PCB News - Impedance characteristics in circuit boards

Impedance characteristics in circuit boards

There will be various signal transmissions in the conductors of the circuit board. When the transmission rate increases, the frequency must increase. If the circuit itself is different due to etching, stack thickness, wire width, and other factors, the impedance will be worth the change and the signal will be distorted. Therefore, the impedance value of the conductor on the high-speed circuit board should be controlled within a certain range, which is called "impedance control". The main factors affecting the impedance of the circuit board are the width of the copper wire, the thickness of the copper wire, the dielectric constant of the medium, the thickness of the medium, the thickness of the pad, the path and winding of the ground wire. Therefore, when designing the PCB circuit board, the impedance of the circuit on the circuit board must be controlled to avoid electromagnetic interference and signal integrity problems such as signal reflection as much as possible, and to ensure the stability of the actual use of the PCB board.

1. The characteristics of the circuit board impedance board:

According to the theory of signal transmission, the signal is a function of time and distance variables, so the signal may change in each part of the connection. Therefore, determine the AC impedance of the transmission line, that is, the ratio of the voltage change to the current change as the characteristic impedance of the transmission line: the characteristic impedance of the transmission line is only related to the characteristics of the signal connection itself. In actual circuits, the resistance of the conductor itself is smaller than the distributed impedance of the system, especially in high-frequency circuit boards. The characteristic impedance mainly depends on the distributed impedance caused by the connected unit distributed capacitance and unit distributed inductance.

2. Impedance control of the circuit board:

The characteristic impedance of the conductor on the circuit board is an important indicator of circuit design. Especially in the PCB circuit board design of high-frequency circuit boards, it is necessary to consider whether the characteristic impedance of the conductor is consistent with the characteristic impedance of the device or signal, and whether it matches. Therefore, in the reliability design of PCB circuit board design, two concepts must be paid attention to.

Third, the impedance matching of the circuit board:

On the circuit board, if there is signal transmission, it is hoped that it can be smoothly transmitted from the power supply to the receiving end with minimal energy loss, and the receiving end can completely absorb it without any reflection. In order to achieve this transmission, the impedance in the line must be equal to the impedance in the transmitter before it is called "impedance matching". Impedance matching is one of the key elements in the design of high-speed PCB circuits. There is an absolute relationship between impedance and routing mode.

For example, walking on the surface layer (Microstrip) or inner layer (Stripline / Double Stripline), the distance from the reference power layer or layer, line width, PCB material, etc. will affect the characteristic impedance value of the line. In other words, the impedance value can only be determined after wiring. At the same time, the characteristic impedances produced by different PCB circuit board manufacturers are also slightly different. Due to the limitation of the circuit model or the mathematical algorithm used, the conventional simulation software cannot consider some discontinuous impedance wiring. At this time, only some resistors can be reserved on the schematic diagram, such as series resistors, to reduce the influence of discontinuous impedance. The real way to solve this problem is to avoid discontinuity in impedance during wiring.

Fourth, the calculation of the impedance of the circuit board:

The proportional relationship between the rising edge time of the signal and the time required to send the signal to the receiving end determines whether the signal connection is regarded as a transmission line. The specific proportional relationship can be explained by the following formula: If the length of the wire on the PCB impedance board is greater than l/b, the wire between the signals can be regarded as a transmission line. From the formula for calculating the equivalent impedance of the signal, the impedance of the transmission line can be expressed by the following formula: wL>R is satisfied at high frequencies (tens to hundreds of MHz). (Of course, in the signal frequency range greater than 109 Hz, considering the skin effect of the signal, this relationship needs to be carefully studied. For a given transmission line, the characteristic impedance is a constant. The signal reflection phenomenon is caused by the driving end and the signal transmission line. It is caused by the inconsistency between the characteristic impedance and the impedance of the receiving end. For CMOS circuits, the output impedance of the signal driving end is relatively small, tens of euros. The input impedance of the receiver is relatively large.