## 2 Common-Mode Noise Suppression Filter Design
### Equivalent Circuit Model  
The equivalent circuit modeling involves the observation and theoretical derivation of the MDCRP filter structure. The side view of the MDCRP structure filter is shown in Figure 5a, in which we can observe the mushroom and DCRP structures. The microstrip line can be divided into five parts where two parts cannot refer to the first ground plane, forming two equivalent inductances of L1 and L2. The two ends of L1 and L2 become equivalent C1, C2, C3, and C4 capacitances with the first ground and mushroom ground planes, and the other three parts are the equivalent models of the coupled microstrip lines of CML1, CML2, and CML3. The two vias in the mushroom structure become the equivalent inductance of L5, which connects the second ground plane. The four vias on the left connect  
Figure 4: The effect of the geometric parameter changes of the MDCRP filter on the CM insertion loss. (**a**) The length of slot 2 (SL2). (**b**) The width of slot 2 (SW2). (**c**) Via quantity (VQ). (**d**) Mushroom length (ML). (**e**) Mushroom width (MW).  
to the second ground plane to become the equivalent inductance of L3. The four vias on the right connect to the second ground plane to become the equivalent inductance of L4.  
The equivalent half-circuit model is shown in Figure 5b. Then, we imported all the relevant PCB and microstrip line parameters into the equivalent circuit model of the Advanced Design System (ADS) for simulation, this produces CM insertion loss (S\({}_{\text{cc21}}\)) parameters. The equivalent components (L1, L2, L3, L4, L5, C1, C2, C3, and C4) were adjusted to achieve CM insertion loss (S\({}_{\text{cc21}}\)) parameters similar to a simulation result, allowing these equivalent components to be effective parameters.  
We modified equivalent component parameters to cause the CM insertion loss (S\({}_{\text{cc21}}\)) parameter change. When the larger value of L1 has better suppression of low-frequency CM noise, the larger value of L2 has worse suppression of CM noise, affecting the resonance frequency change in the low-frequency band. When the value of L3 is larger, the suppression of CM noise in the low-frequency band is worse, and the suppression of CM noise in the high-frequency band is better. A smaller L4 value affects the resonant frequency change in the low-frequency band. When the value of L5 is smaller, it affects the resonance frequency change in the low-frequency band, and the suppression of CM noise in the low-frequency band is worse.  
When the value of C1 is smaller, it affects the change in resonance frequency in the low-frequency band, and the suppression of CM noise is worse. When the value of C2 is smaller, it affects the change in resonance frequency in the low-frequency band, and the suppression of CM noise is smaller, it affects the change in resonance frequency in the low-frequency band, and the suppression of CM noise is worse. When the value of C3 is smaller, the suppression of CM noise in the high-frequency band is better, but the suppression of CM noise in the high-frequency band is worse, but the suppression of CM noise in the low-frequency band is worse and shifts toward low frequencies.  
A verification equivalent half-circuit model by full-wave simulation of the CM insertion loss is shown in Figure 6. The CM insertion loss (S\({}_{\text{cc21}}\)) parameters of both are very similar. The suppression bandwidths are also very similar, which proves the validity of the equivalent circuit model.  
Figure 5: (**a**) The MDCRP structure filter side view. (**b**) Equivalent half-circuit model.