## 4 Implementation System and Results
### Testing with the SAM System  
As described in Figure 15, the SAM system transmits and receives signals based on the signal from the motor encoder (this is also the trigger signal for the P/R and the digitizer). The SAM system consisted of three main devices: a PC and digitizer (NI PXIe-1062Q and NI PXI-5152 8-bit, 2 GS/s digitizer, National Instruments Corporation. Austin, TX, USA), a Newport Driver (Model ESP300, Newport Corporation. Irvine, CA, USA), and our P/R device. Therefore, the higher the pulse frequency from the encoder, the faster the sample scan time. In our design, the trigger module of a P/R device could receive pulses of up to 100 kHz.  
In this test, we used an image acquisition system to test the signal with a sample of one coin. The parameters for setting up the scanning system are described in Figure 18 and Table 6.  
Figure 17: **(a)** The real steel sample with a thickness of 0.25 mm; **(b)** A model of the echo signals from the sample; **(c)** The echo signal of the 50 MHz transducer without a damping resistor; **(d)** The echo signal of the 50 MHz transducer with damping resistors of 47 \(\Omega\).  
On the other hand, we conducted experiments to determine the SNR ratios of two different transducers. This test was done before the system's scan of the coin. The results are described in Table 7.  
In Figure 19, we described the system's final image with the use of damping resistors. The images obtained for systems using suitable damping resistors were of a high definition due to the improved SNR of the echo signal.  
In Figure 19a, we scanned the surface of the coin and used a high-frequency transducer (85 MHz). High-frequency transducers with poor sample penetration are suitable for surface scanning applications. In Figure 19c, the inner layer of the Spartan-6 chip is described. We used a 25 MHz transducer to do this experiment. The ultrasonic waves generated by the 25 MHz transducer penetrate the object better than the 85 MHz transducer. Hence, the SAM system using the 25 MHz transducer obtained a better image than the SAM system with 85 MHz transducer.  
After performing the coin scan, we calculated the contrast-to-noise ratio (CNR) of the obtained image. The results are described Table 8.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Items** & **Transducer** & **R-Damping (\(\Omega\))** & **Step size** & **Number A-Scan** & **Number B-Scan** \\ \hline
1 & 25 MHz & 25 & 100 \(\upmu\)m & 500 & 500 \\
2 & 85 MHz & 50 & 100 \(\upmu\)m & 500 & 500 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Parameters of the SAM system for coin scanning.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Items** & **Transducer** & **SNR (no Damping Resistor)** & **SNR (with Damping Resistor)** \\ \hline
1 & 25 MHz & 6.5 & 7.3 \\
2 & 85 MHz & 7.3 & 9.1 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Compared SNR of echo signal with and without damping resistor.  
Figure 18: Set-up of the transducer with a coin sample.  
Based on the results in Tables 7 and 8, we can see that the damping resistor increased the SNR of the echo signal from the transducer. The CNR of the image was also improved after using damping resistors.