PCB Tech - Analysis of PCB design and signal integrity

(1) Signal integrity analysis

Factors associated with SI: reflection, crosstalk, radiation. Reflection is caused by impedance mismatch along the transmission path; Crosstalk is caused by line spacing; Radiation is related to the high-speed device itself and PCB design.

Transmission line judgment

According to the previous formula for judging high-speed signals, signal frequency and transmission path length should be taken into account to distinguish high-speed and low-speed signals.

Judgment steps:

1) Obtain the effective frequency of signal Fknee and line length L;

2) Using Fknee to calculate the effective wavelength of the signal λknee, namely λknee=C/Fknee;

3) Judge the relationship between L and 1/6xλknee, such as L "1/6xλknee, then the signal is high-speed signal, and vice versa;

The lambda knee = C/Fknee; Where C is the speed slightly lower than the speed of light, Fknee=0.5/Tr (10%~90%). It should also be noted that, for the signal of 100 MB frequency, if there is no ready-made board, the effective frequency Fknee can be estimated, Fknee is about 7 times of Fclock (signal cycle).

If L "1/6xλknee, it is regarded as a transmission line. The transmission line must consider the reflection problem of signal caused by impedance mismatch in the transmission process.

Reflection formula

Signal reflection ρ= (z2-z1)/(Z2+Z1);

Where Z2 is the line impedance behind the reflection point; Z1 is the line impedance before reflection;

The possible value of ρ is ±1, 0, which is absorbed completely at 0 and reflected at ±1. The reflection of the signal is caused by a mismatch in the impedance of the origin, transmission path, and terminal.

Reduced reflection method

In order to minimize the reflection of the signal, you need Z2 and Z1 to be as close as possible. There are several methods for impedance matching: transmitter series matching, receiver parallel matching, receiver partial voltage matching, receiver resistance and capacitance parallel matching, receiver diode parallel matching.

3) Partial pressure matching at the receiving end

4) Parallel matching of resistance and capacitance at the receiving end

Advantages: Low power consumption;

Disadvantages: there is a mismatch between the high and low levels of the receiving end, because of the existence of capacitance, the edge of the signal will change slowly.

(2) Signal loop

The signal loop mainly includes two paths, one is the driving path and the other is the loop path. The signal level measured at the sending end, transmission path and receiving end is essentially the voltage value at the corresponding position on the driving path and return path of the signal. These two paths are very important.

To provide a complete backflow path, note the following:

1. When the signal changes layers, do not change the reference layer. If the signal changes layers from signal layer 1 to signal layer 2, the reference layer is the bottom layer 1.

2. The network properties of the reference layer are not changed during signal layer switching. In other words, the reference layer of signal 1 is power layer 1/ ground 1, and after layer change, the reference layer of signal 1 is power layer 2/ ground 2. The network properties of the reference layer are GND or power, and the access of the return path can be realized by nearby GND or power hole. Here, in the case of high speed, the capacitance and inductive reactance of the through-hole can not be ignored. In this case, the through-hole should be minimized as far as possible, the influence of the impedance change caused by the through-hole itself and the influence on the signal backflow path should be reduced.

3. During signal layover, add a pass hole with the same properties as the reference layer near the signal pass hole.

4. If the network properties of two reference layers are different before and after layer replacement, the two reference layers should be close to each other to reduce the impedance between layers and the voltage drop on the return path.

5. When layer changing signals are dense, a certain distance should be kept between the nearby ground or power supply holes. When layer changing signals are numerous, more holes should be made to the ground or power supply.

(3) crosstalk

The solution to crosstalk is that high-speed signals, clock signals, other data signals, etc., should be spaced to meet the 3W principle.