Research on the Coupling Effect of Communication Display Interface under Strong Electromagnetic Pulse

In the high-altitude electromagnetic pulse (HEMP) environment, high-intensity broadband electromagnetic pulse energy is transmitted to the display interface circuit through the interconnection cable, causing interference or damage to the internal electronic components and chips, resulting in various damage effects such as the display cannot be displayed normally, and even restart. To investigate the coupling effect of strong electromagnetic pulses on the display interface circuit, the existing field-line coupling model is utilized as the basis of this study, with a specific focus on the VGA interface. The influence of different terminal impedances on the coupling effect is examined, aiming to determine the significance of terminal impedances in the simulation estimation of interface circuit coupling response. A HEMP coupling signal loop model is established based on the signal loop defined in the VGA datasheet, replicating the normal working conditions of VGA. HEMP pulse injection experiments are conducted to compare the response waveforms of the simulated VGA interface equivalent impedance circuit model with the measured data. This allows the accuracy of the established equivalent impedance model to be verified using key parameters.


Introduction
In recent years, with the development of pulse weapons, electronic devices have been increasingly exposed to complex electromagnetic effects.When a computer system is subjected to strong electromagnetic pulse (EMP) effects, the interconnecting cables between computers conduct the highintensity, wide-bandwidth electromagnetic pulse energy released by the EMP to the directly connected secondary interface circuits [1].Among the various interfaces of a computer, the display interface is particularly vulnerable to interference and exhibits prominent phenomena.The strong EMP-generated electromagnetic interference often leads to various destructive effects, such as computer blackouts or even restarts [2].To ensure the smooth and normal operation of the computer system under strong EMPs without impacting other interfaces, it is necessary to first simulate and estimate the electromagnetic pulse coupling response of the display interface circuit and determine its damage threshold.Then, the interface needs to be reinforced and protected.
A series of research studies on the electromagnetic interference and protection of video interfaces have been conducted by many scholars internationally [3][4][5].Aiming at the simulation prediction of electromagnetic pulse coupling response of interface circuits, the current field-line coupling simulation model is mainly established in simulation software to obtain the coupling current of electromagnetic pulse radiation field to communication transmission cable.In the simulation model, the transmission line simulates a computer interconnection cable [6].However, due to the lack of corresponding analysis and modeling for the coupling signal loop and the equivalent impedance of the loop of the internal interface circuit of the device, it is difficult to predict the electromagnetic pulse coupling current through simulation [7].Through the simulation of the field-line coupling model, the coupling current of the electromagnetic pulse radiation field to the communication transmission cable is obtained.However, it is found that the end impedance has a certain influence on the coupling current.If the value is incorrect, there will be a deviation between the simulation result and the real value [8].Therefore, it is necessary to analyze the coupling mechanism of the electromagnetic pulse to the interface circuit based on establishing the normal signal circuit of the interface circuit.
Common display interfaces such as VGA, HDMI, DVI, and others are known to have differences in performance, transmission signal format, resolution, and other aspects [9].In this paper, the VGA interface is chosen as the research subject, and a methodical study is conducted on the coupling response of the VGA interface under the HEMP pulse.By analyzing the coupled signal loop and equivalent impedance of the VGA interface circuit under HEMP, the HEMP coupled response of the interface circuit is estimated through simulation.Furthermore, the real coupling waveform is obtained by constructing an electromagnetic pulse effect test platform for the interface circuit, and the accuracy of the simulation model is validated.

Modeling coupling loop of display interface in HEMP
After the detonation of a nuclear weapon at a high altitude, an extremely intense high-altitude electromagnetic pulse (HEMP) radiation field is generated in space.This radiation field propagates to computers and their nearby cables mainly through field-wire coupling, entering the interface circuitry via the cables connected to the subsequent stages of the interface board.By establishing a HEMP fieldwire coupling model in CST simulation software, it was found that the magnitude of HEMP coupling current is influenced not only by factors such as cable length and cable height above ground [10].The magnitude of the terminating impedance at both ends of the cable also significantly affects the accuracy of the simulated estimation of the HEMP coupling current, as shown in Figure 1.As the terminating impedance at both ends of the transmission line increases, the peak value of the HEMP coupling current decreases.The normal operating current of the interface circuit is in the range of several milliamperes (mA), and a current variation on the order of amperes can cause interference or even damage to the interface circuit.Therefore, it is essential to study the HEMP coupling signal loop and loop impedance to improve the accuracy of the simulated estimation of HEMP coupling current and enhance the precision of equipment HEMP protection design.
Before the HEMP-coupled signal loop is established, it is necessary to have the terminating impedance of the VGA interface's normal signal loop determined.The impedance Z L between the signal receiving end and ground is determined as 1.2 kΩ, and the impedance Z S between the signal transmitting end and ground when the interface chip is operating normally is specified as Z S ≥ 6.8 kΩ, as per the data manual [11].
The HEMP-coupled signal loop under VGA coupling is comprised of the signal driving source and its internal impedance in the interface circuit, the parasitic impedance between the microstrip line and ground, the VGA signal line, the internal impedance of the chip receiver, and the chassis ground.The parasitic impedance between the microstrip line and ground in the interface circuit primarily manifests as distributed capacitance to ground, with a magnitude of about tens of pF.As the rising edge of the HEMP-coupled current signal is in the ns range and the spectral component is mainly distributed within 200MHz, the high-frequency impedance of the microstrip line to ground distributed capacitance becomes significantly smaller than the terminating impedance of the normal signal loop due to the highfrequency component.Consequently, the HEMP-coupled current signal is predominantly discharged along the loop composed of the signal line, parasitic capacitance, and ground, while only a small portion of the current is discharged through the normal signal loop consisting of the chip internal impedance and ground [12].The HEMP-coupled current signal loop model of the VGA interface circuit is depicted in Figure 2.However, in the research process, the specific PCB layout of the interface circuit board is typically unavailable, and the simulation software cannot provide these parasitic parameters.Therefore, it is necessary to combine the structure of the HEMP-coupled loop equivalent model with simulation modeling and experimental measurement to obtain the actual HEMP-coupled loop equivalent impedance of the interface circuit.

VGA interface equivalent circuit modeling
The impedance characteristics of the VGA interface need to be determined by establishing a coupled signal loop model for the VGA interface circuit based on the previous text and by establishing an equivalent impedance circuit model through time-domain simulation.The impedance parameters need to be obtained by building an experimental device and conducting HEMP direct injection tests while ensuring the normal operation of the interface circuit board.The response current at the front end of the interface circuit board needs to be obtained.First, the corresponding equivalent circuit model of the HEMP injection source needs to be established, and it needs to be combined with the established equivalent model of the HEMP coupled loop to complete the joint simulation.

HEMP injection source simulation modeling
Based on the actual internal circuit of the HEMP pulse source, the equivalent circuit model of the HEMP pulse injection source is established as shown in Figure 3.In this model, the charging and discharging capacitor is represented by C, the stray inductance generated by the internal wiring and switches of the pulse injection source is represented by L, and the internal resistance of the pulse injection source is represented by R [13].
The HEMP pulse source circuit calibration test is conducted, as shown in Figure 4.The response current of the HEMP pulse injection source is directly injected into a short circuit, 50 Ω, and 10 Ω loads using a current clamp.The peak time and pulse width of the response current waveform saved by the oscilloscope are measured.
The values of the device parameters in the equivalent circuit of the HEMP pulse injection source, including capacitance C = 14 nF, resistance R = 52 Ω, and inductance L = 450 nH, are determined by gradually adjusting them to approach the parameters of the simulated response waveform and the test response waveform.The parameter comparison table of the simulated response waveform and the test response waveform is presented in Table 1.In the table, the first row of each set is the actual measurement, and the second row is the simulation.It can be observed from the measured and simulated results table that the simulation model and parameter settings of the HEMP injection source have been accurately performed and meet the required specifications.

Impedance characteristic injection test of the interface circuit
As shown in Figure 5, a direct injection experiment was conducted on the VGA interface loaded on the computer graphics card.The pulse current was directly injected from the data transmission core wire of the VGA cable, and the response current on the sending core wire of the card was monitored through an oscilloscope and a clamp meter.The response waveform parameters obtained from the direct injection experiment on the VGA interface card were measured, with a clamp factor of 1 and a 20 dB attenuator connected to the front end of the oscilloscope.
An impedance equivalent circuit model is constructed by combining the established VGA interface circuit HEMP coupling loop model.In this model, the capacitance is used for the capacitance of the microstrip line to ground in the equivalent interface transmission loop, and the inductor and the resistance in series are used for the impedance of the equivalent post-stage, which is composed of the real part and the imaginary part.However, the final equivalent impedance circuit model requires continuous adjustment of the distributed capacitance and the impedance of the subsequent shunt circuit.The overall process of simulating the impedance equivalent circuit parameters of the VGA interface is similar to the simulation of the pulse injection source circuit parameters.
Finally, the equivalent impedance circuit of the VGA interface is obtained as shown in Figure 6, with a distributed capacitance C = 13 pF, resistance R = 4 Ω, and inductance L = 775 nH.The comparison of simulation and measured parameters is shown in Table 2.In the table, the first row of each set is the actual measurement, and the second row is the simulation.By comparing the simulation waveform parameters and the test waveform parameters from the above table, we can observe that the simulation results match very well with the measurements of peak time, pulse width, and rise time.This indicates that the impedance equivalent circuit model of the VGA interface is accurate and satisfies the required specifications.

Conclusions
In this study, a simulation estimation of the HEMP coupling response of the interface circuit is provided, and the acquisition of the accurate failure threshold of the interface circuit is very crucial for protection.
The HEMP coupling signal loop model of the VGA interface circuit is proposed through theoretical analysis, model simulation, and practical experiments in this article.The equivalent impedance model of the interface is established based on the experimental data, which verifies the accuracy of the model to a certain extent.Additionally, the importance of establishing an equivalent impedance model for studying the coupling effect of interface circuits under strong electromagnetic pulses is demonstrated to some extent.In subsequent research, higher-level injections will be carried out to obtain the damage threshold of the VGA interface.The interface equivalent model not only helps us to study the coupling effect under a strong electromagnetic pulse but also provides help for the subsequent protection circuit design at the simulation level.

Figure 1 .Figure 2 .
Figure 1.Simulation results of coupling current in transmission lines with different terminal impedances under HEMP irradiation

Table 1 .
Comparison of actual measurement and simulation results of HEMP injection source

Table 2 .
VGA interface card actual test and simulation result parameter comparison tableInjecting voltage (V) Peak value (A)