## 3 Results and Discussion
### Measurements and Discussion  
We have performed measurements of the analog waveforms with the implemented circuit system, which are shown in Figure 13. The measured bipolar HV pulse is shown in Figure 15a. It has a number of cycles of 4, and the peak-to-peak amplitude is 18.8 V due to voltage drop in the grass-clipping diode. A number of periods of HV pulse can be programmed by adjusting a unipolar pulse signal from the FPGA. In addition, an arbitrary waveform of either unipolar or bipolar pulse can be generated by adjusting the signals of _TX_ and _Unipolar pulse_ of Figure 4. During the RX phase, an ultrasonic echo signal is acquired. The measured waveforms shown in Figure 15b illustrate a repetitive HV pulse \begin{table}
\begin{tabular}{c c c} \hline \hline
**Logic Utilization** & **Used** & **Utilization Percentage** \\ \hline Slice registers & 382 & 0.7\% \\ Slice LUTs & 1535 & 5.6\% \\ Fully used LUT–FF pairs & 336 & 21.2\% \\ Bonded IOBs & 25 & 11.5\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of device utilization in FPGA. Figure 14: Place and route (PnR) result in FPGA. and the echo signal, which is amplified and filtered. To verify the performance of analog-signal conditioning, such as amplification and filtering, we have acquired a typical A-mode echo signal from a commercial phantom model (Model 040 GSE, CIRS Inc., Norfolk, VA, USA). The passband of the RX part of the analog interface circuit ranges between 1 MHz and 3 MHz, and the overall voltage gain in the RX part is 40 dB. As shown in Figure 15b, the time interval between each of the ultrasonic echo signals is uniform, and the amplitude of each echo signal is sufficiently large. To demonstrate the performance of the implemented circuit system, this work implemented a flow phantom model, as shown in Figure 16. The flow phantom consists of a tissue-mimicking part, vessel-mimicking channels, a water pump, and a water reservoir. The tissue-mimicking part, which was made of gelatin, includes two vessel-mimicking channels. To verify that the proposed automatic range-gate-positioning scheme tracks the focal depth of the flowing fluid, one of the vessel-mimicking channels was connected to an external tube, and the other channel was filled with only water. The fluidic channels in the phantom are wall-less type, which is in direct contact with fluid. In the process of phantom  
Figure 15: Measurement analog waveforms of (**a**) high-voltage pulser output and (**b**) echo signal from commercial phantom model. implementation with gelatin, a rod was used to make a space for the wall-less channel. To prevent leakage at the edge of the channel, the wall-less channel was connected with a C-flex tube through a reticulated form and tube connectors. In addition, to acquire a Doppler signal with sufficient amplitude, a solute of NaCl was dissolved in water. The solute acts as scatterers in this measurement; that is, when the concentration of NaCl is high enough, ion clusters are created, and the effective cluster size also increases. [27; 28]. We adjusted the NaCl concentration heuristically to acquire sufficient amplitude of flow signals. Figure 17 illustrates the measured digital waveforms from the flow phantom model. During every RX phase, an echo signal (i.e., an ensemble signal) is acquired and filtered in the digital domain, as shown in Figure 7. Two consecutive ensemble signals, which correspond to a BPF output, are presented in Figure 17a,b. These two ensembles look almost the same. A relatively strong scattering occurs at shallow depth, and the following two echo packets with relatively large amplitude correspond to vessel-mimicking channels. In the implementation of the automatic range-gate-positioning scheme in the FPGA, the number of accumulation of adjacent ensemble differences is set to 256. The resultant waveform, the accumulation of adjacent ensemble differences, is shown in Figure 17c. The Doppler amplitude deviation is sufficiently amplified in the channel region where a fluid flows.