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5.Challenges of the power supply and ground noise
Due to the fact that the power supply and ground plane serve as the reference plane and return path for signal lines, the noise from power supply and ground will directly affect the signal. Avoiding the effect of power supply and ground noise on the signal plays an important role in stabilizing the electrical level of power supply, and ensuring the reliability of high-speed signal.
To design the power supply for high-speed PCB, we have to be aware of the power supply tree and analyze the reasonableness of channels for power supply.
First of all, if the trace have to carry a large current, we should set aside a surplus before allotting a width for trace laying. In addition, due to the fact that the resistance exists in the trace, there will be a voltage drop at the trace from the output terminal of power supply to the actual load. But the voltage of high-speed circuit devices, especially Core voltage, is very low generally, so the voltage drop will have a direct effect on the power supply. The capacity of carrying current is related to the width of trace, inner and outer layers, thickness of copper and the allowable temperature rise.
Second, consideration must be given to the impedance of power supply so as to ensure the filtering for power supply. The channels for power supply have resistance and impedance, and the momentary power supply need to be supplied for the high-speed circuit when it is switched at the gate. So it will take some time for the electric current to provide energy for the high-speed circuit being switched at each circuit gate step by step, which can be thought of as a course of multi-step electric charging. 
Drawing 6: Routes for power supply
As shown in the drawing, in the state of high frequency, the current at the pins of device is first supplied by the plate-type capacitor exposed of a power supply and ground plane because the power supply system consisting of them has the lowest impedance and a fastest power supply. But, this plate-type capacitor stores a very small electric quantity that is supplied by small capacitors of which the electric charges are provided by a big energy storage BULK capacitor. Then the switching power supply will charge BULK capacitor via electric current channels. The reasons for the above are that the switching power supply is of low impedance only when the frequency is thousands of Hz; the BULK capacitor is of low impedance only when the frequency is of MHz; the small capacitors are of low impedance only when the frequency is between scores of and hundreds of MHz. So, only by multi-step electric charging can the electric current reach the pins of device to meet the needs for momentary power supply. Furthermore, Cadence provides a PI analysis module used to analyze the impedance of power supply at different power consumption and check whether the selection of capacitors is proper or not.
The theoretical basis for PI simulation tool refers to that the transmission line is used to divide the power supply level in the form of limited elements, and make the power supply and the relevant ground plane into a pair of plate-type capacitors, which are divided into several portions as shown in the drawing:
After frequency domain analysis, the tool is used to analyze the impedance of each small block on the plate-type capacitor and achieve the impedance drawings for each point finally.
If the impedance of some point exceeds the target impedance, the power supply level needs to be re-distributed, or some filtering capacitors need to be added to reduce the target impedance of the point and enhance the ability to filter the waves at the pins of devices.
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08-07-21
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