Design of high frequency PCB wiring

PCB is also known as the Printed Circuit Board (Printed Circuit Board), it can realize the circuit connection between electronic components and function realization, is also an important part of the power supply circuit design. Today, this article will introduce the high frequency circuit wiring skills in PCB design.

Multilayer wiring:

High frequency circuit often has high integration, high wiring density, using multilayer board is not only necessary for wiring, but also an effective means to reduce interference. In the stage of PCB Layout, reasonable selection of a certain number of layers of printed board size, can make full use of the intermediate layer to set shielding, better realize the nearby grounding, and effectively reduce the parasitic inductance and shorten the transmission length of the signal, but also greatly reduce the signal cross interference, all these methods are beneficial to the reliability of high frequency circuit. In the same material, the noise of four layers is 20dB lower than that of two panels. However, there is also a problem, the higher the number of PCB half-layers, the more complex the manufacturing process, the higher the unit cost, which requires that in the PCB Layout, in addition to the selection of the appropriate number of layers of PCB board, also need to carry out reasonable component layout planning, and adopt the correct wiring rules to complete the design.

1, high-frequency circuit device pin between the lead layer alternating less, the better

The phrase "the less interlayer alternations of leads, the better" means that the fewer holes (Via) used in the connection of components are the better. A single pass hole brings about 0.5pF of distributed capacitance, and reducing the number of pass holes significantly increases speed and reduces the possibility of data errors.

2. The shorter the lead between the pins of the high-frequency circuit devices, the better

The radiation intensity of the signal is proportional to the length of the signal line. The longer the high-frequency signal lead, the easier it is to be coupled to the components close to it. Therefore, for high frequency signal lines such as signal clock, crystal oscillator, DDR data, LVDS wire, USB cable, HDMI cable, etc., the shorter the cable is required as far as possible.

3. The less the lead bends between the pins of high-speed electronic devices, the better

It is best to use the lead of high frequency circuit wiring fully straight line, need to turn, can be used 45 degrees fold line or arc turn, this requirement in low frequency circuit is only used to improve the copper foil fixation strength, and in high frequency circuit, meet this requirement can reduce the high-frequency signal external emission and mutual coupling.

4. Pay attention to "crosstalk" introduced by short distance parallel running of signal line

High frequency circuit wiring should pay attention to "crosstalk" caused by short distance parallel routing of signal lines. Crosstalk refers to the coupling phenomenon between signal lines that are not directly connected. Since high-frequency signals are transmitted along the transmission line in the form of electromagnetic waves, signal lines will play the role of antennas, and the energy of electromagnetic fields will be transmitted around the transmission line. The unwanted noise signals generated between signals due to the mutual coupling of electromagnetic fields are called Crosstalk. The parameters of PCB board layer, the distance between signal wires, the electrical characteristics of the driver and receiver end, and the connection mode of signal wires all have a certain influence on the crosstalk. So in order to reduce the crosstalk of high frequency signals, the following points should be done as far as possible when wiring:

(1) If the wiring space allows, insert a ground or ground plane between the two lines with serious crosstalk, which can play the role of isolation and reduce crosstalk;

(2) When time-varying electromagnetic fields exist in the space around the signal line, if parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of the parallel signal line to greatly reduce interference;

(3) Increase the spacing between adjacent signal lines, reduce the parallel length of signal lines, and try to make the clock line perpendicular to the key signal line rather than parallel;

(4) If parallel routing within the same floor is almost unavoidable, the directions of the two adjacent floors must be perpendicular to each other;

(5) In digital circuits, the usual clock signals are those with fast edge changes and large external crosstalk. Therefore, in the design, the clock line should be surrounded by the ground line and more ground holes to reduce the distributed capacitance, so as to reduce crosstalk;

(6) For the high frequency signal clock, try to use the low voltage differential clock signal and include the ground, it is necessary to pay attention to the integrity of the ground punching;

(7) Do not suspend the idle input terminal, but ground it or connect it to the power supply (the power supply is also ground in the high-frequency signal loop), because the suspended line may be equivalent to the transmitting antenna, grounding can inhibit the transmission. Eliminating crosstalk in this way has sometimes proved to be effective immediately.

5, high frequency digital signal ground and analog signal ground to do isolation

When analog ground wire and digital ground wire are connected to the public ground wire, high frequency choke magnetic beads should be used to connect or directly isolate and select the appropriate ground single point interconnection. The ground potential of ground wires of high frequency digital signals is generally inconsistent, and there is often a certain voltage difference between the two directly. Moreover, ground wires of high frequency digital signals are often rich in harmonic components of high frequency signals. When the ground wires of digital signals and analog signal ground wires are directly connected, the harmonics of high frequency signals will interfere with analog signals by means of ground wire coupling. Therefore, under normal circumstances, the ground wire of the high frequency digital signal and the ground wire of the analog signal should be isolated, which can be used in the way of single point interconnection at the appropriate position, or the way of high frequency choke magnetic bead interconnection.