We all understand the importance of maintaining continuous impedance in PCB design. However, there are occasions when the impedance of the PCB board cannot be continuous. In such cases, what should be done?
Characteristic impedance, also known as “characteristic impedance,” is not simply a DC resistance, but rather a concept related to long-term transmission. In the high frequency range, a momentary current is generated between the signal line and the reference plane (power or ground plane) when a signal edge arrives during the signal transmission process due to the establishment of an electric field. If the transmission line is isotropic, there will always be a current I as long as the signal is transmitting. If the output voltage of the signal is V, the transmission line will be equivalent to a resistance, with the size V/I, and this equivalent resistance is known as the characteristic impedance Z of the transmission line. Any changes in the characteristic impedance of the transmission path during the signal transmission process will cause signal reflection at the node with discontinuous impedance. The factors that affect the characteristic impedance include dielectric constant, dielectric thickness, line width, and copper foil thickness.
(1) Gradient line
Some RF device packages are small, with the SMD pad width as small as 12mils, and the RF signal line width sometimes exceeding 50mils. Gradient lines are used, and sudden changes in line width are prohibited. The gradient line is shown in the picture, and the line in the transition part should not be too long.
(2) Corner
If the RF signal line runs at a right angle, the effective line width at the corner will increase, causing impedance discontinuities and signal reflection. To reduce discontinuities, there are two ways to deal with corners: chamfering and filleting. The radius of the arc angle should be large enough, generally R>3W.
(3) Large pad
When there is a large pad on the 50 ohm microstrip line, the large pad acts as distributed capacitance, affecting the characteristic impedance of the microstrip line. To improve this, two methods can be used: thickening the microstrip line medium, and hollowing out the ground plane under the pad, both of which can reduce the distributed capacitance of the pad.
(4) Vias
Vias are metal cylinders plated through the holes between the top and bottom layers of a circuit board. Signal vias connect transmission lines on different layers. Via stubs are the unused parts of a via. Via pads are annular pads that connect vias to top or internal transmission lines. Isolation pads are annular voids within each power or ground plane to prevent shorts to the power and ground planes. The parasitic parameters of the vias can be modeled as the equivalent circuit model of the vias, including parasitic capacitance and inductance. These parasitic effects can degrade signal quality and filtering effect of the power system. Common methods to reduce via impedance discontinuities include using a diskless process and optimizing the diameter of the anti-pad.
(5) Through-hole coaxial connector
Similar to vias, through-hole coaxial connectors also have impedance discontinuities. Common methods for reducing impedance discontinuities in through-hole coaxial connectors include using a diskless process, a suitable lead-out method, and optimizing the diameter of the anti-pad on the PCB board.
Characteristic impedance, also known as “characteristic impedance,” is not simply a DC resistance, but rather a concept related to long-term transmission. In the high frequency range, a momentary current is generated between the signal line and the reference plane (power or ground plane) when a signal edge arrives during the signal transmission process due to the establishment of an electric field. If the transmission line is isotropic, there will always be a current I as long as the signal is transmitting. If the output voltage of the signal is V, the transmission line will be equivalent to a resistance, with the size V/I, and this equivalent resistance is known as the characteristic impedance Z of the transmission line. Any changes in the characteristic impedance of the transmission path during the signal transmission process will cause signal reflection at the node with discontinuous impedance. The factors that affect the characteristic impedance include dielectric constant, dielectric thickness, line width, and copper foil thickness.
(1) Gradient line
Some RF device packages are small, with the SMD pad width as small as 12mils, and the RF signal line width sometimes exceeding 50mils. Gradient lines are used, and sudden changes in line width are prohibited. The gradient line is shown in the picture, and the line in the transition part should not be too long.
(2) Corner
If the RF signal line runs at a right angle, the effective line width at the corner will increase, causing impedance discontinuities and signal reflection. To reduce discontinuities, there are two ways to deal with corners: chamfering and filleting. The radius of the arc angle should be large enough, generally R>3W.
(3) Large pad
When there is a large pad on the 50 ohm microstrip line, the large pad acts as distributed capacitance, affecting the characteristic impedance of the microstrip line. To improve this, two methods can be used: thickening the microstrip line medium, and hollowing out the ground plane under the pad, both of which can reduce the distributed capacitance of the pad.
(4) Vias
Vias are metal cylinders plated through the holes between the top and bottom layers of a circuit board. Signal vias connect transmission lines on different layers. Via stubs are the unused parts of a via. Via pads are annular pads that connect vias to top or internal transmission lines. Isolation pads are annular voids within each power or ground plane to prevent shorts to the power and ground planes. The parasitic parameters of the vias can be modeled as the equivalent circuit model of the vias, including parasitic capacitance and inductance. These parasitic effects can degrade signal quality and filtering effect of the power system. Common methods to reduce via impedance discontinuities include using a diskless process and optimizing the diameter of the anti-pad.
(5) Through-hole coaxial connector
Similar to vias, through-hole coaxial connectors also have impedance discontinuities. Common methods for reducing impedance discontinuities in through-hole coaxial connectors include using a diskless process, a suitable lead-out method, and optimizing the diameter of the anti-pad on the PCB board.
