Power over Coax (PoC) basics
This video describes the basic concept of using power over coax to power remote devices.
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Hello. My name is TK Chin. I am a system manager, supporting FPD-Link high-speed service products from Texas Instruments. In this module, I'm going to talk about the basic concept of sharing DC power and high-speed radio data over a coaxial cable. I will also talk about the basic design elements for this scheme.
PoC is not a new concept. In this diagram, I'm showing you an example of a radio amplifier mounted on the rooftop antenna. The power is being delivered from the receiver at home through the same coaxial cable. In this example, you are seeing that the power and the RF signal from the amplifier are using the same coaxial cable. This scheme is widely used in automotive, camera module, and some other similar remote devices where power access is difficult and the use of PoC is becoming very popular.
Here is how PoC works. At DC, the inductor is a short circuit, while the capacitor is an open circuit. So the power supply at the receiving end is able to inject through the inductor into the commercial cable, and extract at the other end through an inductor, and then use it to power the local amplifier. For the AC high frequency signal path, the capacitor is low impedance, while the impedance of the inductor is very high. Because of this property, the high-speed radio signal is able to transmit from the amplifier through the coaxial cable and then go to the receiver.
The impedance of the inductor is so high, such that it appeared to be transparent and would not be able to degrade the signal. For this scheme to work properly, the PoC inductor must be designed properly to avoid degrading the high-speed signal. At the same time, the DC resistance of the inductor and the wire should be kept low enough to allow effective power distribution.
Let's take a look at the key parameters of the power of the inductor. L is the inductance value. It is chosen to support high impedance for the AC signal. Isat is the saturation current of the magnetic core. When situation occurs, the inductance values significantly dropped, and we'll no longer be able to support the high impedance that you need. So Isat must be chosen high enough to support the full DC current of the circuit.
[? RL ?] is the internal coil resistance of the coil. It is chosen to be low, such that it will not contribute too much IR drop and affect the power distribution. SRF is the serious resonant frequency, beyond which the impedance of the inductor will no longer behave like an inductor, and the [? parasitic ?] [? capacitance ?] [? dominate. ?] SRF must be chosen high enough, such that you support the operating frequency of the circuit.
Here is a summary of the PoC design goals. The power feed inductor provide sufficiently high impedance in order to avoid degrading the AC signal. The inductor must have enough high bandwidth, such that it can support the signal's operating frequencies. You should also have high saturation current, such that it is able to support the full load DC current through the system. Any surprise noise or interfering signal through the inductor must be kept low enough in order to avoid degrading the signal quality.
On the DC path, the IR drop must be kept low in order to provide effective power distribution. It means that the component must be chosen to have low internal DC resistance for the inductor as well as for the cable. To provide higher power through the system, usually higher voltage is used to deliver higher power through the system. When high voltage is being used, the AC coupling capacitor must have a corresponding high voltage rating in order to tolerate the high voltage, as well.
On this slide, I'm showing you a remote camera module implemented with TI's FPD-Link III Serializer and Deserializer [INAUDIBLE] subsystem. In this example, the video high-speed data, the control back channel, and also DC power are all sharing the same cable through the use of the PoC scheme.
For more information about Texas Instrument's FPD-Link III product portfolio, please visit our website at TI.com. You can also contact our FPD-Link support team at E2E high speed interface forum. Thank you very much for watching.
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FPD-Link learning center
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