Research on Manual Routing Design of High Frequency PCB

writer: G January 22, 2019

Pay special attention to the following aspects in the wiring process of high-frequency PCB.

1. The direction of routing

The routing of the circuit is preferably in a straight line according to the flow direction of the signal, and can be completed by a 45° fold line or a circular arc curve when the transition is required, so that the external transmission and mutual coupling of the high frequency signal can be reduced. The wiring of the high-frequency signal lines should be as short as possible. According to the operating frequency of the circuit, the length of the signal line wiring is reasonably selected, which can reduce the distribution parameters and reduce the signal loss. When making a double panel, the wiring is preferably perpendicular, oblique or curved at the adjacent two levels. Avoid parallel to each other, which can reduce mutual interference and parasitic coupling.

The high-frequency signal line and the low-frequency signal line should be separated as much as possible, and if necessary, shielding measures should be taken to prevent mutual interference. For receiving signal input that is weaker, it is easy to be interfered by external signals. You can use the ground wire to shield it and enclose it or shield the high-frequency connector. Parallel routing should be avoided on the same level, otherwise distribution parameters will be introduced, which will affect the circuit. If it is unavoidable, a grounded copper foil can be introduced between the two parallel lines to form an isolation line.

In digital circuits, for differential signal lines, they should be routed in pairs, as far as possible, so that they are parallel, close to each other, and the length is not much different.

2. The form of routing

During the routing of the PCB, the minimum width of the trace is determined by the adhesion strength between the conductor and the insulating substrate and the current strength flowing through the conductor. When the thickness of the copper foil is 0.05 mm and the width is 1 mm to 1.5 mm, a current of 2 A can be passed. The temperature will not be higher than 3 °C, except for some special traces, the width of other wiring on the same level should be as uniform as possible. The spacing of the wiring in the high frequency circuit will affect the size of the distributed capacitance and inductance, thereby affecting signal loss, circuit stability, and signal interference. In high-speed switching circuits, the spacing of the wires will affect the signal transmission time and the quality of the waveform. Therefore, the minimum pitch of the wiring should be greater than or equal to 0.5 mm, and as long as it is allowed, the PCB wiring is preferably a relatively wide line.

The printed conductor should be spaced from the edge of the PCB by a certain distance (not less than the thickness of the board), which not only facilitates installation and machining, but also improves insulation performance.

When wiring is encountered in a line that can only be connected by a large circle, the flying line should be used, that is, the short-line connection is directly used to reduce the interference caused by long-distance wiring.

A circuit containing a magnetic sensing element is sensitive to a surrounding magnetic field, and a high-frequency circuit is susceptible to radiating electromagnetic waves when the wiring is bent. If a magnetic sensing element is placed in the PCB, the wiring corner should be kept at a certain distance therefrom.

Wiring on the same level is not allowed to cross. For possible intersecting lines, it can be solved by "drilling" and "winding", that is, letting a certain lead "drill" from the gap under the pins of other devices such as resistors, capacitors, and triodes, or from a possible crossover. One end of the lead "wounds" past. In special cases, if the circuit is complex, to simplify the design, it is also possible to solve the crossover problem with wire bridging.

When the operating frequency of the high-frequency circuit is high, it is also necessary to consider the impedance matching and antenna effect of the wiring.

3. Routing requirements for power and ground cables

According to the different working current, try to increase the width of the power cord. The high-frequency PCB should use a large-area ground wire and be placed on the edge of the PCB as much as possible to reduce the interference of external signals on the circuit. At the same time, the ground wire of the PCB can be in good contact with the casing, so that the grounding voltage of the PCB is closer to Earth voltage. The grounding method should be selected according to the specific situation. It is different from the low-frequency circuit. The grounding wire of the high-frequency circuit should be grounded or multi-point grounded. The grounding wire is short and thick to minimize the ground impedance. 3 times the standard of working current. The grounding wire of the speaker should be connected to the grounding point of the PCB amplifier output stage. Do not ground it at will.

In the wiring process, some reasonable wiring should also be locked in time to avoid repeated wiring. That is, execute the EditselectNet command to select Locked in the pre-routed properties to lock it no longer.