How PCB Designers Should Consider Signal Levels and Operating Frequency

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How PCB Designers Should Consider Signal Levels and Operating Frequency [Copy link]

PCB designers should first know the direction of the signal current of a circuit schematic. However, this question is very simple. According to international practice, the input terminal is located on the left side of the circuit schematic, and the output terminal is located on the right side of the circuit schematic, which is clear at a glance. Next, the problem that needs to be understood is the magnitude of the signal current in the circuit and the operating frequency. When the signal current magnitude is expressed by gain, the understanding of the current magnitude can also be replaced by the understanding of the signal level. In general, the current at the output end of an analog circuit is greater than the current at the input end; in most cases, the current of a digital circuit is much greater than the current of an analog circuit. When a small signal current circuit is adjacent to a large signal current circuit, or when the input and output ends of the same circuit are close together, the circuit is prone to unstable state or self-excited oscillation, which must be absolutely avoided. For example, in digital circuits, the magnitude of the signal voltage is at the same level as the magnitude of the power supply voltage, while the signal voltage of the high-frequency circuit at the receiving end of radios and televisions is only about 1/180,000 of it. Therefore, even if only 1/500,000 of the digital circuit signal is inserted into the latter high-frequency circuit, it will seriously hinder the reception and viewing of sound signals and television signals. For example, when designing the circuit of a (analog) CD player or a digital CD player on a PCB, there are both analog circuits and digital circuits in the printed circuit board of the entire electrical device. At this time, special attention should be paid to prevent the signal of the digital circuit from interfering with the normal operation of the analog circuit. For this reason, this type of circuit schematic is often first divided into block diagrams with different functions in each part, and clearly marked which wires are signal wires, which wires are power wires, and which wires are ground wires or shielding wires.

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