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Electronic Design

Electronic Design - Impedance matching and 0 ohm resistance for R-F PCB design in embedded systems

Electronic Design

Electronic Design - Impedance matching and 0 ohm resistance for R-F PCB design in embedded systems

Impedance matching and 0 ohm resistance for R-F PCB design in embedded systems

2021-09-17
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Author:Aure

The impedance matching and 0 ohm resistor designed by R-FPCB in embedded system are discussed.

R-FPCB tells you that impedance matching refers to the appropriate combination of signal source or transmission line and load according to the prescription distribution method. According to the impedance of the access form, there are two forms: serial and parallel. According to the frequency of the signal source, the impedance can be divided into low frequency and high frequency.


1. High frequency signals usually use serial impedance matching. The resistance value of the series resistance is 20~75 Ω. The resistance volume is proportional to the signal frequency and inversely proportional to the PCB trace width.

In the mosaic system, when the common frequency is greater than 20M and the PCB trace length is greater than 5cm, serial resistors must be added, such as clock signal, value and address bus signal in the system.

The series matched resistor has two effects: attenuation of high-frequency noise and edge overshoot. If the edge of the signal is very steep, it includes a large number of high-frequency components, which will radiate interference. In addition, it is easy to cause overshoot.

The series resistance of the signal line, the dispersed capacitance and the load input capacitance form an RC circuit, which will reduce the steepness of the signal edge. Reduce high-frequency reflection and self-excited vibration.

When the frequency of the signal is long, the wavelength of the signal is very short. When the wave jitter is equal to the length of the transmission line, the reflection signal superimposed on the original signal will change the style of the original signal.


If the characteristics of the transmission line indicate that the impedance is not equal to the load impedance, it is not matched, The load end will reflect, causing self-excited vibration. Low frequency signals wired in PCB can be directly connected adjacent to each other, usually without adding a series common resistor.

2. Parallel impedance matching is also called "terminal impedance matching", which is generally used for input/output interfaces, mainly referring to impedance matching of transmission cables.

For example, LVDS and RS422/485 use Category 5 twisted pair inputs with a resistance of 100~120 Ω. Video file signals use coaxial cables with a resistance of 75 Ω or 50 Ω, and use flat cables with a resistance of 300 Ω.

The resistance value of the parallel common resistor is related to the medium of the transmission cable and is independent of the length. Its main function is to avoid signal reflection and reduce self-excited vibration. It is worth mentioning that impedance matching can improve the EMI performance of the system.

In addition, in addition to using series/parallel resistors, transformers can also be used for impedance transformation to solve common impedance configurations. Typical examples include Ethernet interface, CAN bus, etc.

R-FPCB

Effect of 0 ohm resistance

1. The simplest method is to use it as a jumper. If you do not need a part of the circuit, just do not directly solder the resistor, which will not affect the appearance.

2. When the parameters of the general circuit are not confirmed, replace them with 0 ohm. During actual adjustment, confirm the parameters, and then replace them with specific digital components.

3. When you want to measure the office current of a local circuit, you can remove the 0 ohm resistance and connect an ammeter, which is very convenient for measuring the current.

4. In PCB wiring, if the cloth does not pass through like this, you can also add a 0 ohm resistor to achieve the jumper effect.

5. In the high-frequency signal network, it acts as an inductor or capacitor, which has the same effect as impedance, and 0 ohm resistance also has impedance. When used as an inductor, it is mainly used to solve EMC problems.

6. Single point grounding, for example, the single point connection of analog grounding and digital grounding share the same grounding.

7. It is equipped with layout circuit and can replace jumper and dial switch. Sometimes the user will change the settings at will, which is easy to cause errors. To reduce the protection cost, a 0 ohm resistor is used instead of other soldering on the jumper and wrench.

8. Try to use system adjustment. For example, divide the system into several parts and use a 0 ohm resistor to separate the power and ground between the boards. When the power or ground is shorted during the adjustment phase, remove the 0 ohm resistor to change the search range.


The above functions can also be replaced by magnetic beads. Although 0 ohm resistance and magnetic bead are somewhat similar in function, they are quite different. The former has special impedance characteristics, while the latter has special inductive reactance characteristics. Magnetic beads are usually used for power and grounding networks and have filtering effects.


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