In the design of PCB board, with the rapid increase of frequency, there will be a lot of interference that is different from the design of low-frequency PCB board. In the actual research, we conclude that there are mainly four aspects of interference, including power noise, transmission line interference, coupling and electromagnetic interference (EMI). Through the analysis of various interference problems of high frequency PCB, combined with the practice of work, the effective solution is put forward.
In the high frequency circuit, the noise of the power supply has an obvious influence on the high frequency signal. Therefore, the first requirement is that the power supply is low noise. Obviously, the power supply has some impedance, and the impedance is distributed throughout the power supply, so the noise will also be added to the power supply. Then we should reduce the impedance of the power supply as much as possible, so it is better to have a proprietary power layer and connecting layer.
In high frequency circuit design, the power supply is designed as a layer, in most cases much better than a bus, so that the circuit can always follow the path with the least impedance. In addition, the power board must provide a signal circuit for all generated and received signals on the PCB. This minimizes the signal circuit and thus reduces noise, which is often overlooked by low-frequency circuit designers.
In PCB, there are only two kinds of transmission lines: ribbon line and microwave line. The biggest problem of transmission lines is reflection, which will cause many problems. For example, load signal will be the superposition of original signal and echo signal, which increases the difficulty of signal analysis. Reflection will cause return loss (return loss), and its influence on the signal is as serious as that of additive noise interference:
The point of impedance discontinuity is the point of transmission line mutation, such as straight corner, through hole, etc., should be avoided as far as possible. The methods are as follows: avoid straight corners of the line, and try to take an Angle of 45° or an arc, or a large curve;
Use as few through-holes as possible because each through-hole is an impedance discontinuity and the outer signal avoids passing through the inner layer and vice versa.
Because any pile line is a source of noise. If the pile line is short, it can be connected at the end of the transmission line; If the pile line is long, it will take the main transmission line as the source, which will generate a lot of reflection and complicate the problem. It is recommended not to use it.
As speed increases, EMI will become more and more serious and will show many aspects (such as electromagnetic interference at interconnects). High-speed devices are particularly sensitive to this, and thus will receive false signals at high speeds, while low-speed devices will ignore such false signals.
With this discussion, we can summarize the following principles that should be followed in the design of HIGH-FREQUENCY PCB:
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