Characteristic impedance of PCB: also known as “characteristic impedance”, it is not a DC resistance, and belongs to the concept of long-line transmission. In the high frequency range, in the process of signal transmission, an instantaneous current will be generated between the signal line and the reference plane (power supply or ground plane) 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 transmitted, and if the output level of the signal is V, the transmission line will be equivalent to a resistor with the size of V/I during signal transmission. This equivalent resistance is called In the process of signal transmission, if the characteristic impedance on the transmission path changes, the signal will reflect at the node with discontinuous impedance. The factors that affect the characteristic impedance are: dielectric constant, dielectric thickness, line width and copper foil thickness.
1. Gradient line
Some RF device packages are small, SMD pad width may be as small as 12mils, while RF signal line width may be more than 50mils, so gradient line should be used, and sudden change of line width is prohibited.
2. Corner
If the RF signal line is at a right angle, the effective line width at the corner will increase, and the impedance will be discontinuous, causing signal reflection. In order to reduce the discontinuity, there are two ways to deal with corners: cutting corners and rounding corners. The radius of the arc should be large enough. Generally speaking, R>3W should be ensured.
3. Large bonding pad
When there is a large pad on the 50 ohm microstrip line, the large pad is equivalent to the distributed capacitance, which destroys the continuity of the characteristic impedance of the microstrip line. At the same time, two methods can be adopted to improve it: firstly, thickening the microstrip dielectric, and secondly, hollowing out the ground plane below the pad, both of which can reduce the distributed capacitance of the pad.
4. via hole
The via hole is a metal cylinder plated outside the through hole between the top layer and the bottom layer of the circuit board. Signal vias connect transmission lines on different layers. The via stub is the unused part of the via. The via pads are annular pads that connect the via to the top or internal transmission line. Isolation disk is an annular gap in each power supply or ground plane to prevent short circuit to power supply and ground plane.
Parasitic parameters of PCB vias are designed.
After rigorous physical theoretical derivation and approximate analysis, the equivalent circuit model of a via can be an inductor with a grounding capacitor connected in series at both ends.
From the equivalent circuit model, it can be seen that the via itself has parasitic capacitance to the ground. Assuming that the diameter of the via counter pad is D2, the diameter of the via pad is D1, the thickness of the PCB board is T, and the dielectric constant of the board substrate is ε, the parasitic capacitance of the via is approximately:
The parasitic capacitance of the via hole can lead to the increase of signal rise time and slow down of transmission speed, thus deteriorating the signal quality. Similarly, there are parasitic inductances in vias. In high-speed digital PCB, parasitic inductances often do more harm than parasitic capacitances. Its parasitic series inductance will weaken the contribution of bypass capacitor, thus weakening the filtering effect of the whole power supply system. L is the inductance of the via, h is the length of the via, and d is the diameter of the central hole. The approximate parasitic inductance of the via is approximately:
It is one of the important factors that cause impedance discontinuity in RF channel. If the signal frequency is greater than 1GHz, the influence of via should be considered. The commonly used methods to reduce the impedance discontinuity of vias are: adopting diskless process, selecting outgoing line mode, optimizing the diameter of anti-pad, etc. Optimizing the diameter of anti-pad is the most commonly used method to reduce impedance discontinuity. As the characteristics of vias are related to the structural dimensions such as aperture, pad, anti-pad, laminated structure, and outgoing mode, it is suggested that HFSS and Optimetrics should be used for optimization simulation according to the specific situation in each design. When the parametric model is adopted, the modeling process is very simple. During the review, PCB designers are required to provide corresponding simulation documents.
The diameter of the via, the diameter of the pad, the depth of the pad, and the anti-pad will all change, resulting in impedance discontinuity, reflection and insertion loss.
5, through-hole coaxial connector
Similar to the via structure, the via coaxial connector also has impedance discontinuity, so the solution is the same as via. The common methods to reduce the impedance discontinuity of through-hole coaxial connector are the same: adopting diskless process, appropriate outlet mode and optimizing the diameter of anti-pad.
