To learn high-speed
PCB board design, you must first know what a transmission line is. The signal will be reflected because the traces on the PCB have a certain impedance, and the impedance on the line does not match the impedance of the output terminal, which will lead to signal reflection. There will be a delay in the transmission of signals in the PCB board. If the timing is not matched, the system will fail. These are all problems caused by transmission lines.
What is a transmission line?
The definition of a transmission line is a signal line with a signal return (composed of two wires of a certain length, one is the signal propagation path, and the other is the signal return path.), a very common transmission line is the trace on our PCB board.
1. When analyzing the transmission line, be sure to contact the return path. A single conductor cannot become a transmission line;
2. Like resistors, capacitors, and inductors, transmission lines are also ideal circuit components, but their characteristics are quite different, and the simulation effect is better, but the circuit concept is more complicated;
3. Transmission lines have two very important characteristics: characteristic impedance and delay.
Transmission Line Impedance
Let’s clarify a few concepts first. We often see impedance, characteristic impedance, and instantaneous impedance. Strictly speaking, they are different, but they are still the same. They are still the basic definition of impedance: the input at the beginning of the transmission line. Impedance is referred to as impedance for short; the instantaneous impedance encountered by the signal at any time is called instantaneous impedance; if the transmission line has a constant instantaneous impedance, it is called the characteristic impedance of the transmission line. Characteristic impedance describes the transient impedance that a signal experiences as it propagates along a transmission line, which is a major factor affecting signal integrity in transmission line circuits. Unless otherwise specified, the characteristic impedance is generally referred to as the transmission line impedance.
transmission delay
Time delay, also known as time delay (TD), usually refers to the time it takes for an electromagnetic signal or an optical signal to pass through the entire transmission medium. The delay on the transmission line refers to the time it takes for the signal to travel through the entire transmission line. Propagation delay, also known as Propagation Delay (PD), usually refers to the time delay of electromagnetic signals or optical signals transmitted in a unit length of transmission medium, which is inversely proportional (reciprocal) relationship to "Propagation Speed", in units of "Ps/inch" or "s/m". It can be seen from the definition that delay = propagation delay * transmission length (L).
Transmission line structure of PCB board
The transmission line structure seen in a typical PCB board consists of wires embedded in or adjacent to a dielectric or insulating material and having one or more reference planes. The metal in a typical PCB board is copper, and the dielectric is a fiberglass called FR4. Two types of transmission lines that are very common in digital designs are microstrip and stripline. Microstrip lines usually refer to the traces on the outer layer of the PCB board, and there is only one reference plane. There are two types of microstrip lines: buried or non-buried. Buried (sometimes called submerged) microstrip lines simply embed a transmission line in a dielectric, but it still has only one reference plane. A stripline is an inner layer trace between two reference planes. Internal traces (stripline) and external traces (microstrip) between different components on the
PCB board.
