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sales@ipcb.com High-frequency PCB sheet characteristics and impedance control High-frequency PCB sheet characteristics and impedance control

High-frequency PCB sheet characteristics and impedance control

2021-11-29
View：175
Author：iPCBer

1. AC Impedance　 AC impedance
AC resistance (Resistance; R), capacitive reactance (CapaciTIve Reactance; Xc), and inductance (InducTIve Reactance; XL) are combined into AC "resistance", referred to as "impedance" (Impedance, representing symbol Z) . The formula is Z=√(R2+(XL-XC)2). This word is related to the signal in high-speed or high-frequency transmission lines (Signal; this word still has other synonyms such as Pulse, Square Wave, Step Wave, etc.). "Charac terisTIc Impedance" (Charac terisTIc Impedance) , Its representative symbol is Zo) is completely different from the former, and its formula is Zo=√(L/C). The AC in the original word refers to AlternaTIng Current, which is the "current of polarity exchange". That is, the current that changes with time, abbreviated as AC. This AC symbol is often used for power supplies that change polarity (Polarity Switching) many times per second, and its waveform is often sine wave, square wave, or triangle wave.

2. Characteristic Impedance
It means that when there is an electronic "signal" waveform in the conductor, the ratio of its voltage to current is called "Impedance". Because the "Resistance" that has been mixed with other factors (such as capacitive reactance, inductive reactance, etc.) in an AC circuit or at high frequencies is no longer just a simple direct current "Ohmic Resistance" (Ohmic Resistance), so in the circuit It should no longer be called "resistance", but should be called "impedance". However, when it comes to the actual use of "Impedance Impedance" AC, it will inevitably cause confusion. For the sake of distinction, the electronic signal has to be called "Characteristic Impedance". When the signal in the circuit board line propagates, the factors that affect its "characteristic impedance" are the cross-sectional area of the line, the thickness of the green material between the line and the ground layer, and its dielectric constant. At present, there are many boards with high frequency and high transmission speed, and the "characteristic impedance" must be controlled within a certain range. In the manufacturing process of the board, the above three important parameters and other cooperation conditions must be carefully considered. 3. Frequency
Refers to various periodic motions, in terms of the number of repetitions per unit time. Electric frequency is usually in units of "Herz" (Herz, the number of repetitions per second or number of cycles). The "signal" in the circuit board circuit is transmitted in a fluctuating manner, so high frequency is necessary for high speed. At high frequencies, the dielectric constant of the sheet is very critical. For example, for high-frequency microwave communication boards above 3 GHz, FR-4 with a dielectric constant of 4.5 must be replaced with a PTFE sheet with only 2.6 to reduce the signal Dissipation and delay.

4. Impedance Match impedance matching
If a signal (Signal) is being transmitted in the electronic circuit, it is hoped that it can be transmitted to the receiving end (Load) smoothly from the source of the power source with the least energy loss. And it is completely absorbed by the receiving end without any reflection. To achieve this kind of good transmission or transmission, the impedance (ZL) in the circuit must be equal to the impedance (Zo) inside the "transmitting end", which is called "impedance matching".