What Is a Transmission Line?

Transmission line A device with a linear structure that transmits electromagnetic energy. It is an important part of the telecommunication system. It is used to transport information-carrying electromagnetic waves from one point to another along the prescribed route of the transmission line.

The quality of the connection lines between the various devices in the sound system will directly affect the sound quality and sound reproduction quality of the sound system. The effect of transmission lines on sound signals is not limited to DC resistance.

According to the characteristics of the transmission medium and structure, transmission lines can be divided into two-line transmission lines, microstrip transmission lines, waveguide transmission lines, surface wave transmission lines, and optical fiber.

4.1 Two-Wire Transmission Line

It consists of two parallel parallel conductive metal wires (usually copper, steel or aluminum wires), a transmission line that transmits transverse electromagnetic waves. According to the structure can be divided into two types of symmetrical and coaxial. Overhead open wires, various twisted-pair cables and star-twisted cables widely used in China are all symmetrical two-wire transmission lines. Coaxial and small coaxial cables are coaxial two-wire transmission lines.

As the frequency increases, the metal loss and dielectric loss of the two-wire transmission line increase rapidly. In addition, the lateral dimension of the transmission line cannot be ignored compared with the wavelength, and stricter requirements are imposed on the manufacturing process and maintenance standards of the equipment. In particular, the open-type electromagnetic field of a symmetrical two-wire transmission line, the coupling between the circuits becomes more serious. Therefore, the transmission frequency is low. China's high-frequency symmetrical cables generally open 60-channel carrier systems below 252kHz; medium-coaxial cables generally open 1800-channel carrier communication systems with a frequency of 8.5MHz.

4.2 Microstrip transmission line

An asymmetric transmission line used in the microwave band to transmit quasi-TEM waves. There are many forms of structure, and the performance and use are also different. The structure form of the standard microstrip is that a layer of a dielectric substrate is closely adhered to a wide ground metal strip, and a narrow metal strip is attached to the other side of the substrate. Standard microstrip line is a transmission line commonly used in microwave integrated circuits.

4.3 Waveguide Transmission Line

A non-TEM wave transmission line consisting of a hollow conductive metal tube in the microwave band. Waveguides are usually made of good conductors such as copper and brass, and the inner wall is often plated with a layer of silver with excellent electrical conductivity, making the tube wall highly conductive. The shape of the waveguide is mainly round, rectangular and oval.

Due to the large conductive area and high conductivity of the wall of the waveguide, the metal heat loss is small, and there is no radiation loss (because the field is closed) and dielectric loss (because there is no solid medium in the tube). Generally used in centimeter and millimeter wave bands.

4.4 surface wave transmission line

A waveguide composed of a single metal conductor with a circular cross section, also known as a high-covered wire. The surface of the conductor is covered with a layer of dielectric material with different electrical characteristics from the inner conductor, which can be hung in the open air to guide the transmission of electromagnetic waves along the surface of the transmission line.

4.5 Optical Fiber Transmission Line

A transmission line that uses an optical fiber as a transmission medium to guide light through the fiber along the path specified by the fiber. According to different transmission modes, it can be divided into two types: single-mode fiber and multi-mode fiber. Optical fiber transmission line has many technical advantages such as large communication capacity, long transmission distance, no electromagnetic interference, strong anti-corrosion ability, light weight, etc. It is a widely welcomed transmission line in the 1970s.

5.1 Uniformity of the transmission line

The shape of the cross section of the transmission conductor, the materials used, the spacing between the conductors, and the medium surrounding the conductors remain uniform over the entire length of the line, which is called a uniform transmission line. Otherwise it is called an uneven transmission line. The primary parameters of the uniform transmission line are evenly distributed on the entire transmission line, and their values do not change with the location of the survey points.

Any deviations that may occur during the manufacturing and construction of a transmission line specify the necessary tolerances. If the non-uniformity deviation does not exceed these regulations, it can be regarded as a uniform transmission line.

5.2 Performance parameters

It is usually described by attenuation coefficient, phase shift coefficient, characteristic impedance, or other parameters corresponding to it. Its value is only related to the structure, geometric size, materials used for manufacturing the transmission line, and the operating wavelength (or operating frequency) of the transmission line, as shown in the table.

Transmission line performance parameters (example), as shown in Figure 1

Crosstalk is also called "intermodulation interference", which mainly comes from the mutual inductance and mutual capacitance formed between two adjacent conductors, as shown in the figure. Crosstalk will become more and more serious as the wiring layout density of printed boards increases. Especially long-distance `bus structures and high-frequency and strong signal lines are more prone to crosstalk. This phenomenon is caused by parasitic parameters such as mutual inductance and mutual capacitance, coupling energy from one transmission line to adjacent transmission lines, so crosstalk is actually a typical EMI problem.

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Crosstalk includes capacitive coupling and inductive coupling. Capacitive coupling (capacitive crosstalk) is usually because a trace is above another trace or above a reference layer. The effect of this crosstalk between parallel lines is smaller, and there will be mutual capacitance effects between the two longer wires. When the voltage on one line changes, capacitive crosstalk occurs on the other line. That is, a small positive pulse will appear, as induced by a change in power supply voltage; inductive coupling (inductive crosstalk) is the effect of antennas caused by the inductance of the wiring and the common impedance between signals on different circuits. Crosstalk occurs when the magnetic field of one wire induces a signal on an adjacent signal. As long as there is a magnetic field caused by the switching current, a transient coupling voltage will be generated. Generally, microstrip lines have more severe crosstalk than striplines. ^[3]

According to the location where crosstalk occurs, crosstalk can be divided into forward crosstalk and backward crosstalk. When the signal is transmitted from the source to the load, forward crosstalk will occur; if the signal is reflected to the source, backward crosstalk will occur. Mutual capacitive coupling is positive for forward crosstalk and negative for backward crosstalk. In general, backward crosstalk affects the system more than forward crosstalk.

Crosstalk occurs not only on clock or periodic signal lines, but also on data, address, control, and LO traces, so it must be avoided as much as possible. Crosstalk values are related to dielectric constant, line width, and spacing.

To avoid crosstalk in PCBs, the following wiring recommendations are recommended.

Provides correct termination termination impedance to eliminate backward crosstalk.

Minimize the length of the wiring.

(3) Avoid parallel wiring layout, and ensure that there is a certain interval between the traces, thereby reducing the coupling between the traces.

Reduce the impedance of the trace and the drive level of the signal.

Try to isolate signals with poor EMI, such as clocks and high-speed interconnects.

Reduce the distance between devices, the device layout is reasonable.

Keep sensitive devices as far away as possible from I / O interconnect interfaces, clocks, and areas susceptible to data interference and coupling.