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Impedance Control is the Characteristic Impedance of a Transmission Line Formed by PCB Traces and Its Associated Reference Planes.

What is Impedance Control

Impedance control has been one of the essential concerns and severe problems in high-speed PCB design.Impedance is the sum of the resistance and reactance of an electrical circuit.The resistance is the opposition to current flow present in all materials.In high-frequency applications,controlled impedance helps us ensure that signals are not degraded as they route around a PCB.Resistance and reactance of an electrical circuit have a significant impact on functionality,as specific processes must be completed before others to ensure proper operation.

Essentially, controlled impedance is the matching of substrate material properties with trace dimensions and locations to ensure the impedance of a trace's signal is within a certain percentage of a specific value. Controlled impedance boards provide repeatable high frequency performance.There're four elements of impedance control as following:

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1. Impedance control of printed circuit board

There are various signal transmissions in the conductors in the circuit board. When it is necessary to increase its frequency in order to increase its transmission rate, if the circuit itself is different due to factors such as etching, stack-up thickness, wire width, etc., the impedance value will change. Its signal is distorted. Therefore, the impedance value of the conductor on the high-speed circuit board should be controlled within a certain range, which is called “impedance control”. The main factors affecting the impedance of PCB traces are the width of the copper wire, the thickness of the copper wire, the dielectric constant of the medium, the thickness of the medium, the thickness of the pad, the path of the ground wire, and the wiring around the wire. Therefore, the impedance of the traces on the board must be controlled when designing the PCB to avoid signal reflection and other electromagnetic interference and signal integrity problems as much as possible, and to ensure the stability of the actual use of the PCB. The calculation method of the impedance of the microstrip line and strip line on the PCB can refer to the corresponding empirical formula.

2,The characteristic impedance control of the printed circuit board

The characteristic impedance of the wires on the printed circuit board is an important indicator of circuit design. Especially in the PCB design of high-frequency circuits, it is necessary to consider whether the characteristic impedance of the wire is consistent with the characteristic impedance required by the device or signal, and whether it matches or not.

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3. Impedance characteristics of printed circuit board

According to the theory of signal transmission, the signal is a function of time and distance variables, so every part of the signal on the connection may change. Therefore, determine the AC impedance of the connection, that is, the ratio of the voltage change to the current change as the characteristic impedance of the transmission line which is only related to the characteristics of the signal connection itself. In the actual circuit, the resistance of the wire itself is smaller than the distributed impedance of the system, especially in the high-frequency circuit, the characteristic impedance mainly depends on the distributed impedance brought by the unit distributed capacitance and unit distributed inductance of the connection. The characteristic impedance of an ideal transmission line depends only on the unit distributed capacitance and unit distributed inductance of the connection.

4, Printed circuit board impedance matching

In the circuit board, if there is signal transmission, it is hoped that from the sending end of the power supply, it can be smoothly transmitted to the receiving end under the condition of minimum energy loss, and the receiving end will completely absorb it without any reflection. To achieve this kind of transmission, the impedance in the line must be equal to the internal impedance of the transmitter to be called “impedance matching”. When designing high-speed PCB circuits, impedance matching is one of the design elements. The impedance value has an absolute relationship with the wiring method. For example, whether it is designing on the surface layer or the inner layer, the distance from the reference power layer or ground layer, the trace width, and the PCB material will affect the characteristic impedance value of the trace. In other words, the impedance value can only be determined after wiring, and the characteristic impedance produced by different PCB manufacturers is also slightly different. Generally, simulation software cannot be considered into some wiring conditions with discontinuous impedance due to the limitation of the circuit model or the mathematical algorithm used. At this time, only some terminations, such as series resistance, can be reserved on the schematic diagram to ease the effect of impedance discontinuity. The real solution to the problem is to try to avoid impedance discontinuities when wiring.

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How Many Types of Impedance Controlled PCB

The common characteristic impedance controls are divided into: single-ended (line) impedance control, differential impedance control, coplanar impedance control and so on.

  1. Single-ended (line) impedance control: refers to the measured impedance of a single signal line.
  2. Differential impedance: refers to the impedance measured in two transmission lines of equal width and equal spacing during differential drive.
  3. Coplanar impedance: refers to the impedance measured when the signal line is transmitted between GND/VCC around it (the distance between the signal line and GND/VCC on both sides is equal).

 

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How to Ensure the Value Matching of Impedance Controlled PCB

3 steps to ensure the value correct of your PCB impedance controlled.

  1. Calculate the value by Polar before production.

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  1. Measure the width of trace anddielectric layer in process
  2. Make an coupon, Test it by impedance test Instrument

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Applications of Controlled Impedance

Controlled Impedance should be considered for PCBs used in fast digital applications such as:

  • Telecommunications
  • Computing 100MHz and above
  • High Quality Analog Video
  • Signal Processing
  • RF Communication

Why Control Impedance on PCBs

When the signal requires a specific impedance to operate normally, controlled impedance should be preferred. In high-frequency applications, the impedance of the entire electronic board must be kept constant to protect the transmitted data from damage and maintain the clarity of the signal. The longer the trace or the higher the frequency, the more adaptability is required. At this stage, any loose operation will increase the switching time of electronic equipment or circuits and cause unexpected errors.

Once the component is mounted on the circuit, it is difficult to analyze the uncontrolled impedance. Components have different tolerance capabilities, depending on their batch. In addition, their specifications are affected by temperature changes, which may cause malfunctions. In this case, when the problem is actually caused by inappropriate trace impedance, replacing components seems to be the solution.

This is why the trace impedance and its tolerance must be checked as early as possible in PCB design. Designers must work hand-in-hand with manufacturers to ensure compliance with component values.

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