\,\upmu\)W. To power the WuRx nodes, 3.0 V batteries are used.