Como fazer o layout de PCB de alta frequência
PCB de alta frequência refere-se à frequência eletromagnética das placas de circuito especiais superiores para alta frequência (frequência superior a 300 MHz ou comprimento de onda inferior a 1 metro) e microondas (frequência superior a 3 GHz ou comprimento de onda inferior a 0.1 metros) na área de PCB, está no substrato de micro-ondas placas laminadas revestidas de cobre no uso de placas de circuito rígidas comuns fabricadas usando alguns dos processos ou no uso de métodos de tratamento especiais e na produção de placas de circuito. O projeto do circuito da placa de circuito de alta frequência é um processo muito complexo, o layout de cada linha deve estar no lugar, o próximo se concentrará em métodos de layout de placas de circuito de alta frequência.
Como fazer o layout de PCB de alta frequência？
1. Fiação de placa multicamada
Os circuitos de alta frequência são frequentemente altamente integrados, densidade de fiação, o uso de placa multicamadas é necessário para a fiação, mas também um meio eficaz de reduzir a interferência. Na fase de Layout de PCB, uma escolha razoável de um certo número de camadas de tamanho de placa de circuito impresso, pode fazer uso total da camada intermediária para configurar a blindagem, uma melhor percepção da proximidade do aterramento, e efetivamente reduzir a indutância parasita e encurtar o comprimento da transmissão do sinal, mas também reduz significativamente a interferência cruzada do sinal, etc., todos os quais são benéficos para a confiabilidade dos circuitos de alta frequência.
2. Dispositivos eletrônicos de alta velocidade entre os pinos do cabo dobram quanto menos, melhor
A fiação do circuito de alta frequência do cabo é melhor usar uma linha reta, a necessidade de virar, disponível dobra de 45 graus ou giro em arco, este requisito nos circuitos de baixa frequência é usado apenas para melhorar a resistência da adesão da folha de cobre, enquanto nos circuitos de alta frequência para atender a este requisito pode reduzir os sinais de alta frequência para o exterior do lançamento e acoplamento entre si.
3. Pinos do dispositivo de circuito de alta frequência entre o condutor, quanto mais curto, melhor
A intensidade da radiação do sinal é proporcional ao comprimento da linha de sinal, sinal de alta frequência leva mais tempo, mais fácil é acoplado aos componentes próximos a ele, então para sinais como o relógio, cristal, Dados DDR, Linhas LVDS, Linhas USB, Linhas HDMI, e outras linhas de sinal de alta frequência são necessárias tanto quanto possível, quanto mais curta a linha, Quanto melhor.
4. O quarto truque: pinos do dispositivo de circuito de alta frequência entre a alternância da camada de chumbo, quanto menos, melhor!
O chamado “less alternation between the layers of the lead, Quanto melhor” means that the components used in the connection process of the hole (Via) the less the better. According to the side, a hole can bring about 0.5pF of distributed capacitance, reduce the number of holes can significantly improve speed and reduce the possibility of data errors.
5. Pay attention to the signal line close to the parallel alignment of the introduction of “Crosstalk”
High-frequency circuit wiring should pay attention to the signal lines in close proximity to the parallel alignment of the introduction of “Crosstalk”, crosstalk refers to the coupling phenomenon between the signal lines are not directly connected. As high-frequency signals along the transmission line is transmitted in the form of electromagnetic waves, the signal line will play the role of the antenna, the energy of the electromagnetic field will be emitted around the transmission line, the signal due to the mutual coupling of the electromagnetic field and the resulting undesired noise signal is called crosstalk. parameters of the PCB board layer, the spacing of the signal line, the driving end and the receiving end of the electrical characteristics, as well as the signal line termination method of the crosstalk have a certain impact. Então, in order to reduce the crosstalk of high-frequency signals Therefore, in order to reduce the crosstalk of high-frequency signals, it is required to do the following as much as possible when wiring:
Under the condition that the wiring space allows, insert a ground line or ground plane between two lines with serious crosstalk, which can play the role of isolation and reduce crosstalk. When the space around the signal line itself there is a time-varying electromagnetic field, if you can not avoid parallel distribution, parallel signal lines can be arranged on the opposite side of a large area of “chão” to significantly reduce the interference.
Under the premise of wiring space permits, increase the spacing between adjacent signal lines, reduce the parallel length of the signal lines, clock lines as perpendicular as possible with the key signal lines rather than parallel. If the parallel alignment within the same layer is almost unavoidable, in two adjacent layers, the direction of the alignment must be perpendicular to each other.
Em circuitos digitais, the clock signal is usually a fast-edge signal, external crosstalk. Portanto, no design, the clock line should be surrounded by ground wires and more ground holes to reduce the distribution capacitance, thereby reducing crosstalk. Clock on high-frequency signals try to use low-voltage differential clock signals and packet ground way, need to pay attention to the integrity of the packet ground hole punching.
Idle not used inputs do not hang, but will be grounded or connected to the power supply (power in the high-frequency signal loop is also the ground), because the suspended line may be equivalent to the transmitting antenna, grounding will be able to inhibit the emission. Practice has shown that this approach to eliminate crosstalk can sometimes be immediately effective.
6. IC block power supply pins to increase the high-frequency decoupling capacitance
Each integrated circuit block power supply pin near the increase of a high-frequency decoupling capacitance. Increase the power supply pin high-frequency decoupling capacitance, can effectively inhibit the power supply pin on the high-frequency harmonics form interference.
7. Isolation of high-frequency digital and analog signal ground lines
Analog ground, digital ground line to the public ground with high-frequency choke bead connection or direct isolation and choose a suitable place for single-point interconnection. High-frequency digital signal ground potential is generally inconsistent, there is often a certain voltage difference between the two directly; e, high-frequency digital signal ground is often with a very rich high-frequency signal harmonic components, when directly connected to the digital signal ground and analog signal ground, high-frequency signal harmonics will be coupled through the ground to the analog signal to interfere with the way.
Em geral, the high-frequency digital signal ground and analog signal ground is to do the isolation, can be used in the appropriate location of a single point of interconnection, or the use of high-frequency choke bead interconnection.
8. Avoid the formation of the loop alignment
Various types of high-frequency signal alignment try not to form a loop, if you can not avoid the loop area should be as small as possible.
9. Must ensure good signal impedance matching
Signal in the transmission process, when the impedance mismatch, the signal will occur in the transmission channel signal reflection, reflection will make the synthesized signal overshoot, resulting in the signal fluctuations in the vicinity of the logic threshold.
Eliminate the reflection of the fundamental approach is to make the transmission signal impedance is well-matched, due to the load impedance and the transmission line of the characteristic impedance of the larger the difference between the reflection of the larger, so should be as much as possible to make the signal transmission line of the characteristic impedance of the load impedance and load impedance is equal; ao mesmo tempo, but also to pay attention to the PCB on the transmission line can not be a sudden change or corners, as far as possible to maintain the transmission line at all points of the impedance continuity, or in the transmission line between the various segments will be a reflection.
10. Maintain the integrity of the signal transmission
Maintain the integrity of the signal transmission, to prevent the “ground bounce phenomenon” caused by the division of the ground.
