PCB Tech - Transmission line in high-speed PCB design

To learn high-speed PCB design, you must first know what a transmission line is. The signal will reflect because the traces on the PCB have a certain impedance, and the impedance of the line does not match the impedance of the output terminal, which will cause the signal to reflect. The signal transmission in the PCB will have a delay. If the timing does not match, the system will strike. 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, the other is the signal return path.), the most common transmission line is also the trace on our PCB board.

1. Analyze the transmission line, be sure to contact the return path, a single conductor cannot become a transmission line; 2. Like resistance, capacitance, and inductance, a transmission line is also an ideal circuit element, but its characteristics are quite different, which is used for simulation The effect is better, but the circuit concept is more complicated; 3. The transmission line has two very important characteristics: characteristic impedance and time delay.

The concept and structure analysis of transmission line in high-speed PCB design

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 always the same. They are still the basic definition of impedance:

The input impedance at the beginning of the transmission line is referred to as impedance for short;

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 the characteristic impedance of the transmission line.

The characteristic impedance describes the transient impedance that the signal receives when propagating along the transmission line, which is a major factor affecting the signal integrity in the transmission line circuit.

If there is no special instructions, the characteristic impedance is generally used to collectively refer to the transmission line impedance.

Transmission delay

Time delay is also called time delay (TD), which usually refers to the time it takes for electromagnetic signals or optical signals 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 is also called propagation delay (PD). It usually refers to the time delay of electromagnetic signal or optical signal transmission in a unit length of transmission medium. It is inversely proportional to the "propagation speed" (reciprocal), and the unit is "Ps/inch" or "S/m".

It can be seen from the definition 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 dielectric or insulating materials 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 lines usually refer to the traces on the outer layer of the PCB, and there is only one reference plane. There are two types of microstrip lines: buried or non-buried. Buried (sometimes called submerged) microstrip line 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.

Do you know all about transmission lines in high-speed PCB design?

To learn high-speed PCB design, you must first know what a transmission line is. The signal will reflect because the traces on the PCB have a certain impedance, and the impedance of the line does not match the impedance of the output terminal, which will cause the signal to reflect. The signal transmission in the PCB will have a delay. If the timing does not match, the system will strike. 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, the other is the signal return path.), the most common transmission line is also the trace on our PCB board.

1. Analyze the transmission line, be sure to contact the return path, a single conductor cannot become a transmission line; 2. Like resistance, capacitance, and inductance, a transmission line is also an ideal circuit element, but its characteristics are quite different, which is used for simulation The effect is better, but the circuit concept is more complicated; 3. The transmission line has two very important characteristics: characteristic impedance and time delay.

The concept and structure analysis of transmission line in high-speed PCB design

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 always the same. They are still the basic definition of impedance:

The input impedance at the beginning of the transmission line is referred to as impedance for short;

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 the characteristic impedance of the transmission line.

The characteristic impedance describes the transient impedance that the signal receives when propagating along the transmission line, which is a major factor affecting the signal integrity in the transmission line circuit.

If there is no special instructions, the characteristic impedance is generally used to collectively refer to the transmission line impedance.

Transmission delay

Time delay is also called time delay (TD), which usually refers to the time it takes for electromagnetic signals or optical signals 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 is also called propagation delay (PD). It usually refers to the time delay of electromagnetic signal or optical signal transmission in a unit length of transmission medium. It is inversely proportional to the "propagation speed" (reciprocal), and the unit is "Ps/inch" or "S/m".

It can be seen from the definition 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 dielectric or insulating materials 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 lines usually refer to the traces on the outer layer of the PCB, and there is only one reference plane. There are two types of microstrip lines: buried or non-buried. Buried (sometimes called submerged) microstrip line 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.