Conduct wiring according to the 50 ohm impedance line width, try to exit the wire from the center of the pad, make the wire straight, and try to walk on the surface layer. Make a 45 degree angle or circular arc routing at the place where you need to turn. It is recommended to turn on both sides of the capacitor or resistor. If you encounter device wiring matching requirements, please strictly follow the reference value length on the datasheet. For example, the requirements for the wiring length between an amplifier tube and a capacitor (or the wiring length between inductors), etc.

1. General practices

During PCB design, in order to make the design of high-frequency circuit boards more reasonable and have better anti-interference performance, the following aspects should be considered:

 

(1) When routing high-frequency circuit boards in PCB design with a reasonable number of layers, using the middle inner layer plane as the power and ground layers can effectively reduce parasitic inductance, shorten the length of signal lines, and reduce cross interference between signals.

 

(2) Routing method: The wiring must be turned at a 45 ° angle or circular arc, which can reduce the transmission of high-frequency signals and mutual coupling.

 

(3) The shorter the routing length, the better. The shorter the parallel distance between two lines, the better.

 

(4) The smaller the number of vias, the better.

 

(5) Inter layer wiring direction The inter layer wiring direction should take the vertical direction, that is, the top layer is in the horizontal direction, and the bottom layer is in the vertical direction, which can reduce interference between signals.

 

(6) Adding copper to ground can reduce interference between signals.

 

(7) Packet processing of important signal lines can significantly improve the anti-interference ability of the signal. Of course, packet processing can also be performed on interference sources to prevent them from interfering with other signals.

 

(8) The signal line cannot be looped and needs to be routed in a daisy chain manner.

2. Wiring Priorities

 

Priority of key signal lines: simulate the priority wiring of key signals such as small signals, high-speed signals, clock signals, and synchronization signals

Density priority principle: start wiring from the components with the most complex connection relationship on the single board. Start wiring from the area with the densest connection on the board

Note:

A. Try to provide special wiring layers for key signals such as clock signals, high-frequency signals, and sensitive signals, and ensure their minimum loop area. If necessary, methods such as manual priority wiring, shielding, and increasing safety spacing should be adopted. Ensure signal quality.  

B. The EMC environment between the power supply layer and the stratum is poor, so it is necessary to avoid arranging signals sensitive to interference.

C. Networks with impedance control requirements should be wired as much as possible according to line length and width requirements.

 

3. Clock wiring

 

Clock lines are one of the factors that have the greatest impact on EMC. The clock line should be less perforated, and parallel routing with other signal lines should be avoided as much as possible. It should be kept away from general signal lines to avoid interference with signal lines. At the same time, avoid the power supply part on the board to prevent mutual interference between the power supply and clock.

If there is a dedicated clock generator chip on the board, it should not be wired underneath, and copper should be laid underneath it. If necessary, it can also be specially cut. For many chips, there are reference crystal oscillators, and the wires should not be routed below these crystal oscillators, and copper should be laid for isolation.

 

4. Right-angle routing

 

Right angle routing is generally a situation that needs to be avoided in PCB wiring, and has almost become one of the criteria for measuring the quality of wiring. So how much impact does right angle routing have on signal transmission? In principle, right-angle routing can change the linewidth of transmission lines, resulting in impedance discontinuity. In fact, not only right angle wiring, sharp angle wiring, and sharp angle wiring may cause impedance changes.

The impact of right-angle routing on signals is mainly reflected in three aspects:

One is that the corner can be equivalent to a capacitive load on the transmission line, slowing down the rise time;

The second is that impedance discontinuity can cause signal reflection;

The third is the EMI generated by the right angle tip.

 

5. Acute angle

 

(1) For high-frequency currents, when the corners of a wire exhibit a right angle or even an acute angle, the magnetic flux density and electric field intensity are relatively high near the corner, which can radiate strong electromagnetic waves. Moreover, the amount of inductance here will be relatively large, and the inductive reactance is also larger than that of an obtuse angle or a rounded corner.

 

(2) For bus wiring of digital circuits, the corners of the wiring present obtuse angles or rounded corners, and the area occupied by the wiring is relatively small. Under the same line spacing conditions, the total width of the line spacing is 0.3 times less than that of a right angle turn.

6. Differential routing

 

See: Difference distribution line and impedance matching

 

Differential signal is increasingly used in high-speed circuit design, and the most critical signals in the circuit often use differential structure design. Definition: Generally speaking, the driver sends two equal and inverted signals, and the receiver compares the difference between the two voltages to determine whether the logical state is "0" or "1". The pair of wires carrying differential signals is called differential wiring.

 

The most obvious advantages of differential signals compared to ordinary single ended signal routing are reflected in the following three aspects:

A. Strong anti-interference ability, because the coupling between two differential routing lines is very good. When there is external noise interference, they are almost simultaneously coupled to both lines, while the receiver only cares about the difference between the two signals, so external common mode noise can be completely eliminated.

B. It can effectively suppress EMI. Similarly, due to the opposite polarity of two signals, the electromagnetic fields radiated by them can cancel each other out. The closer the coupling, the less electromagnetic energy released to the outside world.

 

C. Accurate timing positioning. Due to the fact that the switching change of the differential signal is located at the intersection of two signals, unlike ordinary single ended signals, which rely on high and low threshold voltages to determine, it is less affected by process and temperature, which can reduce timing errors, and is more suitable for circuits with low amplitude signals. The current popular LVDS (low voltage differential signaling) refers to this small amplitude differential signal technology.

 

For PCB engineers, the most important concern is how to ensure that these advantages of differential routing can be fully utilized in actual wiring. Perhaps anyone who has been exposed to Layout will understand the general requirements for differential routing, which are "equal length and equal distance".

Equalization is to ensure that two differential signals maintain opposite polarity at all times, reducing common mode components; Equidistant is mainly to ensure that the differential impedance of the two is consistent and reduce reflection. "The principle of being as close as possible" is sometimes one of the requirements of differential routing.

 

 

7. Serpentine line

 

Serpentine is a type of routing method often used in Layout. Its main purpose is to adjust the delay to meet the system timing design requirements. Designers should first have the understanding that serpentine wires can damage signal quality and change transmission delay, and should be avoided when wiring. However, in practical design, in order to ensure sufficient signal retention time or reduce the time offset between the same group of signals, it is often necessary to intentionally wire wrap.

 

Note:

Paired differential signal lines are generally routed in parallel, with minimal through-hole drilling. When it is necessary to drill holes, the two lines should be drilled together to achieve impedance matching.

A group of buses with the same attributes should be routed side by side as much as possible, with equal lengths as possible. The vias from the patch pad should be as far away from the pad as possible.