12 Layers High Impedance PCB Board
The circuit lines on PCB board can be equivalent to the capacitance, resistance and inductance connection structure in series and parallel.The typical value of series resistance is 0.25-0.55 ohms/foot, which is usually very high when parallel due to the insulation layer. When parasitic resistance, capacitance, and inductance are added to the actual PCB connection, the final impedance on the connection is called characteristic impedance Zo.The wider the wire diameter, the closer it is to power source/ground, or the higher the dielectric constant of the isolation layer, the smaller the characteristic impedance.If the impedance of the transmission line does not match the impedance of the receiving end, the output current signal will be different from the final stable state of the signal, which will cause the signal to be reflected at the receiving end, which will return to the transmitting end and reflect back again.
As the energy diminishes, the amplitude of the reflected signal decreases until the voltage and current of the signal becomes stable.This effect is called oscillations, and the oscillations of the signal are often seen on the rising and falling edges of the signal.
What is the transmission line effect?
Based on the transmission line model defined above, it can be concluded that the transmission line will bring the following effects to the whole circuit design.
· reflection signal
· Delay & Timing errors
· False Switching
· Overshoot and Undershoot
· Induced Noise (or crosstalk)
· EMI radiation
Generally, transmission line effect refers to the distortion of various signals, such as signal reflection, interference, ringing effect, antenna effect, attenuation and superposition, that occurs during the transmission of high-frequency electromagnetic wave in conductive medium (such as circuits in PCB board, communication cable, etc.).At present the most discussed signal distortion is caused by the circuit impedance mismatch.