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PCB design always has a few impedance discontinuities, how to do?
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PCB design always has a few impedance discontinuities, how to do?

Characteristic impedance: Also known as "characteristic impedance", it is not DC resistance, belongs to the concept of long line transmission. In the high frequency range, a transient current will be generated between the signal line and the reference plane (power or ground plane) along the place where the signal arrives during signal transmission 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 being transmitted. However, if the output voltage of the signal is V, the transmission line will be equivalent to a resistance of size V/I in the process of signal transmission. 

This equivalent resistance is called the characteristic impedance Z of the transmission line. In the process of transmission, if the characteristic impedance of the transmission path changes, the signal will reflect at the node where the impedance is discontinuous. The factors affecting the characteristic impedance are: dielectric constant, dielectric thickness, wire width, copper foil thickness.


1. Gradient line

Some RF devices have small packaging, SMD pad width may be as small as 12mils, and RF signal line width may be more than 50mils, to use a gradient line, prohibited line width mutation. The line in the transition section of the gradient line should not be too long.

2. Corner

If the RF signal line is at a right Angle, the effective line width at the corner will increase, resulting in discontinuous impedance and signal reflection. To reduce discontinuity, there are two ways to treat corners: cut corners and round corners. The radius of the arc Angle should be large enough, generally speaking, to ensure that: R> 3W.

3. Large welding pad

When there is a large pad on the 50 euro fine strip line, the large pad is equivalent to the distributed capacitance, which destroys the characteristic impedance continuity of the microstrip line. Two methods can be taken at the same time to improve: first, the microstrip line medium thickened, and secondly, the ground plane below the pad is hollowed out, which can reduce the distributed capacitance of the pad.

4, through the hole

The through hole is a metal cylinder plated in the through hole between the top and bottom of the circuit board. Signal holes connect transmission lines on different layers. The hole stump is the unused part of the hole. Through-hole pads are circular spacers that connect through-holes to top or internal transmission lines. Isolation panels are annular gaps within each power supply or ground to prevent short circuit to the power supply and ground. After rigorous physical theory derivation and approximate analysis, the equivalent circuit model of the hole can be taken as a grounding capacitor connected in series at each end of an inductor.

From the equivalent circuit model, it can be seen that there is parasitic capacitance to the ground through the hole itself. Assuming that the diameter of the backweld plate through the hole is D2, the diameter of the backweld plate through the hole is D1, the thickness of the PCB board is T, and the dielectric constant of the substrate is ε, then the parasitic capacitance through the hole is approximately:

The parasitic capacitance through the hole can cause the signal rise time to be prolonged and the transmission speed to be slowed down, thus deteriorating the signal quality. Similarly, there is also a parasitic inductance through the hole. In high-speed digital PCB, the harm caused by the parasitic inductance is often greater than the parasitic capacitance.

Its parasitic series inductance will reduce the contribution of the bypass capacitor and thus reduce the filtering effectiveness of the whole power system. Suppose L is the inductance of the hole, h is the length of the hole, and d is the diameter of the central hole. The parasitic inductance of the through-hole approximation is approximately:

a. Hole is one of the important factors that cause impedance discontinuity on RF channel. If the signal frequency is greater than 1GHz, the influence of hole should be considered.

b. The commonly used methods to reduce the impedance discontinuity through the hole are: using no-plate process, selecting the outlet mode, optimizing the diameter of the backplate, etc. Optimizing the diameter of back pad is one of the most commonly used methods to reduce impedance discontinuity. Because the pore characteristics are related to the size of the aperture, pad, back pad, laminated structure, outlet mode and other structures, it is recommended that each design should be optimized by HFSS and Optimetrics according to the specific situation.

c. When parameterized models are used, the modeling process is simple. During the review, the PCB designer should provide corresponding simulation documents.

d. Through hole diameter, pad diameter, depth, back pad, will bring changes, resulting in impedance discontinuity, reflection and insertion loss severity.

5, through hole coaxial connector

Similar to the through-hole construction, the through-hole coaxial connector also has impedance discontinuity, so the solution is the same as the through-hole. The common methods to reduce the impedance discontinuity of through-hole coaxial connectors are as follows: using the disk-free process, appropriate outlet mode, and optimizing the diameter of the backplate.

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