In the design of PCB board, with the rapid increase of frequency, there will be many interference different from that of low frequency PCB board. Moreover, with the increasing frequency and the miniaturization and low cost of PCB board, the contradiction between these interference is becoming more and more complex. In practical research, we can conclude that there are four main aspects of interference, including power noise, transmission line interference, coupling, electromagnetic interference (EMI). By analyzing various interference problems of high frequency PCB and combining with work practice, an effective solution is put forward.
1. Power noise
In the high frequency circuit, the noise of power supply has an obvious effect on the high frequency signal. Therefore, the first requirement is that the power supply is low noise. Here, clean ground and clean power are equally important. Why? The power characteristics are shown in Figure 1. Obviously, the power supply has a certain impedance, and the impedance is distributed throughout the power supply, so the noise will be superimposed on the power supply. Then we should reduce the impedance of the power supply as much as possible, so it's better to have a special power supply layer and connection layer. In the design of high frequency circuit, the power supply is designed in the form of layers, which is much better than that in the form of buses in most cases, so that the circuit can always follow the path with the smallest impedance. In addition, the power board has to provide a signal loop for all generated and received signals on PCB, which can minimize the signal loop and reduce noise, which is often ignored by low frequency circuit designers.
There are several ways to eliminate power noise in PCB design.
(1) Pay attention to the through holes on the board: the through holes make it necessary to etch the openings on the power supply layer to allow space for the through holes to pass through. If the opening of the power supply layer is too large, the signal circuit will be affected, the signal will be forced to bypass, the area of the circuit will increase, and the noise will increase. At the same time, if some signal lines are concentrated near the opening and share this circuit, the common impedance will cause crosstalk.
(2) Connecting wires need enough ground wires:
Each signal needs its own special signal loop, and the area of the loop is as small as possible, that is to say, the signal and the loop should be parallel.
(3) The analog and digital power supply should be separated: High frequency devices are generally very sensitive to digital noise, so they should be separated. Connect at the entrance of the power supply. If the signal crosses the analog and digital parts, a loop can be placed at the signal crossing to reduce the area of the ring road. Spanning between digital and analog for signal loop.
(4) Avoid overlapping of separate power supply between different layers: otherwise circuit noise can easily be coupled through parasitic capacitance.
(5) Isolation of sensitive elements, such as PLL.
(6) Placing the power cord: In order to reduce the signal loop, the noise can be reduced by placing the power cord on the side of the signal line, as shown in Figure 4.
2. Transmission line
There are only two kinds of transmission lines in PCB: stripline and microwave line. The biggest problem of transmission line is reflection, which will cause many problems, such as the overlap of the original signal and the echo signal, which will increase the difficulty of signal analysis. Reflection can cause echo loss, and its effect on signal is as serious as that of additive noise interference.
(1) The signal reflected back to the source will increase the system noise, making it more difficult for the receiver to distinguish the noise from the signal.
(2) Any reflected signal will basically reduce the quality of the signal and change the shape of the input signal. In principle, the main solution is impedance matching (for example, the interconnection impedance should match the system impedance very well), but sometimes the calculation of impedance is more difficult, you can refer to some transmission line impedance calculation software.
The methods of eliminating transmission line interference in PCB design are as follows:
(a) Avoiding impedance discontinuity of transmission lines. The point of discontinuity of impedance is the point of sudden change of transmission line, such as right corner and through hole, which should be avoided as far as possible. The methods are as follows: Avoid the right corner of the line, and take 45 degree angle or arc as far as possible. Large corner can also be used. Use as few holes as possible, because each hole is an impedance discontinuity.
(b) Do not use pile lines. 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 length of the pile line is long, the main transmission line will be used as the source, which will produce great reflection and complicate the problem. It is recommended not to use it.
(1) Common impedance coupling: It is a common coupling channel, that is, the interference source and the interfered equipment often share some conductors (such as loop power supply, bus, common ground, etc.).
(2) Field common-mode coupling will cause the common-mode voltage of the emitter on the loop and common reference plane formed by the disturbed circuit. If the magnetic field is dominant, the common-mode voltage generated in the series earth circuit is the area of Vcm=-(B/t)* If it is an electromagnetic field, when its electric field value is known, its induced voltage is Vcm=(L*h*F*E)/48. The formula is applicable to L(m) = 150 MHz or less, beyond which the calculation of maximum induced voltage can be simplified as: Vcm = 2*h*E.
(3) Differential mode field coupling: refers to the direct radiation received by a pair of guided wires or a lead on a circuit board and its loop. This coupling will be greatly reduced, so two wires can be twisted together to reduce interference.
(4) Inter-line coupling (crosstalk) can make any line equal to the unwanted coupling between parallel circuits, which will seriously damage the performance of the system. It can be divided into capacitive crosstalk and sensory crosstalk. The former is because the parasitic capacitance between the lines makes the noise on the noise source coupled to the noise receiving line through current injection. The latter can be imagined as signal coupling between the primary stages of an unwanted parasitic transformer. The magnitude of the inductive crosstalk depends on the proximity of the two loops, the area of the loop and the impedance of the load affected.
(5) Power line coupling: refers to the AC or DC power lines are subject to electromagnetic interference, the power line will transmit these interference to other equipment.
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