## 5 Test Results and Analyses  
Use a spectrum analyzer to measure the quality of its output signal, that is, the SNR. The test block diagram is shown in Figure 11.  
Figure 10: The relationship diagram of amplitude error when the time error is \(\Delta t\). According to this scheme, the specifically designed circuit board and the actual test connection diagram are shown in Figure 12.  
When using a spectrometer to test the SNR, due to the limitation of the dynamic range of the spectrometer, it is necessary to observe the power of the output signal and the power of the output noise under different attenuation degrees. Because of the system noise figure in the spectrum analyzer and the distortion performance of the input mixer, the dynamic range of the spectrum analyzer is inconsistent with its measurable range, and appropriate test parameters need to be set according to the specific situation [24]. For the test of this subject, the GA4063 type spectrum analyzer was used. It has a measurable range of +30 dBm to \(-148\) dBm. In addition, the unit of spectrum analyzer is dBm, which is different from the unit of SNR, dB. So, after obtaining the test results, we need to calculate the SNR through Formula (11). The SNR is equal to the power of the signal minus the power of the noise. \[SNR(dB)=10\log_{10}\frac{P_{signal}(mW)}{P_{noise}(mW)}=10\log_{10}\frac{10^ {\frac{P_{signal}(dBm)}{10}}}{10^{\frac{P_{noise}(dBm)}{10}}}=P_{signal}( dBm)-P_{noise}(dBm) \tag{11}\]  
Firstly, we measured the signal before and after the filter. The result is shown in Figure 13.  
Figure 11: Block diagram of dynamic parameter SNR test. Figure 12: (**a**) The specifically designed circuit board and (**b**) the actual test connection diagram. In Figure 13, the frequency of the tested signal is 10 KHz. The span of the spectrum analyzer is set as 32 KHz, which can include the second, third and fourth harmonics. From Figure 13a, it can be found that the second and third harmonic components are a bit high. However, in Figure 13b, the harmonic components of the signal are reduced a lot after the filter. So, the signal should be tested after the filter. When outputting a sine signal with a frequency of 10 KHz, the test result of SNR is shown in Figure 14.  
Firstly, when the attenuation factor is set to 30 dB, the measured power value of the input signal is 5.49 dBm as shown in the frequency standard in Figure 14a; then, adjust the reference point to a suitable position and reduce the attenuation factor. When the attenuation factor is set to 0 dB, the noise of the input signal can be measured as \(-\)98.00 dBm by observing the marking line of the spectrum analyzer. So, the SNR is 103.49 dB when the TIDAC circuit outputs a signal of 10 KHz that can be obtained. In addition to the 10 KHz sine signal, the other groups of frequency values were tested within the measuring range of the spectrum analyzer. The corresponding SNR when the TIDAC circuit and the single DAC circuit output a sine wave of the same frequency are respectively measured. The specific results are shown in Table 1.  
Figure 14: Test result of 10 KHz sine wave SNR. (**a**) Signal power test. (**b**) Noise power test. Figure 10:0 Test result of 10 KHz sine wave. (**a**) Signal before filter. (**b**) Signal after filter. It can be found from Table 1 that when the output signal frequency is low, the SNR test result of the single DAC circuit is better than the test result of the TIDAC circuit. However, when the frequency gradually increases, the amplitude of the single DAC decreases faster, and the noise increases more. So, the SNR is not as good as the signal output by the TIDAC circuit. The reason is that, at low frequencies, the number of sampling points in each cycle of the two circuits is enough, and the output signal is mainly affected by noise. TIDAC increases the noise because of the introduction of more circuits, so the SNR is lower. With the frequency increase, the sampling number of the output signal of the single DAC circuit decreases in each cycle.