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Characteristic impedance: also known as “characteristic impedance”, it is not a DC resistor and belongs to the concept of long line transmission. In the high frequency range, during signal transmission, a transient current will be generated between the signal line and the Plane of reference (power supply or ground plane) at the place where the signal reaches due to the establishment of electric field.
If the transmission line is isotropic, as long as the signal is transmitted, there will always be a current I, and if the output voltage of the signal is V, during the signal transmission process, the transmission line will be equivalent to a resistance of V/I. This equivalent resistance is called the characteristic impedance Z of the transmission line.
During signal transmission, if the characteristic impedance on the transmission path changes, the signal will reflect at nodes with discontinuous impedance.
The factors that affect characteristic impedance include dielectric constant, dielectric thickness, line width, and copper foil thickness.
【1】 Gradient line
Some RF devices have smaller packaging, and the SMD pad width may be as small as 12 mils, while the RF signal line width may reach over 50 mils. Gradient lines should be used to prevent sudden changes in line width. The gradient line is shown in the figure, and the transition line should not be too long.
【2】 Corner
If the RF signal line in a PCB is routed at a right angle, the effective line width at the corner will increase, and the impedance will be discontinuous, causing signal reflection. To reduce discontinuity, there are two methods to process corners: chamfering and rounding. The radius of the arc angle should be large enough, generally speaking, to ensure that R>3W. As shown on the right side of the figure.
【4】 Through-hole
PCB vias are metal cylinders plated between the top and bottom layers of a circuit board. Signal vias connect transmission lines on different layers. The residual pile through the hole is the unused part of the hole. Through hole pads are circular gaskets that connect through holes to the top or internal transmission lines. The isolation disk is a circular gap within each power supply or ground plane to prevent short circuits to the power supply and ground plane.