To learn high-speed PCB design, you must first know what a transmission line is. Signals will reflect because the traces on the PCB have a certain impedance. If the impedance on the line does not match the impedance at the output end, it will cause signal reflection. There will be a delay in the transmission of the signal in the PCB. If the timing does not match, the system will go out of service. These are all problems caused by the transmission line.
What is a transmission line?
The definition of a transmission line is a signal line with signal return (composed of two wires of a certain length, one is the signal propagation path, and the other is the signal return path.), and the most common transmission line is the trace on our PCB board.
1. When analyzing the transmission line, you must contact the return path. A single conductor cannot be a transmission line;
2. Like resistors, capacitors, and inductors, transmission lines are also an ideal circuit element, but their characteristics are very different. They are used for simulation with better results, 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 basic definition of impedance:
The input impedance at the beginning of the transmission line is referred to as impedance;
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 encountered by the signal when it propagates along the transmission line, which is a major factor affecting the signal integrity in the transmission line circuit.
Unless otherwise specified, characteristic impedance is generally used to collectively refer to transmission line impedance.
Transmission delay
Time delay(TD), which usually refers to the time it takes for an electromagnetic signal or an optical signal to pass through the entire transmission medium. The time delay on the transmission line refers to the time it takes for the signal to pass through the entire transmission line.
Propagation delay (PD), usually refers to the time delay of electromagnetic signals or optical signals in a transmission medium per unit length, which is inversely proportional to the “propagation speed” (inverse) and is expressed in “Ps/inch” or “s/m”.
From the definition, we can see that time delay = propagation delay * transmission length (L).
PCB transmission line structure
The transmission line structure seen in a typical PCB is composed 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 is copper, and the dielectric is a glass fiber called FR4. The two most common types of transmission lines in digital design are microstrip and stripline.
Microstrip usually refers to the traces on the outer layer of the PCB and has only one reference plane. There are two types of microstrip: buried or non-buried. Buried (sometimes called submerged) microstrip simply embeds a transmission line in a dielectric, but it still has only one reference plane. Stripline refers to the inner layer trace between two reference planes.