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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