# Radar Emitter Structure Inversion Method Based on Metric and Deep Learning
## 5 Experiments  
### Simulation and Analysis of the Modelled RF Signal  
First, the spurious information introduced by DDS signal source models with different modulation types is verified using experiments. The frequency of the reference clock source is \(f_{\rm c}=2\) GHz, the number of bits in the phase accumulator is \(N=15\), the number of bits for phase truncation is \(B=6\), the pulse width is \(T=10^{-6}\) s, and the sampling rate is \(f_{\rm s}=1\) GHz. Thus, \(K_{\rm CW}=409.6\), \(K_{\rm LFM}=409.6\), and \(K1_{\rm LFM}=0.0819\). According to the parameters set in the model, \(f_{\rm CW}=f_{\rm BPSK}=f_{\rm LFM}=f_{\rm QPSK}=20\) MHz and \(B_{\rm LFM}=10\) MHz can be obtained. The frequency variation range of the LFM is 20\(\sim\)30 MHz.  
Figure 9 shows four modulation types of source signals generated by the ideal signal source and four DDS source models. It can be observed from the spectrum that the phase truncation error of the DDS model has a certain influence on the spectrum of the four modulation type signals. In addition, the presence of spurious information can be clearly observed in the spectrum.  
Figure 9: Spectrum diagram of the source signals of four modulation types. (**a**) CW; (**b**) BPSK; (**c**) LFM; (**d**) QPSK; (**e**) DDS_CW; (**f**) DDS_BPSK; (**g**) DDS_LFM; (**h**) DDS_QPSK.  
The RF structure information of the different classes of RES models is then verified using experiments. Figure 10 illustrates the spectrum diagram of the RF signals produced by different classes of RES models. It is clear that the three classes of RES models exhibit distinct differences in the spectrum purity of RF signals of the four modulation types. Each RF signal carries different RF structure information, with the RF signal generated by the RES1 model having the smallest amount of RF structure information. This is because the amplifier of the RES1 model is relatively ideal and only exhibits one stage of amplification. The measured S parameter is used for the amplifier modeling of RES2 and RES3 models, and the RF structure information is obvious. The structure differences caused by the usage mode, working principle, and material composition of each module are also verified. The generated RF signal also carries various levels of unique RF structure information. In addition, there are obvious differences in the spectrum purity of RF signals with various modulation types under the same RES model, and each RF signal carries different signal information.  
Finally, the individual information introduced by different REI models under the same class of RES models is validated experimentally. As shown in Figure 11, as observed from the spectrum of RF signals (DDS_CW) generated by four different REI models under the RES2 model, the RF signals of four different REI models under the same RES2 model carry different individual information, and there is less individual information than RF structure information, reflecting the subtle differences among individuals.  
Figure 10: Spectrum diagram of RF signals with different classes of RES models: (**a**) RES1_DDS_CW; (**b**) RES1_DDS_BPSK; (**c**) RES1_DDS_LFM; (**d**) RES1_DDS_QPSK; (**e**) RES2_DDS_CW; (**f**) RES2_DDS_BPSK; (**g**) RES2_DDS_LFM; (**h**) RES2_DDS_QPSK; (**i**) RES3_DDS_CW; (**j**) RES3_DDS_BPSK; (**k**) RES3_DDS_LFM; (**l**) RES3_DDS_QPSK.