# An NFC on Two-Coil WPT Link for Implantable Biomedical Sensors under Ultra-Weak Coupling
## 5 Experimental Results
### Measurement Results  
Figure 12a,b shows the measured spectrum of the input and output signals of the uplink receiving filter, respectively, when the coil separation \(d\) is 20 mm. Figure 12a is measured at the output of the LNA in the uplink receiving filter. The measured results are measured using a spectrum analyzer (DSA815, RIGOL, Beijing, China). By comparing Figure 12b with Figure 12a, we can see that the received uplink signal power is improved from \(-66.28\) dBm to \(-11.15\) dBm. The power of the power carrier interference is suppressed from 9.51 dBm to \(-43.47\) dBm. Thus, the uplink filter improves the uplink SIR from \(-75.79\) dB to 32.32 dB, which is about 108 dB. As seen, there are several peaks between 125 kHz and 2 MHz. They are the noise induced by the DC-DC convertors in the prototype. The noise has negligible impact on the uplink demodulation, since its power is much lower than the received uplink signal power. We measured the SIR after the uplink receiving filter at different coil separations. Figure 13 shows the measured and calculated uplink SIR versus the coil separations. As seen, when the coil separation is the maximum (40 mm) in the operating range, the SIR is the minimum. The value of the minimum SIR is above 13 dB, which is sufficient for demodulating. In addition, there is only a 5-dB offset between the measured SIR and the calculated SIR, which implies that our circuit modeling is consistent with the practical situation of the prototype. Figure 14 shows the waveforms of three captured signals, which are the output of the uplink PA in the in-body transmitter (not yet merged into the power carrier), the \(V_{\text{load}}\) (the power carrier waveform on the in-body coil) and the output of the uplink receiving filter in the out-body receiver. The output of the uplink PA is a BPSK-modulated square wave signal. The \(V_{\text{load}}\) estimates the ripples of \(V_{\text{load}}\) induced by the in-body uplink PA. The uplink modulation caused negligible ripple (about \(\sim\)5% according to the oscilloscope download data) on \(V_{\text{load}}\). Through the filtering and the amplifying,  
Figure 12: (**a**) The spectrum of the uplink received signal before the uplink receiving filter; and (**b**) the spectrum of the uplink received signal after the uplink receiving filter. Figure 13: Measured and calculated uplink SIR at the output of the uplink filter versus the coil separation \(d\). the uplink signal can be clearly visible, and the modulated phase inverting is significant, while the interference of the power carrier is sufficiently suppressed by the uplink receiving filter. Direct ADC sampling on the uplink carrier (125 kHz) is applied, and the match-filter for demodulation is directly performed right after ADC. The match-filter cross-correlates the 125-kHz local sinusoidal signal with the ADC output. Figure 15a shows the ADC output. As seen, there exists parasitic amplitude modulations, and it is hard to detect the phase changing directly from the uplink carrier. Figure 15b shows the correlation results of the matched filter. As seen, after the matched filtering, the phase changing is exposed clearly, as shown in Figure 15b. Figure 16 shows the measured results of the uplink BER and the coupling coefficient over different coil separations. As seen, when the coil separation is within the operating range, the BER of the uplink can be down to zero in half an hour. When the separation is 50 mm (\(k=0.008\)), the bit error is measured. The large distance demonstrates the capacity of reliable data transmission under ultra-weak coupling. To examine the effect of tissue on the data links, the BER was measured with beef tissue (of a thickness of 10 mm) tightly covering the in-body coil, as shown in Figure 10. The measured BER of the uplink with the beef is also plotted in Figure 16. The results show that the tissue induces a negligible impact on the BER. Figure 14: Waveforms in our prototype when the coil separation is 20 mm.