The conducted interference between electronic devices on an aircraft is mainly caused by coupling of common ground impedance. This article analyzes the effect of the impedance of the ground plane on electromagnetic interference, and proposes relatively simple and very effective measures to reduce ground loop interference, including the interference caused by minimizing the feedthrough capacitors minimize coupling path through.

In electronic systems and electronic equipment, the currents of several devices and circuit units often flow through a common ground line or ground plane. When these currents flow through the ground plane, a voltage drop is generated, causing mutual influence between devices. On an aircraft, this common ground plane is the basic structure of the aircraft. In the case of DC, the DC resistance of the aluminum alloy ground plane is very small, but as the frequency increases, the skin effect will increase this impedance, and high-frequency impedance is an important factor causing electromagnetic interference. Especially today, replacing some backward materials with advanced composite materials has caused major technological changes in aircraft design. However, composite materials have large RF impedance and poor conductivity, which will directly affect the expanded application of composite materials on aircraft. Therefore, how to reduce the electromagnetic interference caused by ground impedance is a problem of great concern to electromagnetic compatibility designers. However, correctly understanding the relationship between material impedance and electromagnetic interference is a very important issue for aircraft electromagnetic compatibility design.

1 Ground plane impedance

The impedance of the ground plane is not only related to the frequency, but also to the thickness, relative conductivity and relative magnetic permeability of the material. The DC resistance of the metal ground plane can be calculated by formula (1):

                                                                     (1)

Where R_DC-DC resistance per unit area, μΩ/sq;

t–thickness of the metal ground plane, mm;

σr-conductivity of the metal ground plane relative to copper.

The RF impedance of the metal ground plane can be calculated by formula (2):

                                                        (2)

Where R_RF-RF impedance per unit area, μΩ/sq;

ƒ-frequency, MHz;

μr–magnetic permeability of the metal ground plane relative to copper;

δ–material skin depth, mm.

The skin depth of any metal can be calculated by formula (3):

                                                           (3)

On the grounding surface of a metal object, within any frequency range, the ground impedance Z_g between any two points can be calculated by formula (4)

                           （4）

Where d–the distance between the two points on the grounding surface;

λ–wavelength;

b–the width of the grounding surface.

If the width b of the grounding surface is greater than the distance between the two points, and the distance d<0.05λ, then the ground impedance Z_g between the two points can be approximately calculated by formula (5):

                                                     (5)

For ease of calculation, the impedance values of the copper grounding surface are listed in Table 1 according to formula (5).

As for other metal ground planes, the ground impedance Z_g within the RF range can be easily calculated by formula (6):

                    (6)

Table 1 Copper ground plane impedance

In actual circuit design, the reasonable application of feedthrough capacitors minimize coupling technology can effectively reduce the common mode interference caused by ground plane impedance and improve the electromagnetic compatibility of the system.

2 Generation of ground current

The current flowing through the ground plane includes the current on the neutral line of the power supply and the ground current of the grounded load. As shown in Figure 1, part of the interference signal does not flow back through the predetermined conductor DA, but flows back through the ground plane CFE. This is the interference signal generated by the interference source forcing a common mode current to flow on the ground plane. Let’s calculate the size of the ground current 1_g.

(Example 1) If a video amplifier outputs a pulse signal, the signal source is grounded, the voltage is 40V at a frequency of 10MHz, and the load impedance is 50Ω. The wire inductance is L₁=L₂=0.35μH, the parasitic capacitance from the circuit to the ground is C₄=2pF, the load circuit board is floating, and the parasitic capacitance from it to the ground is C₅=10pF. If the ground plane is a 1mm thick aluminum plate, what is the common mode voltage generated?

The wire inductance is 0.35μH, corresponding to an inductive reactance of 21Ω at 10MHz, then the current I₁≈I₂≈I₃≈0.6 A.

I_g≈11mA

Looking up Table 1, the impedance of a 1mm thick copper ground plane at 10MHz is 1.16mΩ. Substituting μr=1,σr=0.6 of aluminum into equation (6), we can get the impedance of the aluminum ground plane as 1.5mΩ, so the common-mode interference voltage generated is 17μV.

If the circuit board is grounded, 1_g≈0.7A, then the common-mode interference voltage generated is 1mV.

In actual electromagnetic compatibility design, by adopting the feedthrough capacitors minimize coupling technology, the flow of common-mode current on the ground plane can be effectively reduced, and the common-mode interference voltage generated by the ground current can be reduced, thereby improving the system’s anti-interference ability.

3 Analysis of common ground impedance coupling

Common ground impedance coupling is an important way of electromagnetic coupling. The following is a brief analysis of this issue. As shown in Figure 2, the ground current 1_g flows through the ground plane to generate a common mode voltage U_i. This voltage acts as an electromagnetic interference source, causing interference current to flow in the loops ABCDEFCHA and ABCIJFGHA. If the circuit is grounded, that is, the signal reference point and the load reference point are connected to their respective housings, the voltage U_i appears between the two grounding points. This voltage causes current sheets I₁ and I₂ in the two signal cables respectively. However, the impedances through which I₁ and I₂ flow are different, so the differential mode voltage U_o is caused at the load input due to imbalance.

Figure 1 Formation of ground current

Figure 2 Common ground impedance coupling

Two devices are installed on the aircraft, and the current flowing through these common impedances can become an independent signal.

The voltage U_i formed by the interference current on the common ground impedance Z_g is partially coupled to the input end of the victim receiver through the ground loop path. We define the ratio of the voltage U_o generated at the input end of the victim circuit to the interference voltage U_i generated on the common ground impedance as ground loop interference. The ratio of U_o to U_i is related to many factors, such as the diameter and length of the connecting wire, circuit impedance, circuit structure and grounding condition of the circuit. In an unbalanced system, when the reference points of the signal and the load are both grounded, U_i is divided into two parts by the signal source impedance and the load impedance. Under the condition that the signal source impedance and the load impedance are the same, the voltage on each impedance is U_i/2. Therefore, at low frequencies, half of the common mode interference voltage is coupled to the input end of the victim receiver. At this time, if the interference frequency is the same as the operating frequency of the sensitive equipment, it will cause serious interference.

The design method of feedthrough capacitors minimize coupling can effectively reduce the common-mode interference caused by common ground impedance coupling, reduce the generation of differential-mode voltage, and fundamentally improve the anti-interference performance of the system.

4 Frequency response characteristics of sensitive equipment

The frequency response usually has a suppressive effect on the interference effect, unless the interference falls entirely within the passband. In fact, most electromagnetic interference problems occur in the cutoff band of the impedance end. Therefore, the cutoff band performance of the victim amplifier is very important. In many applications, the cutoff band performance of the amplifier is similar to that of an ordinary filter. When the cutoff frequency is exceeded, the gain of the amplifier begins to decrease. Short-circuit stray capacitance is the main reason for the reduction in cutoff band performance. The relative gain G of the amplifier in the cutoff band can be calculated using formula (7):

                              (7)

Where f_I–interference impedance frequency;

f_C–amplifier cutoff frequency:

n–amplifier number.

By feedthrough capacitors minimize coupling the reasonable design, high-frequency interference caused by common ground impedance coupling of sensitive equipment can be effectively suppressed, thereby improving the frequency response characteristics of the amplifier and reducing the overall interference level of the system.