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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".

It is an unexpected and undesirable interference, including the appearance of some undesirable transients (Transients) signals, which will interfere with the operation of electronic communication equipment or other electronic machines and affect their normal functions. For example, in the early TVs that were not protected against RFI, when a foot-pedal motorcycle was started nearby, the sparks (Spark) electromagnetic waves emitted by the spark plugs would cause temporary chaos in the picture after being transmitted to the TV. If the inner wall of the TV plastic case is treated with chemical copper or nickel-containing paint, a shielding layer can be used to guide the electromagnetic waveguide to the "grounding layer" to reduce RFI interference. As for some "high-frequency" welding workshops, it is also necessary to ground their buildings with metal meshes to avoid the interference of the high-frequency electromagnetic waves emitted from the surrounding electronic appliances. Operators near the airport channel may even cause interference to the aircraft landing radar instruments, which poses a great threat to flight safety and must be strictly guarded.

6. Reflection
In common sense, it means that the mirror reflects the incident light. However, when the high-speed signal is propagated in the computer motherboard, it means that the "signal" is sent by the Driver through the signal line and broadcast on the Receiver. If the impedance values of the three can be matched, the energy of the signal can reach the Receiver smoothly. Once there is a problem with the quality of the signal line, which causes the "characteristic impedance" value to exceed the limit, it will cause part of the signal energy to fold back to the Driver, also known as "reflection".

7. Rise Time
This word is an important property of square wave logic signal (Signal) or pulse (Pulse). The "clock" (clock pulse) system composed of the ordinate as the voltage (such as 5V in the early days and 3.3V now, and possibly 2.5V in the future), and the abscissa as the time; the generation of square waves in its propagation In theory, it should rise vertically from the low state to the "high state", but in fact it rises on a certain slope. The time taken to raise the slope from 10% to 90% height is called Rise Time, and the common unit is 10-9 seconds Nono-Second, abbreviated as NS, translated as nanosecond.

8. Ultra High Frequency (UHF)
Refers to the "ultra ultrashort wave" with a frequency between 300MHz and 3GHz or a wavelength between 1 m and 10 cm, which is called UHF. Such as TV, car phone, big brother phone, etc. belong to this range, and the circuit board FR-4 used in it can still achieve the mission. As for microwave electronic products with higher frequencies, Teflon PCB are required (see the special article of the 50th issue of the Circuit Board Information Magazine for details). EMI Electromagnetic Interference; electromagnetic interference RF Radio Frequency; radio waves, radio frequency RFI Radio-Frequency Interference; radio frequency interference SHF Super High Frequency; The big brother of wireless or satellite communication) UHF Ultra-High Frequency; Ultra-high frequency electromagnetic waves refer to radio waves with a frequency between 300 and 3,000 MHz, or "ultra ultra short waves" with a wavelength between 1 and 10 dm (10 to 100 cm). They are commonly used Yu Gao Picture Display TV. VHF Very High Frequency; Very high frequency electromagnetic waves (ie ultrashort waves) refer to radio waves with a frequency between 30 and 300 MHz, with a wavelength between 1 and 10 m, used for broadcasting and television.