1. What is a transmission line
? We often use the term transmission line, but when it comes to its specific definition, many engineers are hesitant to speak, and they seem to understand...
We know that transmission lines are used to transmit signals from one end to the On the other end, the following figure illustrates the general characteristics of all transmission lines, so it can be understood as follows: a transmission line consists of two conductors of a certain length, one for the signal propagation path and the other for the signal return path.
1. When analyzing the transmission line, it is necessary to contact the return path. A single conductor cannot become a transmission line
. 2. Like resistance, capacitance and inductance, transmission line is also an ideal circuit component, but its characteristics are quite different, which are used for simulation effects. Better, but the circuit concept is more complicated
3. The transmission line has two very important characteristics: characteristic impedance and time delay
2. Transmission line classification
The often used twisted pair and coaxial cables are transmission lines.
For PCBs, there are often two types of microstrip lines and strip lines.
Microstrip lines usually refer to the routing of the outer layer of the PCB, and there is only one reference plane.
The stripline refers to the inner layer between the two reference planes. Line
The following figure shows the schematic diagram of the microstrip line and the stripline and its impedance calculation formula. It can be seen from this formula that the impedance is related to those factors, but in practical engineering applications, some professional software is used for impedance calculation, such as Polar
3. Transmission line impedance
Let’s first clarify a few concepts. We often see impedance, characteristic impedance, and instantaneous impedance. Strictly speaking, they are different, but they are still the basic definition of impedance.
The transmission line The input impedance of the starting end is referred to as impedance for short
. The timely 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 the characteristic impedance
of the transmission line. state impedance, which is a major factor affecting signal integrity in transmission line circuits. If there is no special description, the characteristic impedance is generally used to refer to the transmission line impedance
. In simple terms, the transmission line impedance can be described by the above formula, but if we go deeper, we need to analyze the behavior of the signal in the transmission line. Dr. Eric Bogatin in His book "Signal Integrity: Simplified" has a very detailed description, and the reader can find the original for a detailed study. Here is only a brief description:
when the signal moves along a transmission line with the same cross-section, it is assumed that the 1V The step function is added to this transmission line (such as connecting a 1V battery to the transmission end of the transmission line, and the voltage is across the transmission line and the return), once connected, this voltage step wave along the line at the speed of light Propagation, its speed is usually about 6 inches/ns. This signal is the voltage difference between the transmit line and the loop, which can be measured from any point on the transmit line and an adjacent point on the loop.
讯号能量在第一个0.01ns前进了0.06英寸，这时发送线路有多余的正电荷(由电池提供)，而回路有多余的负电荷，正是这两种电荷差维持着这两个导体之间的1V电压差，且这两个导体间也形成了一个电容器。在下一个0.01ns中，又要将下一段0.06英寸传输线的电压从0调整到1V，这必须再加一些正电荷到发送线路，与加一些负电荷到接收线路。每移动0.06英寸，必须把更多的正电荷加到发送线路，而把更多的负电荷加到回路。每隔0.01ns，必须对传输线路的另外一段进行充电，然后信号开始沿着这一段传播。电荷来自传输线前端的电池，当讯号沿着这条线移动时，就给传输线的连续部份充电，因而在发送线路和回路之间形成了1V的电压差。每前进0.01ns,就从电池中获得一些电荷(±Q)，恒定的时间间隔(±t)内从电池中流出的恒定电量(±Q)就是一种恒定电流。流入回路的负电流实际上与流出的正电流相等，而且正好在信号波的前端，交流电流藉由上、下线路组成的电容，结束整个循环过程。
讯号传递时，会在传输线内建立一个电场，而这讯号传递的速度取决于在讯号与回路周围金属材质的电荷充放电与磁场生成速度。
对电池来说，当信号沿着传输线传播，并且每隔0.01ns对连续0.06英寸传输线段进行充电。从电源获得恒定的电流时，传输线看起来像一个阻抗器，并且它的阻抗值恒定，这可称为传输线路的浪涌阻抗(surge impedance)。同样地，当信号沿着线路传播时，在下一步之前(0.01ns之内)，把这一步的电压提高到1V所需供应的能量(电流)，这就涉及到瞬时阻抗的概念。
如果信号以稳定的速度沿着传输线传播，并且传输线具有相同的横截面，那么在0.01ns中每前进一步需要相同的电荷量，以产生相同的信号电压。此时，信号着这条线前进时，会遭遇同样的瞬时阻抗，这被视为传输线的一种特性，被称为特性阻抗。如果信号在传递过程的每一步的特性阻抗相同，那么该传输线可认为是可控阻抗(controlled impedance)传输线。
瞬时阻抗或特性阻抗，对信号传递质量而言非常重要。在传递过程中，如果下一步的阻抗和上一步的阻抗相等，工作可顺利进行，但若阻抗发生变化(阻抗不匹配)，那会出现一些问题。为了达到最佳信号质量，设计目标是在信号传递过程中尽量保持阻抗稳定，首先必须保持传输线特性阻抗的稳定，因此，可控阻抗板的生产变得越来越重要。另外，其它的方法，如余线(stub)长度最短化、末端去除和整线使用，也用来保持信号传递中瞬时阻抗的稳定。
四. 传输线阻抗的计算
设计一个预定的特性阻抗，需要不断调整线宽、介质厚度和介电常数。如果知道传输线长度和材料的介电常数，就可以计算出特性阻抗以及其它参数
求解特性阻抗的途径有三种：
1. 经验法则；
2. 解析近似；
3. 采用数值仿真的场求解器。
这里只看看经验法则，其中两种还是交给专业的软件或者PCB人员吧 ：）
对于50ohm 微带线：w=2h, 对于50Ohm 带状线： b=2w
经验法则：FR4上50Ω微带线的线宽w等于介质厚度h的两倍。50Ω带状线，两平面间总介质厚度b等于线宽w的两倍。