PCB Technical - High-frequency board characteristics and impedance control

1. ACimpedance AC impedance

Alternating current combines resistance (Resistance; R), capacitive reactance (Xc), and inductive reactance (XL) to form an AC "resistance", referred to as "impedance" (Impedance, representing the 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 Pulse, square wave SquareWave, StepWave order wave, etc.) "Characteristic Impedance" (CharacteristicImpedance, which represents the symbol). Zo) is completely different from the former, and its formula is Zo=√(L/C). AC in the original word refers to AlternatingCurrent, 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. 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 must be selected first for high speed. At high frequencies, the dielectric constant of the high-frequency board is very critical. For example, for high-frequency boards above 3GHz/high-frequency microwave boards/high-frequency microwave radio-frequency boards/high-frequency microwave communication boards, the dielectric constant is 4.5 FR -4 must be replaced with a PTFE high-frequency board of only 2.6 to reduce signal dissipation and delay.

3. ImpedanceMatch 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".

4. 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".

5. RiseTime rise time

This word is an important property of square wave logic signal (Signal) or pulse (Pulse). The ordinate is the voltage (such as the early 5V and the current 3.3V, and the possible 2.5V in the future), and the abscissa is the "clock" (clock pulse) system composed of 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% of the height is called RiseTime, and the common unit is 10-9 seconds Nono-Second, abbreviated as NS, translated as nanosecond.

6, CharacteristicImpedance 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 high-frequency circuit board circuit propagates, the factors that affect its "characteristic impedance" are the cross-sectional area of the circuit, the thickness of the green material between the circuit and the ground layer, and its dielectric constant. At present, there are many high-frequency high-frequency board boards with high transmission speed, and the "characteristic impedance" must be controlled within a certain range. Therefore, in the manufacturing process of high-frequency boards, the above three important parameters and Other conditions of cooperation.

7. RadioFrequency Interference (RFI) radio frequency interference

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 a shielding layer is treated with chemical copper or nickel-containing paint on the inner wall of the TV plastic case, the electromagnetic waveguide can be guided 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.

8. UltraHigh Frequency (UHF)

Refers to the "ultra-ultra-short wave" with a frequency between 300MHz and 3GHz, or a wavelength between 1m and 10cm, 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, high-frequency board plates are needed. Electromagnetic interference radio frequency RFRadioFrequency; radio waves, radio frequency RFIRadio-FrequencyInterference; radio frequency interference

SHFSuperHighFrequency; ultra-high frequency (ie microwave with frequency between 3 ~ 30GHz; wavelength of 1 ~ 10cm, can be used for long-distance wireless or satellite communications) UHFUltra- High Frequency; Ultra-high frequency electromagnetic waves refer to radio waves with a frequency of 300 to 3,000 MHz, or "ultra ultra short waves" with a wavelength of 1 to 10 dm (10 to 100 cm), and are often used in high-definition televisions. VHFVeryHighFrequency; Very high frequency electromagnetic waves (ie, ultrashort waves) refer to electric waves with a frequency between 30 and 300 MHz, with a wavelength between 1 and 10 m, used for broadcasting and television.