Does PCB design always have impedance discontinuities? These tips can help you solve your worries!

To clarify the following important concepts, we can often see an impedance, a characteristic impedance, and a transient impedance. 1oz vs 2oz Strictly managed, they are different, but all the same, their development is still the basic problem definition of impedance:

A) The input impedance at the beginning of the transmission line is abbreviated as impedance;

b) The instantaneous impedance encountered by the signal at any time is called the instantaneous impedance;

If the transmission line has a constant instantaneous impedance, it is called a characteristic impedance transmission line.

Characteristic impedance describes the transient impedance of a signal propagating along a transmission line and is the main factor affecting the integrity of a signal in a transmission line circuit.


Characteristic impedance

Characteristic impedance: also known as "characteristic and impedance", it is not through the DC resistance, belongs to the concept of long-term transmission. how to test a circuit board with a multimeter In the high frequency region, the signal data transmission process, the signal along the arrival of the place, the signal line and the reference learning plane (power or ground plane) due to the establishment of an internal electric field, will produce an impact can be instantaneous current.

If the transmission line is isotropic, then as long as the signal is transmitted, there will always be a current i. If the output voltage of the signal is V, then when the signal is transmitted, the transmission line is equal to the resistance, the size is V/I, and this equivalent resistance is called the characteristic impedance transmission line Z.

In the process of signal transmission, if the characteristic impedance of the transmission path changes, the signal will reflect at the node where the impedance is discontinuous.


Impedance discontinuity solution

The factors that affect the characteristic impedance are: dielectric constant, dielectric thickness, line width, copper foil thickness.


Gradient line

Some RF devices have small packages, SMD pad widths can be as small as 12 mil, RF signal line widths can be greater than 50 mil. Gradient lines should be used to prevent sudden changes in line width. The gradient line is shown, and the transition line should not be too long.



If a right Angle is taken between RF signal lines, the effective absorption line width at the corner will increase, and the impedance is discontinuous, causing the relevant signal to reflect through. In order to reduce discontinuity, there are two ways to analyze a corner economy: cutting corners and rounding corners. The radius of the arc Angle should be large enough, generally speaking, enterprises should ensure that: R > 3W. As shown on the right.


Big pad

When the 50 euro microstrip line has a large pad, the large pad is equivalent to a distributed capacitor, which breaks the characteristic impedance continuity of the microstrip line. There are two improvement methods: one is to thicken the microstrip line, and the other is to hollow the ground plane under the liner, which can reduce the distributed capacitance of the liner. Down.



The through-hole is a metal cylinder plated on the outside of the through-hole between the top and bottom layers of the circuit board. Signal holes connect transmission lines on different layers. The through hole residue is the unused part of the through hole. Through-hole pads are annular pads that connect through-holes to top or internal transmission lines. Isolation disks are annular gaps in each power layer or ground layer to prevent short circuits between the power layer and ground layer.

Parasitic parameters of the pore

The equivalent circuit model can be modeled as grounding capacitance in series at both ends of the inductor, as shown in the figure.

The equivalent circuit model of the hole

It can be seen from the equivalent circuit model that the through-hole itself has parasitic capacitance to the ground. Assume that the diameter of the backweld pad through the hole is D2, the diameter of the backweld pad through the hole is D1, and the thickness of the PCB is T, that is, we often see that the dielectric constant of the board substrate is ε, and the parasitic capacitance through the hole is similar to:

The parasitic capacitance through the hole can directly cause a signal to rise in development time as well as prolong, slow down the transmission speed, and thus continuously deteriorate the quality of signal control. Similarly, there is also parasitic inductance through the hole, and in high-speed digital PCB, the harm caused by parasitic inductance often needs to be greater than the parasitic capacitance.

Its parasitic series inductance will weaken the contribution of the bypass capacitor, thus weakening the filtering effect of the whole power supply system. Suppose L is the inductance of the hole, h is the length of the hole, and d is the diameter of the center hole. The size of the parasitic inductance is related to:

Through-hole is one of the important factors leading to the impedance discontinuity of RF channel. If the signal frequency is greater than 1GHz, the effect of the hole should be considered.

The common methods to reduce the impedance discontinuity of the through-hole include: adopting the disc-free process, selecting the outlet mode, optimizing the diameter of the protective pad, etc. Optimizing the flip-flop diameter is a common method to reduce impedance discontinuity. Since the characteristics of the through-hole are related to the size of the aperture, liner, reverse liner, stack structure and outlet mode, it is recommended to optimize and simulate each design using HFSS and Optimetrics software.

When parametric model is adopted, the modeling process is simple. During the review, the PCB designer is required to provide the corresponding simulation documents.

The diameter of the hole, the diameter of the pad, the depth, and the reverse pad will all develop and bring about great changes, resulting in system impedance discontinuity, and a serious impact on reflection and insertion losses.


Through hole coaxial connector

Similar to the through-hole structure, the through-hole coaxial connector also has an impedance discontinuity, so the solution is the same as the through-hole structure. The common methods to reduce the impedance discontinuity of the through-hole coaxial connector are: using the disk-free process, selecting the appropriate outlet mode, optimizing the diameter of the protective pad.