1. Design and application of anti-RF interference feed-through filter

With the rapid development of RF and microwave electronic technology, the operating frequency band is becoming increasingly tight, the power of electronic equipment is getting higher and higher, and the frequency and amplitude of electromagnetic interference are also getting higher and higher. The interference frequency usually reaches hundreds of MHz, or even above GHz. Since the higher the frequency of voltage or current, the easier it is to generate radiation, it is these high-frequency and high-amplitude RF interference signals that cause the problem of radiation interference to become increasingly serious. Therefore, there is an urgent need for a filter that can have a large attenuation on high-frequency signals of radiated interference, that is, an anti-RFI feed-through filter (abbreviated as anti-RFI feed-through filter or anti-RFI filter). The principle of feed-through filter shows that this type of filter can effectively isolate signals of different frequency bands and reduce unnecessary RF interference signals entering electronic equipment. The effective filtering frequency range of ordinary anti-EMI filters is from a few kHz to MHz, while the effective filtering frequency of anti-RFI filters ranges from kHz to more than GHz. The RFI anti-interference filter designed in this paper can effectively improve the ability of the DC power supply of electronic equipment to suppress radio frequency interference, and can well suppress 100 kHz to 10 GHz radio frequency interference. It is easy to use, has a large working current, high voltage resistance, and a small size. It adopts a through-the-wall threaded installation method, which can cut off the radiation interference path inside and outside the module and effectively suppress the conducted interference.

2.Principle and working principle of feed-through filter

Anti-RFI filter is a low-pass filter, generally using π-type, T-type or T-type equivalent circuit, as shown in Figure 1. The principle of feed-through filter shows that the anti-RFI filter can effectively suppress high-frequency interference signals through a specific circuit structure. Generally, the anti-interference filter is a reflective filter, that is, the interference signal is reflected to the source instead of being conducted through the filter.

Figure 1 Equivalent circuits of anti-interference low-pass filters: (a) π type, (b) T type, (c) Γ type

The working principle of the filter is exactly the opposite of that of the filter usually used for gating signals. The filter used for gating signals hopes to be in the best matching state with the signal source to achieve efficient transmission of useful signals, while the anti-interference filter needs to be in the maximum mismatch state with the impedance of the source to block the transmission of RF signals.

According to the attenuation characteristics of the Chebyshev filter prototype, the suppression characteristics of the low-pass filter in Figure 1 are calculated:

                            (1)

                         (2)

                                                    (3)

where ω₀ is the normalized cutoff angular frequency, ω is the normalized angular frequency, LA(ω₀) is the out-of-band suppression of the low-pass filter, L_r is the attenuation value within the passband, and n is the order. For a double-terminal Chebyshev low-pass prototype filter with resistors at both ends, assuming that its passband attenuation value is L_r=0.2 dB, g₀=1,w¢ =1, the values of other components can be calculated using the following formula:

                                                                                           (4)
                                                            (5)
                                                   (6)
(7)
(8)
                                                         (9)
                                                 (10)
According to the order n=3, the passband attenuation value is 0.2 dB, |ω₀/ω|–1=10, a suppression degree greater than 60 dB can be obtained. However, this is an ideal situation, that is, all inductors and capacitors are ideal components, or they are ideal components within the frequency range of 100 kHz to 10 GHz.

The actual equivalent model of inductors and capacitors is shown in Figure 2. It can be seen that as the operating frequency increases, the inductor will show capacitance, while the capacitor will show inductance, that is, both the inductor and the capacitor have a certain self-resonant frequency. The larger the inductor and the capacitor value, the lower the self-resonant frequency, which may reach the MHz level. If the self-resonant frequency is exceeded, the characteristics of the circuit will completely change, so the expected suppression effect cannot be achieved by designing according to ideal components. It is necessary to use ideal components as much as possible to suppress extremely high interference frequencies, that is, the equivalent capacitance term C_s is mainly reduced in Figure 2a, and the inductance term L is reduced in Figure 2b. Therefore, this ideal condition must be met as much as possible in the structural design and component selection of the filter to achieve a good suppression effect.

Figure 2 High-frequency equivalent circuit: (a) Inductor equivalent model, (b) Capacitor equivalent model

3 Principle of feed-through filter and design

To obtain the ideal suppression effect, select non-capacitive inductors and non-inductive capacitors respectively. In order to meet this condition, corresponding measures are taken respectively. The following is a detailed analysis of the characteristics of inductors and capacitors, and the filter design is carried out by taking advantage of the material and structural characteristics to avoid the shortcomings.

The principle of feed-through filter shows that the filter can effectively eliminate radio frequency interference signals by accurately designing the structure and materials of its inductor and capacitor elements.

3.1 Inductor analysis and design

For the inductor equivalent model of Figure 2a, the equivalent capacitance of the inductor is composed of the inter-turn capacitance, inter-layer capacitance and lead capacitance of the winding. Therefore, to reduce the equivalent capacitance, the number of turns and layers of the winding needs to be reduced. Then the minimum number of turns is 1 turn and a single layer; does it mean that there is no equivalent capacitance after reducing it to 1 turn? Of course not, because the input and output pins will have electric field coupling, which will also form the equivalent capacitance in Figure 2a. To eliminate this coupling capacitance, it is necessary to avoid the direct spatial electric field coupling between the input and output leads of the inductor. Therefore, we use the coaxial inductor structure of Figure 3, and the shell of the coaxial inductor is well grounded, so that the electric field coupling path of the input and output leads of the inductor can be isolated to eliminate the coupling capacitance formed by the input and output leads of the inductor. This design complies with the principle of feed-through filter, which can effectively isolate signals of different frequency bands and reduce unnecessary RF interference signals.

Figure 3 Inductor structure

By taking this measure, the equivalent capacitance of the inductor is basically eliminated. Looking at R₁ in Figure 2a, R₁ is composed of the magnetic loss and copper loss of the inductor. Since the magnetic loss varies with the frequency, it is equivalent to a variable RF resistor, but R₁ is in the mΩ range under DC and low frequency, which can be almost ignored; under RF, the resistance will reach tens of ohms or even hundreds of ohms. The equivalent resistor absorbs and consumes the RF signal in the form of heat, and will not generate reflection, avoiding the self-excited oscillation of the RF circuit and making the circuit work stably, which is required to suppress interference signals. The conduction path of interference signals can be blocked by reflection and absorption. Inductance and capacitance reflect interference signals, and resistance consumes and absorbs interference signals. Therefore, R₁ is beneficial to the interference suppression of the filter and does not need to be eliminated. Instead, it should be increased. Together with the inductance L, it forms the series impedance term of the circuit, that is,Z=R+jX=ωL₀µ＂+jωL0µ＇. The inductance term in Figure 1 is replaced by the series impedance term and simulated. The results show that except for the overall improvement of suppression, the suppression effect of the filter has no adverse effect (as shown in Figure 6).

Therefore, when selecting inductor materials, it is necessary to select materials with the highest possible magnetic permeability and magnetic loss. The complex magnetic permeability of magnetic materials is composed of µ＇ and µ＂ (i.e.µ=µ＇–jµ＂).µ＇ provides inductance, and µ＂ produces high-frequency absorption, which is equivalent to high-frequency resistance. Therefore, high µ＇ can increase L in Figure 2a, and high µ＂ can increase R₁ at high frequencies in Figure 2a, which also increases high-frequency series impedance and improves interference suppression capabilities. Using appropriate materials can usually increase the suppression degree of shortwave to centimeter wave bands by more than 10 dB.

Due to the use of the inductor structure of Figure 3, it is also necessary to pay attention to the withstand voltage and insulation issues when selecting the inductor material. After all, the safety of the circuit is the primary issue, so NiZn material with high surface resistance is selected, and NiZnCu soft magnetic material has a magnetic permeability of 1000~3000, a surface resistance of 10⁸Ω, and a broadband µ＂characteristic, which is the preferred material for anti-RFI filters.

3.2 Capacitor analysis and selection

According to the equivalent circuit of Figure 2b, to effectively reduce or even eliminate L, the capacitor can work at a higher frequency. Because any pin will introduce high-frequency inductance, if the high-frequency inductance can be eliminated in conjunction with the structural design of the device, the purpose of reducing L can be achieved. In terms of structure, consider using a circular tube capacitor. The inner and outer walls of the circular tube capacitor form the two electrodes of the capacitor. If the electrodes of the outer wall are directly welded to the shell of Figure 3, then L in the equivalent circuit of Figure 2a is completely eliminated,

The capacitor can work at an extremely high frequency. R₁ and R_e in Figure 2b belong to the loss of capacitors, which will not affect the suppression of interference signals in the anti-interference filter.

In order to improve the rectangularity of the filter, we use the circuit of Figure 1a, that is, two capacitors and an inductor, and the capacitor uses a tubular π-type dual capacitor structure, as shown in Figure 4.

Figure 4 Circular Tube π-Type Dual Capacitor

3.3 Filter Design

Combining the inductor of Figure 3 with the tubular dual capacitor of Figure 4 forms the the π-type anti-RFI principle of feed-through filter, and the appearance is shown in Figure 5. The feed-through filter principle utilizes the synergistic effect of inductance and capacitance, and can effectively suppress radio frequency interference signals through precise structural design.

Figure 5 Filter Appearance Structure

4 Simulation Analysis and Experimental Verification: Design Example

The following is an anti-RFI filter required by the user, and the specific parameter index requirements are shown in Table 1.

Table 1 Design example index requirements

According to the low-pass prototype in Figure 1, Serenade 8.7 is used for simulation analysis, taking C=2×7.5 nF, L=0.5 μH, and the results are shown in Figure 6. At f=100 kHz, the suppression IL＞5 dB, at f=5 MHz, the suppression IL＞20 dB; at f=50 MHz, the suppression IL＞60 dB. In Figure 6, the upper curve is the simulation result based on the ideal inductor element, and the lower curve is the simulation result after adding resistance to the inductor element, that is, after adding the loss parameter, the suppression at each frequency point will increase, which is exactly what we want.

Figure 6 π-type filter simulation curve

According to the above parameters, a tubular π-type dual capacitor C≥2×7.5 nF is selected, the size is ∅3×∅2.5×10 mm, the magnetic core is NiZnCu material, the magnetic permeability is 1500, the size is ∅2.4× ∅1.1×9 mm, the center conductor is ∅1 mm, the installation thread is M6, and the sample is made as shown in Figure 5. The test results of the sample no-load insertion loss are shown in Table 2.

Table 2 No-load insertion loss (suppression degree) of the filter

The test results show that the frequency of 100 kHz~100 MHz is highly consistent with the simulation. The test is divided into two cases: In the first case, the filter is directly connected to the input and output for testing (unshielded test), and the actual test value above 100 MHz is smaller than the simulation value. In order to find the cause, we disconnected the input and output ends of the test fixture for testing. The isolation of the input and output ends of the fixture is basically consistent with the suppression of the installed filter, proving that the isolation of the test fixture is not high, that is, the decrease in the filter suppression at high frequencies is caused by the electromagnetic radiation of the input and output ends of the test fixture. In the second case (tested by the shielded cavity), we improved the test fixture, shielded the input and output ends through the cavity, cut off the electromagnetic coupling of the input and output ends, and the effect changed greatly. The suppression at the high frequency end was greatly improved. The test results are shown in Table 2.

The results in Table 2 show that the anti-RFI filter designed in this paper can effectively suppress RF conducted interference and achieve the expected design effect. According to GJB1518 “General Specification for Radio Frequency Interference Filters”, the identification and inspection were carried out and the inspection was qualified. The comparison results of the two test methods also remind us how to use the anti-RFI filter correctly, that is, to play the role of the anti-RFI feedthrough filter most effectively, the filter must be installed on the cavity of the device to shield the input and output ends, so that the RF interference signal can pass through the filter and be suppressed. Therefore, when using the filter, it should be installed in accordance with Figure 7.

Figure 7 Installation and use of the filter

This article introduces the component characteristics and design process of the π-type anti-RFI feed-through filter in detail, and explains the realization method of the equivalent capacitance and inductance of the ideal inductor and capacitor. The principle of feed-through filter shows that the structural design of the filter must be a coaxial structure (i.e., feed-through) to achieve effective RF interference signal suppression effect. Through the analysis of the characteristics of materials and components, it is found that the anti-RFI feed-through filter needs to select magnetic materials with high magnetic permeability, large high-frequency loss, and high surface resistance, and needs to select disc through-core capacitors or tubular through-core capacitors with good high-frequency characteristics, and the structure of the filter is designed to be coaxial (i.e., feed-through) to achieve good RF interference signal suppression effect. The LC202-1WB1EE3 anti-RFI feed-through filter designed according to GJB1518 “General Specification for Radio Frequency Interference Filters” has passed the appraisal inspection and can effectively suppress various RF interferences in the range of 100 kHz to 10 GHz, and can be widely used in various RF and microwave amplification circuit modules.