Microwave Tech - Solve the High-Frequency PCB board power supply noise interference

In the high-frequency PCB board, one of the most important interference is power supply noise. Based on the systematic analysis of the characteristics and causes of power supply noise on high-frequency PCB board, and combined with engineering applications, some effective and simple solutions are put forward.

Distributed noise is due to the inherent impedance of the power supply itself. In the high-frequencycircuit board, power supply noise has a great influence on the high-frequency signal. Therefore, a low noise power supply is needed first. Clean land is just as important as clean power, Common mode field interference. Refers to the noise between the power supply and the ground, which is caused by the interference of a power supply by the loop formed by the interfered circuit and the common-mode voltage caused by the common reference surface, the value of which depends on the relative strength of the electric and magnetic fields.

1., Analysis of power supply noise

Power supply noise refers to the noise generated by the power supply itself or induced by disturbance. Its interference is manifested in the following aspects:

1) Distributed noise is caused by the inherent impedance of the power supply itself. In the high-frequency circuit, power supply noise has a great influence on the high-frequency signal. Therefore, a low noise power supply is needed first. Clean land is just as important as clean power.

An ideal power supply has no impedance and therefore no noise. However, in the actual situation, the power supply has a certain impedance, and the impedance is distributed on the entire power supply, so the noise will also be superimposed on the power supply. Therefore, the impedance of the power supply should be reduced as much as possible, and it is best to have a special power layer and ground layer. In the high-frequency circuit design, it is generally better to design the power supply as a layer rather than as a bus, so that the loop can always follow the path with the lowest impedance. In addition, the power board has to provide a signal loop for all generated and received signals on the PCB, thus minimizing the signal loop and thus reducing noise.

2) Power cable coupling. After the AC or DC power cables receive electromagnetic interference, the power cables transmit the interference to other devices. This is the indirect interference of the power supply noise to the high-frequency circuit. It should be noted that the noise of the power supply is not necessarily generated by itself, but may also be the noise induced by external interference, and then the noise is superimposed with the noise produced by itself (radiation or conduction) to interfere with other circuits or devices.

3) Common mode field interference. Refers to the noise between the power supply and the ground, which is caused by the interference of a power supply by the loop formed by the interfered circuit and the common-mode voltage caused by the common reference surface, the value of which depends on the relative strength of the electric and magnetic fields.

On this channel, a drop in the Ic will cause a common-mode voltage in the series current loop, affecting the receiving part. If the magnetic field dominates, the value of the common-mode voltage generated in the series earth loop is:

Vcm = -- (△B/△t) * S (1)

In Equation (1), δ B is the change of magnetic induction intensity, Wb/m2; S is the area, m2.

If it is an electromagnetic field and its electric field value is known, its induced voltage is:

Vcm = (L*h*F*E/48) (2)

Formula (2) is generally applicable below L=150/F, where F is the electromagnetic wave frequency MHz.

If this limit is exceeded, the calculation of the maximum induced voltage can be simplified as:

Vcm = 2*h*E (3)

3) Differential mode field interference. Interference between the power supply and the input and output power cables. In the actual PCB design, the author found that its proportion in the power supply noise is very small, so it can not be discussed here.

4) Interline interference. Interference between power lines. When there is mutual capacitance C and mutual inductance M1-2 between two different parallel circuits, if there is voltage VC and current IC in the interference source circuit, then the interfered circuit will appear:

A. The voltage coupled through the capacitive impedance is

Vcm = Rv* c1-2 *△Vc/△t (4)

In Formula (4), Rv is the parallel value of the interference circuit's local resistance and remote resistance.

B. Series resistance through an inductive coupling

V = M1-2*△Ic/△t (5)

If there is common mode noise in the interference source, the interference between lines is generally in two forms: common mode and differential mode.

2, Countermeasures to eliminate noise interference of power supply

According to the above analysis of the different forms of power noise interference and its causes, can be targeted to destroy the conditions of its occurrence, can effectively suppress the interference of power noise. The solutions are:

1) Pay attention to the through-hole on the plate. Through holes require etched openings in the power layer to leave space for through holes to pass through. If the opening of the power supply layer is too large, the signal circuit will be affected and the signal will be forced to bypass, the circuit area will increase and the noise will increase. At the same time, if some signal lines are concentrated near the opening and share this section of the circuit, the common impedance will cause cross-talk.

2) Place the power supply noise filter. It can effectively suppress the internal noise of the power supply and improve the anti-interference and security of the system. And it is a two-way rf filter, not only can filter out the noise interference from the power cord (to prevent the interference of other equipment), but also can filter out the noise generated by itself (to avoid the interference of other equipment), to the series mode, common-mode interference has an inhibitory effect.

3) Power isolation transformer. By separating the power loop or the common-mode ground loop of the signal cable, it can effectively isolate the common-mode loop current generated at high-frequency circuit board.

4) Power regulator. The recovery of a cleaner power supply can greatly reduce the power noise level.

5) Wiring. The input and output lines of the power supply should not be placed at the edge of the media board. Otherwise, radiation is likely to be generated and other circuits or devices may be interfered with.

6) Separate analog and digital power supplies. High-frequency devices are generally very sensitive to digital noise, so they should be separated and connected to the power inlet. If the signal is to span both analog and digital parts, a loop can be placed across the signal to reduce the loop area.

7) Avoid overlapping of separate power sources between different layers. Stagger it as much as possible, otherwise, power supply noise can easily pass through parasitic capacitive coupling.

8) Isolation of sensitive components. Some elements, such as the phase-locked loop (PLL), are very sensitive to power supply noise and should be kept as far away from the power supply as possible.

9) Enough ground wires are needed for the connection. Each signal needs to have its proprietary signal loop, and the loop area of the signal and loop should be as small as possible, that is, the signal and the loop should be parallel.

10) Place the power cord. To reduce the signal loop, the noise can be reduced by placing the power line on the edge of the signal line.

11) To prevent the interference of the power supply noise to the circuit board and the cumulative noise caused by the external interference to the power supply, a bypass capacitor can be connected on the interference path (except radiation) so that the noise can be bypassed to the ground to avoid interference with other equipment and devices.

3, Conclusion

Power supply noise is produced from the power supply directly or indirectly to the, and interference, the circuit in suppressing the influence of it on the circuit, should follow a general principle, that is: on the one hand, as far as possible to prevent the power supply noise on the influence of the high-frequency circuit board, on the other hand, also want to minimize the impact on the power supply or circuit, so as not to deteriorate the power noise.