## 3 Experimental Results, Verification, and Discussion
In addition, the blue solid horizontal line in the figure signifies the average peak value of the noise base, which is approximately \(-\)76 dB and \(-\)66 dB for each channel, indicating that channel 2 also possesses a relatively high level of noise. Figure 8(c1) illustrates the result of directly synthesizing the two channels after impleeting the PLM and normalization processing within the first 15 ns, and it can be found that the intensity non-uniformity has been improved. Figure 8(c2,c3) depict the fitted signals based on the data in Figure 8(c1) at frequencies of 300 MHz and 4.9 GHz, respectively. The solid blue dots represent the directly synthesized data, and the solid red lines represent the fitted signal. However, it can be noted that the collected data do not match the fitted signal accurately, with corresponding ENOB values of only \(-\)0.9181 and 0.3285 bits, respectively. Figure 8(c4) shows the power spectrum information of the directly synthesized signal after using the PLM and normalization. The top four frequencies and power information with the strongest power are marked in the figure, with 4.9 GHz having the highest power, followed by 300 MHz. Therefore, the fitting effect of 4.9 GHz is relatively better than that of 300 MHz, with a power difference of 3.76 dB between the two frequencies. Nevertheless, due to the insignificant difference and the unclear main frequency, the signals fitted with them do not match the collected data very well. The next higher power components are 900 MHz and 4.6 GHz. Thus, the nonlinear distortion of the directly synthesized signal  
Figure 8: Experimental results of 21.1 GHz signal collected by the PADC system. (**a–d**) Data of channel 1, data of channel 2, synthesized signal directly, and synthesized signal after shift correction. is significant. By observation, it can be found that the average peak of the noise base is about \(-\)71 dB. Figure 8(d1) presents the result of channel 2 being shifted 12 positions forward and then combined with channel 1 to synthesize a signal. The calculated ENOB is 4.1184 bits, further substantiating the effectiveness of using shift synthesis to recover the original signal compared to Figure 8(c1). Figure 8(d2) shows the synthesized result of two channel signals being processed by the PLM and normalization, and then shifted. The ENOB reaches 4.1913 bits, which is slightly higher than that without using the PLM and normalization processing. However, the increase is not significant, as there is no significant power difference between the signals collected by the two channels. In addition, the ENOB of the synthesized signal is reduced compared to both channel 1 and channel 2 due to the inaccurate alignment of pulse peaks collected by the two channel EADCs, which has a greater impact on high-frequency synthesized signals. The next step will be to calibrate it using the FROG method to achieve a precise alignment between the pulse peaks and EADC [25]. Figure 8(d3) demonstrates the shift synthesized signal and corresponding fitted signal after applying the PLM and normalization, wherein the processed data match the fitted signal very well. Figure 8(d4) shows the power spectrum information of the normalized shift synthesized signal, indicating the top four frequencies and power information with the strongest power. The frequency corresponding to the highest power is 300 MHz, followed by 4.9 GHz, 900 MHz, and 4.6 GHz. The power of the main frequency is higher than the other three frequencies by more than 30.95 dB, but the maximum power is only \(-\)3.286 dB, which is relatively small compared to Figure 7(d4), suggesting that there are more spectral components and a larger proportion of power. For instance, the difference between the second and third power is only 1.13 dB, resulting in a slightly lower ENOB for the synthesized signal compared to channel 1 and channel 2. Additionally, the average peak noise of the synthesized signal is also approximately \(-\)71 dB, which is related to the noise of the two channels.