(1) The power cord is an important way to EMI access circuit. Through the power cord, the external interference can be passed into the internal circuit, affecting the RF circuit indicator. In order to reduce electromagnetic radiation and coupling, the side, secondary side, and load side rings area of the DC-DC module are required. The power supply circuit has more complex, and its large current loop must be as small as possible. The power cord and ground line always have to be placed very close.
(2) If the switching power supply is used in the circuit, the peripheral layout of the switching power supply should comply with the principle of shorting the short power reflow path. Filter capacitors are close to switching power supply related pins. Use a common mode inductor, close to the switching power supply module.
(3) The power cord of the long distance on the single board cannot close or pass through the output of the cascading amplifier (gain than 45dB) and the input. Avoiding a power cord into an RF signal transmission path, which may cause self-excited or reducing the sector isolation. The long distance power cord needs to be added with high frequency filter capacity, even in the middle, also increase the frequency filter capacitor.
(4) The power entry of the RF PCB is combined with three filter capacitors, and the respective advantages of these three capacitors are used to filter out low, medium and high frequencies on the power line, respectively. For example: 10uf, 0.1uf, 100pf. And in accordance with the order of the power supply from the order of the large to small.
(5) Use the same set of power to give the small signal cascader amplifier feed, first start from the end level, in turn, power supply, so that the EMI generated by the end-level circuit is smaller. And each of the power filters has at least two capacitors: 0.1uf, 100PF. When the signal frequency is higher than 1 GHz, it is to increase 10PF filter capacitance.
(6) Commonly used to small power electronics filters, filter capacitors are close to the triode pin, and the high-frequency filter capacitor is closer to the pin. The triode is lowered. If the triode in the electronic filter is a high-frequency tube, it works in the zone, the layout of the peripheral device is unreasonable, and high frequency oscillation is easily generated in the power output. The linear regulator module may also have the same problem because there is a feedback loop in the chip, and the internal triode works in the zone. The high-frequency filter capacitor is required in the layout of the high-frequency filter capacitor to reduce the distribution inductance, destroy the oscillation conditions.
(7) The copper foil size of the PCB Power part conforms to the current thereof, and considers the margin (general reference to 1A / mm line width).
(8) The input and output of the power cord cannot be cross.
(9) Pay attention to power supply decoupling, filtering, preventing different units from generating interference through power cords, and power cords should be isolated between power lines. The power cord is isolated from other strong interference lines (such as CLK).
(10) The power supply wiring of the small signal amplifier requires copper skin and ground vias to avoid other EMI interference, and thereby deteriorate the quality of the signal.
(11) Different power layers should avoid overlapping in space. Mainly in order to reduce interference between different power supplies, in particular, the overlap problems of the power supply plane must be avoided, and the intermediate separatation can be considered when it is difficult to avoid it.
(12) The PCB plate layer is distributed to simplify subsequent wiring processing, for a four-layer PCB board (circuit board in WLAN), in most applications, the top layer of the circuit board and the RF lead, the second layer Systemically, the power supply portion is placed in the third layer, and any signal line can be distributed in the fourth layer.
The second layer uses a continuous ground layout for establishing an impedance controlled RF signal pathway, it is also easy to obtain as short as possible, and provide a height of electrical isolation of the layer and the third layer, so that between two layers Coupling. Of course, other slabs can also be used (especially when the circuit board has different layers), but the above structure is a proven successful example.
(13) Large-area power supplies can make the VCC wiring easily, but this structure is often a fuse that causes system performance deterioration, and connects all power leads together on a large plane will not be avoided. Noise transmission between. Conversely, if you use a star topology, the coupling between different power pins will be reduced.
The above figure shows a star-connected VCC wiring scheme, which takes the evaluation board from the MAX2826 IEEE 802.11A / G transceiver. A primary VCC node is established in the figure, which leads the power cord of different branches from that point to power the power pin of the RF IC. Each separate lead supply pin provides spatial separation between the pins, thus to reduce the coupling between them. In addition, each lead also has a certain parasitic inductor, which is best we want, which helps filter out high frequency noise on the power line.
When using a star topology VCC lead, it is necessary to take the appropriate power decoupling, while the decoupling capacitor exists a certain parasitic inductor. In fact, capacitance is equivalent to a series-connected RLC circuit, and the capacitor is dominant at a low frequency band, but at the self-excited oscillation frequency (SRF):
Thereafter, the impedance of the capacitor will present an inductance. From this, it can be seen that the capacitor has only decoupling when the frequency is close or lower than its SRF, and the capacitance is lowered in these frequency points.
A typical S11 parameter under different value values is given. From these curves, it can clearly see SRF, and it can also be seen that the greater the capacitance, the better the decoupling performance provided at a lower frequency (the lower the impedance presented) .
Place a large capacity capacitor at the primary node of the VCC star topology, such as 2.2μF. The capacitor has a lower SRF, and the establishment of a stable DC voltage is effective for eliminating the low frequency noise. Each power pin of the IC requires a low capacity capacitor (eg 10nF) to filter out high frequency noise that may be coupled to the power cord. For power pins that are powered by noise sensitive circuits, you may need to connect two bypass capacitors. For example, by one 10pF capacitor is parallel with a 10nf capacitor, it provides a wider frequency range decoupling to eliminate the impact of noise on the supply voltage. Each power pin should be carefully examined to determine how much of the decoupling capacitors and the actual frequency at which the circuit susceptible to interference noise.
Good power decoupling technology combines a rigorous PCB layout, VCC lead (star topology), can lay a solid foundation for any RF system design. Although there will be other factors in the actual design, there will be other factors of system performance indicators, but it has a "noise" power source is the basic element of optimizing system performance.
Reprinted from Verid Electronic Market Network.
Our other product:
