## 3 Measurement Results of System Evaluation  
### Measurement Results  
We first checked if the SoC blocks operate well in terms of amplification, digitization, and wireless transmission, by checking the operation of each block individually and together. We then verified if the automatic switching of the frequency band works well with the SR-Rx operation, when there is interference at either 27 MHz or 432 MHz band. Finally, we measured the power consumption of each functional block and compared it with the results of the iTDS-1. Figure 9a shows measurement results of the AFE and the Tx, during a single sampling period acquiring data from four 3-axial magnetometers. Four 3-axial magnetometers generate 12 analog voltages that are digitized by the 13-bit SAR ADC and packetized by the FPGA. Following OOK/FSK modulation at 250 kbps, the Tx transmits the data wirelessly at either 27 MHz or 432 MHz. The data packet also includes battery voltage information to alert the user of the remaining charge. At 27 MHz, the data packet has start/stop bits for the universal asynchronous receiver/transmitter (UART) interface after recovery by the custom 27 MHz super-heterodyne receiver. At 432 MHz, the packet has a predefined 5-byte header to communicate with the CC1110 transceiver, as shown in Figure 9a [41]. The default output power at 27 MHz and 432 MHz is set to \(-\)3 dBm and \(-\)5 dBm, respectively. The frequency spectrum in Figure 9b was measured at the output of the 13-bit SAR ADC with 128 Hz, 1.8 V peak-to-peak sine wave input, to evaluate the performance of the ADC, which results in 62.8 dB signal-to-noise-distortion ratio (SNDR) and 10.1 effective number of bits (ENOB). The differential nonlinearity (DNL) measurement result in Figure 9b shows that the DNL lies within \(\pm\)0.54 LSB at 10.1 bits ENOB. The AFE and the ADC are turned on for half of the sampling period,while the high power blocks, such as magnetic sensors and the Tx are aggressively duty cycled at 2.3% and 9%, respectively, to save power and increase the operating time of the iTDS-2. Figure 10a shows the measurement results of the SR-Rx, with the transmitted/recovered data and the corresponding quench signal. These results show that the SR-Rx successfully recovers the 27 MHz OOK carrier at 5 kbps. Figure 10b shows how the quench period changes according to the 27 MHz carrier amplitude. The quench signal is turned off after 40 \(\upmu\)s if there is no oscillation or as soon as an oscillation is detected, and turned on again after 20 \(\upmu\)s that was found as sufficient time to suppress the residual oscillation. Figure 10c shows that the oscillator is turned off after 16 peaks are detected by the FPGA. Figure 9: Measurement result of the AFE, Tx, and ADC: (**a**) a time domain waveform of the AFE and Tx within a single sampling period; (**b**) Output spectrum of a 128 Hz, 1.8 V\({}_{\text{P-P}}\) sine wave input after fast Fourier transform and differential nonlinearity of the ADC over 10.1 bits ENoB. A Schmitt trigger is located between the oscillator and the FPGA to recognize only meaningful oscillations out of the noise floor. The FPGA recognizes the period of the quench signal and recovers the data accordingly. The data bit is recovered as '1' when the quench period is less than 56 \(\upmu\)s, to have a 4 \(\upmu\)s margin, and as '0' otherwise. When the strength of the 27 MHz RF carrier exceeds \(-\)73 dBm, which is the sensitivity of the SR-Rx, 16 oscillations occur in less than 56 \(\upmu\)s and the counter resets the quench signal early enough to recover the data bit as '1'. Figure 11 demonstrates the operation of the dual-band radio when there is interference, larger than the predefined threshold level, at either 27 MHz or 432 MHz band. On the left side, the SR-Rx detects 27 MHz interference, and the Tx changes the communication band from 27 MHz to 432 MHz. On the right side, the TDS-UI detects 432 MHz interference by the built-in 432 MHz Rx and delivers the predefined 8-bit Ack packet to the iTDS-2 via 27 MHz band.