Power over coax (PoC) works by transmitting both data and power over a single coaxial cable, reducing wiring, weight, and complexity for in-vehicle systems like cameras and high-definition displays. PoC can be crucial for meeting fuel efficiency standards and supporting the growing number of cameras and increasing display sizes and resolutions in modern vehicles.
Cabling simplification makes vehicles easier to produce and maintain. The use of coaxial cabling can significantly reduce the challenges associated with electromagnetic interference (EMI) in densely packed high-speed automotive communication and control systems and wiring harnesses. That leads to more reliable and consistent transmission of critical signals.
The need for high-speed connectivity in automobiles is growing with the increasing use of high-resolution radar, LIDAR ,and cameras to support advanced driver assistance systems (ADAS) and increasing autonomous driving. The latest generations of PoC can also support the need for real-time driver interfaces using standards like flat panel display (FPD) link.
PoC is such a powerful concept that it’s been embodied in numerous standards, including proprietary approaches from IC makers and open-source implementations from standards organizations. Each embodiment delivers signal transmission speeds and power delivery capabilities suited for specific applications (Table 1).
The initial introduction of some of these standards predates the development of PoC. For example, FPD-Link III and later versions can employ PoC. The Gigabit Multimedia Serial Link (GMSL) standard is another example of the emerging use of PoC. GMSL is optimized for automotive camera and sensor systems. GMSL1 doesn’t inherently support PoC. Newer and faster generations of the standard, GMSL2 and GMSL3, have integrated PoC.
SerDes and PoC
Serializer and deserializer (SerDes) interfaces are a common element in PoC implementations, like those listed in Table 1 above. The use of SerDes enables high-frequency digital signals and DC power to be superimposed on a coaxial cable.
SerDes enables the conversion of high-speed parallel digital output signals from video cameras, LIDAR, and other sources into serial data streams that can be sent over a single wire. PoC further reduces the cable count since that single cable supports both data and power. In addition, many PoC implementations use two communication channels for bidirectional transfer of digital information.
Two channels are better than one
In PoC systems, the forward and back channels use frequency division multiplexing (FDM) to carry data in different frequency ranges over the same coaxial cable that delivers DC power. FDM is used to divide the total available bandwidth into separate, non-overlapping frequency ranges for the two channels.
The forward, or downstream, channel carries the high-speed data from the camera or other sensor to the central ADAS system. Transmission frequencies are typically 50 to over 1,000 MHz and vary depending on the specific PoC implementation and application requirements.
The back, or upstream, channel is used primarily for control signals being sent to the sensor from the central ADAS system. Typical frequencies are 1 to 40 MHz.
Specialized filter circuits at both ends of the coax cable allow the equipment to distinguish between the DC power, the forward data, and the back-channel data.
Filters are a key component
In a typical PoC filter, bias tee inductors are essential components for separating DC power from high-frequency signals on a single coaxial cable. In a PoC system, these inductors block the AC signal from leaking into the power supply and, conversely, prevent power supply noise from affecting the signal.
The inductor’s ability to pass DC current while presenting high impedance to AC signals makes it a crucial element in this filtering and power injection process. Specific inductors are used at both the transmitting and receiving ends to separate the superimposed power and data signals, ensuring reliable communication without degrading signal quality.
To guarantee signal integrity, the filter solution must be capable of carrying the current to deliver power without saturating the inductor(s) or exceeding the ratings of any of the filter components.
To maintain signal integrity and a good signal-to-noise ratio, PoC filter solutions must be designed for high impedance (typically > 1 kΩ) compared to the lower (50 Ω) characteristic impedance of the coax channel. This high impedance can be maintained over the full frequency range of the PoC system using multiple cascaded inductor-capacitor (LC) stages (Figure 1).
Summary
There are multiple standards and implementations of PoC in automotive applications. All contribute to lower solution weight and improved performance, including meeting challenging fuel efficiency standards. PoC supports bidirectional communication with devices like cameras and LIDAR. Key components enabling PoC include SerDes and cascading filters.
References
Ensuring communication quality and filtering suitable for PoC, TDK
Navigating GMSL: How Pixel and Tunnel Modes Enhance System Performance, Analog Devices
PoC Filter Solutions for ADAS, Coilcraft
PoC system requirements for inductors and noise suppression, Murata
Power over Coax (PoC) basics, Texas Instruments
Power over Coaxial Cable Optimization and Signaling Trade-off, Broadcom
Power over coaxial in automotive: A simplified path to advanced vehicle systems, SAE International
Related EE World content
FAQ on cable impedance: 50 Ω versus 75 Ω
Beyond traditional filtering: how Power-over-Coax is reshaping EMI control
What are the seven variations of Camera Link Machine Vision connectivity?
How do car camera bus and FPD link compare?
FAQ on the bias tee
Leave a Reply
You must be logged in to post a comment.