A Study on the EMI Effect of Impedance

Mismathcing for LVDS Single Transmission Line

in Vehicle Camera System

Byeongchan Jo1, Kibum Jung1 1E&R Co., Ltd, A-304, 387, Simin-daero, Dongan-gu, Anyang-si, Gyeonggi-do, Republic of Korea

Abstract – LVDS(low-voltage differential signaling) is

transmission interface used in serial data transmission. We measured the TDR of LVDS transmission line using FPCB and RF connector and compared the radiated emission level of

vehicle camera system applied FPCB and RF connector for LVDS transmission line. As a result, we found that the impedance matching of transmission line is very important for

the radiated emission level.

Index Terms — LVDS, impedance matching, vehicle camera system, radiated emission.

1. Introduction

LVDS(low-voltage differential signaling) that introduced

in the mid-1990s, is the most popular differential data

transmission standard in a various industry fields which

because LVDS has many other benefits. For example, it

operates at low power, is able to run at very high speeds,

generates low EMI noise and removes high common mode

noise [1]. For all of the above reasons, LVDS standard has

been applied in various fields. Recently, because LVDS

signal can transfer to single ended transmission line as well

as differential transmission line, its application field is

becoming more and more diverse.

In the automotive industry, a research is underway to

change a low-resolution analog camera imaging system to a

high-quality digital camera imaging system using LVDS or

Ethernet interface.

In this paper, we measure the impedance of a single

transmission line for an automotive camera imaging system

using LVDS interface. Also we measure and analyze the

effect of impedance mismatch on radiated emissions.

2. Configuration of LVDS system

(1) Configuration of LVDS transmission lines

LVDS is transmission interface widely used in serial data

transmission. Generally, LVDS transmission lines uses

differential pair matched with differential impedance of

100Ω [2]. Differential pair transmission lines achieve low

EMI noise due to high common mode noise rejection and

have strong immunity against external noise [3].

In recent years, as the main signal line to LVDS

transmission line uses 50Ω coaxial cable and the return path

line of signal terminates 50Ω resistor to ground plane of PCB,

LVDS signal can transfer to single ended transmission line.

Fig. 1 shows that two configurations of LVDS transmission

line.

Fig. 1. Scheme of LVDS transmission line

(2) Configuration of vehicle camera system

Fig. 2 shows LVDS transmission line configuration in

vehicle camera system.

Fig. 2. Example of LVDS transmission line configuration for

vehicle camera system

In case 1, LVDS signal line is connected by soldering for

the inner conductor of coaxial cable and FPCB signal line.

Also, the shielded mesh wire of coaxial cable connected by

soldering to FPCB ground line. Therefore, the shield opening

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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length of coaxial cable by soldering is about 3.1mm. In case

2, LVDS signal and ground line are directly connected to the

inner conductor and housing of RF connector. As in case 1

and 2, the differences in the connection method of the LVDS

transmission lines cause the impedance difference of the

LVDS signal line.

3. Measurement results

(1) TDR measurement

A time-domain reflectometer (TDR) [4] measures the

reflection of transmission lines such as PCB traces, cables,

and connectors. The reflection is caused by impedance

mismatch of transmission lines. If the impedance of

transmission line is constant and an end of the line is

properly terminated, there will be no reflections.

Fig. 3 shows the configuration for measuring the TDR of

an LVDS signal line consisting of a single transmission line.

Fig. 3. TDR measurement configuration of LVDS

transmission line

Fig. 4 shows the TDR measurement result for case 1 and 2

of Fig. 3.

Fig. 4. TDR measurement result

In case 1 of Fig. 4, as the inner conductor of coaxial cable

is soldered to the FPCB, impedance mismatch occurs

severely. On the other hand, in case 2 of Fig. 4, impedance

mismatching is minimized as compared to case 1 because the

coaxial cable connected to the RF connector.

(2) Radiated emission measurement

Radiated emissions measurement of vehicle camera

system was performed by the ALSE method of CISPR 25 [5].

Fig. 5 shows the radiated emission measurement result. In

Fig. 5, it can be found that the radiated noise level is more

than 20 dB difference due to the impedance mismatch to the

LVDS signal transmission line.

Fig. 5. Radiated emission measurement result

4. Conclusion

In this paper, we measured the TDR of LVDS signal line

using a single transmission line and compared the radiated

emissions level of vehicle camera system applied a single

transmission line for LVDS interface. As a result, we found

that the impedance matching of LVDS transmission line is

very important. Therefore, when applying LVDS system to

vehicle camera system, the impedance matching of

transmission line must be fully considered in order to reduce

the radiated emission level.

References

[1] Electrical characteristics of low-voltage differential signaling (LVDS)

interface circuit, TIA/EIA-644, National Semiconductor Corp., ANSI/TIA/EIA, 1996.

[2] LVDS owner’s manual 4rd edition – low voltage differential signaling,

National Semiconductor Corp., 2008. [3] H. Johnson, M. Graham, “High-speed signal propagation advanced

black magic,” Prentice-Hall 2002.

[4] Robert H. Cole, “Time domain reflectometry,” Annual review of physical chemistry 28:283-300, 1977.

[5] CISPR 25 Ed. 4.0, “Vehicles, boats and internal combustion engines-

radio disturbance characteristic-limits and methods of measurements for the protection of on-board receivers,” 2016.

2018 International Symposium on Antennas and Propagation (ISAP 2018)October 23~26, 2018 / Paradise Hotel Busan, Busan, Korea

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