## 1 Introduction
The authors of [15] proposed a reconfigurable filter based on three varactor-loaded compactDGSs, in which the bandstop frequency range was adjustable from 1.8 GHz to 2.9 GHz and from 2.9 GHz to 8.1 GHz. In the design described in [16], the authors symmetrically etched the DGS filter using a UH-shaped patterned ground plane to form three mutually coupled resonators for wideband CM noise suppression. It effectively reduced the CM noise in the frequency range from 3.6 GHz to 9.1 GHz. In [12; 13; 14; 15; 16], these designs took advantage of a simple structure and a low fabrication cost, but none could achieve an ultra-wide bandstop frequency range, even with three DGS units. Furthermore, the authors of [17] designed a three-pole DGS filter to reduce the CM noise in the frequency range from 3.2 GHz to 12.4 GHz. However, the physical size of the filter was 10 mm \(\times\) 10 mm, which is not practical for a compact circuit design. Details of the CM suppression filters are listed in Table 1, including the lower frequency (\(f_{L}\)), upper frequency (\(f_{H}\)), central frequency (\(f_{C}\)), and fractional bandwidth (FBW), where the FBW is defined as the absolute bandwidth divided by the central frequency and is expressed as a percentage of the central frequency. It is given by [19]  
\[\text{FBW}=2\times\frac{f_{1}-f_{2}}{f_{1}+f_{2}} \tag{1}\]  
where \(f_{1}\) and \(f_{2}\) are the upper frequency (\(f_{H}\)) and lower frequency (\(f_{L}\)) of the stopband, respectively. After that, CM suppression filters in [18; 19; 20] obtained a bandstop range using a \(-\)10 dB definition. In CM rejection filters based on a complementary split-ring resonator design [18], the authors proposed three glide symmetry implementations to achievea maximum FBW of 26.8%. A reconfigurable common-mode (CM) filter with a wide tunable bandwidth and frequency range was proposed in [19]. It obtained three tunable CM suppressions of frequency range from 1.9 to 3 GHz, 2.5 to 5.9 GHz, and 3.3 to 7 GHz, achieving a maximum FBW of 75.5%. In the enhanced coupled DGS design [20], the filter used only two C-shaped DGSs to achieve wideband CM noise suppression. Two C-shaped DGSs were the same size and in opposite directions on the ground plane and were symmetrical to the centerline of the differential microstrip. The simulation results for each half-structure indicated two resonant frequenciesclose to 8 GHz. The enhanced coupling mechanism of the two C-shaped DGSs shifted the first-order resonant frequency to 4.5 GHz and the second-order resonant frequency to 7 GHz. They suppressed the CM noise in the frequency range from 3.7 GHz to 10.8 GHz using the definition of \(-\)10 dB, as shown in the simulated result; the stopband FBW reached 97.9%. The physical sizes were 7 mm \(\times\) 7 mm. However, the simulated results showed that the second-order transmission zero did not bring an extended stopband to suppress the CM noise in the higher-frequency domain due to an insufficient coupling magnitude between the two DGS units. As an extension design of [20], we propose an ultra-wideband CM suppression filter with an asymmetrical magnified coupled DGS in this paper. We take advantage of the second-order transmission zero produced by the asymmetrical magnified coupled DGS, which provides a more pronounced coupling effect than [20]. Next, the proposed CM suppression filter's design details are presented in Section 2, including full-wave simulations using high-performance electronic design automation (EDA) software ANSYS HFSS 2022 R1 and equivalent circuit modelsusing the circuit simulation tool Agilent ADS. Section 3 presents measurements to validate the simulated results, including frequency and time domain measurements. Section 4 presents discussions. Finally, the conclusion is providedin Section 5.