Design Article
Combine power feed and data link via cable for remote peripherals
Tsun-kit Chin and Dac Tran, Texas Instruments
11/9/2011 10:52 PM EST
In many communication systems, remote devices are connected to main units through cable assemblies. The data and control commands are exchanged between each remote device and the main units. Very often these devices are placed in locations with limited power access. Camera modules used in automotive safety (backup cameras) and security surveillance applications are good examples of remote devices. The most convenient way to deliver power to them is through the cable assembly. This article outlines the different methods to send power through a cable assembly, and discusses an implementation to combine both power and data over a shared transmission medium.
Power feed with copper wires
Many transmission standards have incorporated the power distribution capability in the transmission medium. Universal Serial Bus (USB) is an example in which 5V is distributed through a pair of copper wires in the USB cable assembly. It is widely used for providing power to remote peripherals in PC and consumer applications. Using copper wires is the most cost-effective and robust method for delivering power to remote devices.
The figure below illustrates the use of a dedicated pair of copper wires to provide power to a remote camera, while the captured image is serialized by the DS90UB901Q and sent through a separate twisted pair to the DS90UB902Q deserializer at the main processing unit. One pair of wires is used to transport the video data and control information through the FPD (Flat Panel Display Link)-Link III serializer and deserialzer (SerDes), while the other pair is used to carry power.
Power feed with coaxial cable
Sharing a single transmission medium between signal and power eliminates the need for a second pair of wires. Often a second pair may be deemed inconvenient, or cannot be used for certain applications. The use of power and signal over a shared medium is not a new concept and has been widely used in television signal amplifiers located in roof-top antennas (see figure below).
Power is delivered by the same coaxial cable that carries the antenna signal to the TV receiver located in the house. As illustrated in the figure, DC power is injected into the coaxial cable through an inductor L1 at the source, and extracted at the destination with inductor L2. The power return path runs through the braided shield of the coaxial cable. As long as L1 and L2 provide sufficiently high impedance, they appear as an open circuit to the high frequency TV signals, and do not affect signal fidelity.
Power feed with shielded twisted pair cable
The concept of sharing power and signal over one transmission medium can be applied to high-speed serial links such as TI’s FPD-Link II/III and Channel Link II/III SerDes families. The figure below shows a remote camera implemented with the DS90UB901Q serializer and DS90UB902Q deserializer, with power and data transmitted over one shielded twisted pair cable.
The power feed is applied equally to the two wires of the differential pair through a pair of inductors at L3. Power is extracted equally from the two wires at the destination through L4. Any switching noise from the power supply is applied equally at the two wires and they appear as common-mode signals. The differential receiver of the deserializer is capable of rejecting common-mode signals and minimizing the effect from the power supply’s switching noise. The DC return path is carried by the cable’s outside braided shield.
In applications where the braided shield cannot be used as the DC return path, power and return can be applied to the two wires of the differential pair. The following figure illustrates the arrangement with a differential power feed. Any switching noise from the power supply will appear differentially and superimposed onto the high-speed differential signals. In the presence of a large amount of supply noise, this approach is more vulnerable to performance degradation due to the potential for signal-to-noise erosion.
Requirements of power feed network
The figure below illustrates a conceptual design for power and serial data over a shared data pair. The power feed inductors L3 and L4 are shunting components on both ends of the cable. They are designed to present sufficiently high AC impedance (ZDIFF) so they do not materially lower the cable impedance, and do not adversely affect its ability to transmit the differential signal. The inductors must also be able to withstand the DC current load without causing magnetic saturation, whereby the inductance collapses and significantly lowers the inductor’s impedance and negatively impacts the cable’s impedance.
L3 can be built with two inductors in separate packages. However, it is advantageous to use a pair of mutually coupled inductors with both coils wound on a common ferrite core. In the above figure, L3 and L4 are differential chokes with a center tap where the DC power is injected or extracted. The DC currents through the two windings of L3 create magnetic fluxes of opposite polarity. The magnetic fluxes cancel each other and avoid magnetic saturation. The use of mutually coupled inductors helps to reduce the physical size of the ferrite core.
Power feed with copper wires
Many transmission standards have incorporated the power distribution capability in the transmission medium. Universal Serial Bus (USB) is an example in which 5V is distributed through a pair of copper wires in the USB cable assembly. It is widely used for providing power to remote peripherals in PC and consumer applications. Using copper wires is the most cost-effective and robust method for delivering power to remote devices.
The figure below illustrates the use of a dedicated pair of copper wires to provide power to a remote camera, while the captured image is serialized by the DS90UB901Q and sent through a separate twisted pair to the DS90UB902Q deserializer at the main processing unit. One pair of wires is used to transport the video data and control information through the FPD (Flat Panel Display Link)-Link III serializer and deserialzer (SerDes), while the other pair is used to carry power.
Power feed with coaxial cable
Sharing a single transmission medium between signal and power eliminates the need for a second pair of wires. Often a second pair may be deemed inconvenient, or cannot be used for certain applications. The use of power and signal over a shared medium is not a new concept and has been widely used in television signal amplifiers located in roof-top antennas (see figure below).
Power is delivered by the same coaxial cable that carries the antenna signal to the TV receiver located in the house. As illustrated in the figure, DC power is injected into the coaxial cable through an inductor L1 at the source, and extracted at the destination with inductor L2. The power return path runs through the braided shield of the coaxial cable. As long as L1 and L2 provide sufficiently high impedance, they appear as an open circuit to the high frequency TV signals, and do not affect signal fidelity.
Power feed with shielded twisted pair cable
The concept of sharing power and signal over one transmission medium can be applied to high-speed serial links such as TI’s FPD-Link II/III and Channel Link II/III SerDes families. The figure below shows a remote camera implemented with the DS90UB901Q serializer and DS90UB902Q deserializer, with power and data transmitted over one shielded twisted pair cable.
The power feed is applied equally to the two wires of the differential pair through a pair of inductors at L3. Power is extracted equally from the two wires at the destination through L4. Any switching noise from the power supply is applied equally at the two wires and they appear as common-mode signals. The differential receiver of the deserializer is capable of rejecting common-mode signals and minimizing the effect from the power supply’s switching noise. The DC return path is carried by the cable’s outside braided shield.
In applications where the braided shield cannot be used as the DC return path, power and return can be applied to the two wires of the differential pair. The following figure illustrates the arrangement with a differential power feed. Any switching noise from the power supply will appear differentially and superimposed onto the high-speed differential signals. In the presence of a large amount of supply noise, this approach is more vulnerable to performance degradation due to the potential for signal-to-noise erosion.
Requirements of power feed network
The figure below illustrates a conceptual design for power and serial data over a shared data pair. The power feed inductors L3 and L4 are shunting components on both ends of the cable. They are designed to present sufficiently high AC impedance (ZDIFF) so they do not materially lower the cable impedance, and do not adversely affect its ability to transmit the differential signal. The inductors must also be able to withstand the DC current load without causing magnetic saturation, whereby the inductance collapses and significantly lowers the inductor’s impedance and negatively impacts the cable’s impedance.
L3 can be built with two inductors in separate packages. However, it is advantageous to use a pair of mutually coupled inductors with both coils wound on a common ferrite core. In the above figure, L3 and L4 are differential chokes with a center tap where the DC power is injected or extracted. The DC currents through the two windings of L3 create magnetic fluxes of opposite polarity. The magnetic fluxes cancel each other and avoid magnetic saturation. The use of mutually coupled inductors helps to reduce the physical size of the ferrite core.
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sharps_eng
11/12/2011 4:50 PM EST
Did I miss something or did this article overlook PoE? That is a system that already has all the details sorted, with low-cost parts here or on their way. I think it should have been mentioned (PoE Power-over-Ethernet).
I have used this SERDES technology and it it is amazingly effective and reliable. I would certainly consider it again for reducing the bulk of shorthaul multibit connections. You have to watch the balance between the diff pairs though, or serious EM emissions will result!
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tkc1
1/18/2012 2:27 PM EST
Hi Sharps_eng,
Yes, PoE has been around for many years, and is specified to ethernet uses to power remote devices. Ethernet uses signal transformer with bandwidth of about 100-200MHz to let the 10/100/1000Mbps ethernet signal passes through, and injects DC power at the center tap of the transformer. For SerDes with Gbps line rate, high bandwidth transformer is not cost-effective, and AC coupling is commonly used, and thus another scheme to inject power as shown in this paper. Yes. Balance is important, and common mode DC injection into the P and N signals is intended to keep the balance.
regards,
tsun-kit.chin@ti.com
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wfbrowniii
11/16/2011 9:11 AM EST
I used to work in the Broadcast TV radio business and designed many a everything over one coax link. DC power, on-off shift keying for a data/control channel, 70MHz if with multiplexed FM video and up to 4 audio channels and 2Gig uWave. It was far more analog than most people want to tackle now a days though.
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tkc1
1/18/2012 2:31 PM EST
Putting everything into one pipe is great, but certainly challenging to engineers to make sure all these pieces running in the same pipe does like each other. And you are right, they are all analog problems.
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zeeglen
11/16/2011 12:56 PM EST
Interesting article, but as mentioned above did not include other forms of phantom power such as used for CATV and T1.
As a lab experiment I once used the center taps of the transformers on a working Ethernet Cat3 cable as tip/ring of a POTS. Ringing voltage did not affect the Ethernet function. The major concern was no guarantee of voiceband common mode rejection between the two individual pairs, so we never pursued the idea.
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tkc1
1/18/2012 2:43 PM EST
Power over cable is not a new idea. My roof top has an amplifier that draws power through the coax cable. You mentioned another good example on the old voice-band POTS lines that power our phones with 48V from central office through voice band transformers. Same basic theory behind, but implementation depends on the speed and signal bandwidth. SerDes receivers have good CMRR to reject supply noise. Balance is the key for differential signaling in the same pair.
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