Design of ultra-wideband EMI filter

　　In the past decade, the frequency range of shielded rooms used as microwave experimental infrastructure has been continuously expanded. The high-end frequency has increased from 1GHz to 18GHz or even 40GHz. It is expected that the future trend will increase to 60GHz or even 100GHz. In order to ensure the shielding effectiveness of the shielded room in the entire applicable frequency range, that is, to prevent interference signals from being introduced into or out of the shielded room due to the introduction of power lines or signal lines, this requires that the power filter and signal filter of the shielded room are in the same state. The frequency range has specified insertion loss.

　　The ultra-wideband EMI filter introduced in this article uses the electrical loss or magnetic loss of the dielectric or magnetic medium to convert the high-frequency interference signal into heat to achieve the filtering effect. The electromagnetic medium we fill in the filter has a weak absorption effect on low-frequency electromagnetic waves and will not cause significant attenuation of useful signals.

2. Design ideas for ultra-wideband EMI filters

　　Ultra-wideband EMI filters use the LC reflective filtering principle at the low end of the frequency and the absorption filtering principle of high-performance absorbing materials at the high end of the frequency. During the filter design process, the low-frequency end of the filter is first computer modeled based on the passband cutoff frequency, stopband insertion loss, rated current and leakage current provided by the demander, so that the required inductance and capacitance can be obtained. number and corresponding component values, and then draw the corresponding circuit diagram. Since the EMI filter only needs to meet the required cutoff frequency and insertion loss and has no special frequency response restrictions, the low-frequency end modeling uses a Chebyshev filter response with a simple circuit and fewer components, which can reduce the filter's Volume and weight.

　　The low-frequency end can only solve the frequency band below 100MHz. In the frequency band above 100MHz, due to the influence of distributed parameters such as the distributed inductance of the wires in the circuit and the distributed capacitance of the inductor coil, the performance of the LC filter circuit is degraded or even completely failed. The high-frequency end processing method is to process a section of hollow coaxial line, and fill the absorbing material with high magnetic loss and electrical loss between the inner and outer conductors of the coaxial line to attenuate the high-frequency interference signal in the propagation path. The dielectric or magnetic media filled between the inner and outer conductors of the coaxial line, such as ferrite, conductive carbon black, etc., are mostly conductors, which will cause a short circuit between the inner and outer conductors of the coaxial line. For this reason, an insulating layer needs to be added between the inner and outer conductors.

　　The LC filter circuit at the low-frequency end has good insertion loss performance in the frequency band below 100MHz. However, since the inductor coil and capacitor in the circuit are lumped parameter components, when the operating frequency reaches 100MHz, the distributed capacitance and capacitor in the inductor coil The distributed inductance in will become the dominant parameter, significantly worsening the insertion loss performance of this type of filter. At high frequencies, coaxial lines filled with absorbing materials have good insertion loss characteristics. If it is required to have good EMI suppression performance from low frequency 10kHz to microwave band 40GHz, two filters need to be used in series, thus forming an ultra-wideband EMI of reflective filtering at the low end of the frequency and absorption filtering at the high end of the frequency. Filter design ideas.

3． Application examples of ultra-wideband EMI filter design

　　Let's take the power filter as an example. Assume that the demander has the following technical requirements: passband cutoff frequency fp=1kHz, stopband starting frequency fs=10kHz, attenuation in the passband less than 3dB, attenuation in the stopband greater than 100dB, and resistance The band is to be extended to an upper frequency limit of 40GHz.

　　The low-frequency end part is processed first and Chebyshev approximation is used for modeling. The Chebyshev filter is also called an equal ripple filter. The attenuation of this filter exhibits equal fluctuation characteristics in the passband. The size of the fluctuation marks the maximum deviation of the attenuation from the ideal uniform characteristics; the attenuation in the transition band has a ratio of Butterworth filter has a faster growth rate; the attenuation in the stop band will show a monotonically increasing trend without considering the distribution parameters. According to the above technical requirements, it can be determined that the order of the Chebyshev filter is 5, and the connection methods of components are divided into two types: T type and π type. The odd-numbered components of the T-shaped circuit are inductors, and the even-numbered components are capacitors. This requires a larger number of inductors. In the actual production of the filter, the main factor that affects the volume of the filter is the size of the inductor coil. Using a T-shaped circuit can easily cause The filter is bulky and difficult to place, so most components are connected using π-type circuits. The odd-numbered components of the π-type circuit are capacitors, and the even-numbered components are inductors. The circuit diagram is shown in Figure 1.