Key points of RF circuit design

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Stacking of circuit boards

Four-layer board, six-layer board, eight-layer board commonly used structure:

The second layer as the complete ground plane, the important signal line on the top layer (including the RF cable), so that the impedance is well controlled.

You can calculate single-ended impedance with polar or some software comes with an impedance calculator.

Impedance control after our principle design and simulation, one of the notable things in layout is impedance control. As we all know, we should try to ensure that the characteristics of the route is 50 ohms, which is mainly related to the line width, in this case, is two and a half, in polar using surface coplanar line model for impedance calculation, we can get a set of ideal values: Height (H) = 39.6mil, Track (W) =30mil, Track (W1) =30mil,thickness=1oz=1.4mil, Separation (S) =7mil, dielectric (Er) = 4.2, the corresponding characteristic impedance is 52.14 ohm, meet the requirements. As highlighted in the line is such an RF route. Placement of RF componentsAs far as possible to make the length of the line is short, the more compact components placed the better (except for special requirements), at the same time, it will be as far as possible to ensure that the placement of the components of the wiring is very advantageous (do not make the line around to go around). For example, is the RF power amplifier (pa,power amplifier) around the device placement, we see that the distance between components is very small. the problems that should be paid attention to in RF RouteAs mentioned earlier, the length of the RF line to be as short as possible, the line width in strict accordance with the calculated value to set. In the line is especially to note that the RF line does not have any sharp vertices, in the transition of the line, it is best to use an arc to achieve, such as the next, in the multilayer board of the line, there may be important RF lines to produce inevitable crossover, then we will use the most we do not want to use: Vias. In this way, there will be a part of the RF signal line to the bottom layer or even the middle layer, but no matter which layer, the RF line must have a reference plane, then a noteworthy problem is not to cross the layer, or not to make the ground plane discontinuous. over-hole placementThe placement of vias is really a complicated matter, this article only discusses that kind of grounding vias. First, the RF line next to the ground is best to pass through the hole to wear, to the bottom or middle layer of the ground plane, this can be any interference signal or radiation has the shortest access to the ground, but the hole and RF signal line distance can not be too close, otherwise it will seriously affect the RF signal quality,  In the actual design process can be flexibly grasped, for example, we see that the highlight of the line two layer of the signal is a lot of holes. Secondly, in the larger area of the ground plane, we usually place a lot of vias for connecting different layers of ground. This in the RF circuit wiring, should pay attention to is bigger than the hole to have no regular dozen, the best can be made into a diamond, so as to suppress all kinds of interference to the maximum. RF Circuit Power Design Considerations(1) The power cord is an important way of EMI access circuit. Through the power line, external interference can be passed into the internal circuit, affecting the RF circuit indicators. In order to reduce electromagnetic radiation and coupling, it is necessary to minimize the loop area of the first side, two sides and load side of the DC module. The power circuit, no matter how complex the form, its large current loop should be as small as possible. The power cord and ground are always placed very close. (2) If switching power supply is used in the circuit, the peripheral device layout of the switching power supply should conform to the principle of the shortest return path of each power. The filter capacitor should be close to the switching power related pins. Use common-mode inductors close to the switching power supply module. (3) The power cord on a single board can not be approached at the same time or through a cascade amplifier (gain greater than 45dB) near the output and input end. Avoid the power cord becoming an RF signal transmission path that may cause self-excitation or reduce sector isolation. Long-distance power lines need to add high-frequency filter capacitors at both ends, even in the middle of the high-frequency filter capacitor. (4) RF PCB Power inlet combination of three filter capacitors, using the respective advantages of these three capacitors to filter out the power line of low, medium and high frequency. For example: 10UF,0.1UF,100PF. and follow the input pins of the power supply in order from large to small. (5) The same group of power supply to the small signal cascade amplifier feed, should first start from the last stage, in turn, forward-level power supply, so that the last stage of the circuit generated by EMI to the front stage less impact. And at least two capacitors per level of power filter: 0.1UF,100PF. When the signal frequency is higher than 1GHz, the 10PF filter capacitor should be added. (6) commonly used to small power electronic filter, filter capacitance near the transistor pin, high-frequency filter capacitance closer to the pin. Transistor selection cutoff frequency is low. If the transistor in the electronic filter is a high-frequency tube, working in the amplification area, the peripheral device layout is not reasonable, at the power output can easily produce high-frequency oscillation. Linear regulators may also have the same problem because the chip memory is in the feedback loop, and the internal transistor is working in the amplification area. In the layout, high-frequency filter capacitors are required to be close to the pins to reduce the distributed inductance and destroy oscillation conditions. (7) The power portion of the PCB size of the copper foil in line with the maximum current flow, and consider the allowance (the general reference is 1a/mm line width). (8) The input and output of the power cord cannot be crossed. (9) Pay attention to the power supply decoupling, filtering, to prevent the different units through the power line interference, power wiring should be separated from each other. The power cord is isolated from other strong interference lines (such as CLK) with ground wires. (10) The power wiring of the small signal amplifier needs the copper skin and the grounding hole to isolate, avoids other EMI interference to enter, thus worsens this level signal quality. (11) The different power layers should avoid overlapping in space. Mainly in order to reduce the interference between different power supply, especially some voltage difference between the power supply, the power plane overlap problem must try to avoid, it is difficult to avoid when you can consider the middle interval strata. PCB board layer distribution simplifies subsequent wiring processing,A four-layer PCB board (commonly used in WLANs), where components and RF leads are placed on the top of the board in most applications, the second layer is systematically, the power supply is placed on the third layer, and any signal lines can be distributed on the fourth floor. The second layer uses a continuous ground plane layout that is necessary to establish an impedance-controlled RF signal path, which facilitates the shortest possible ground loops and provides a high degree of electrical isolation for the first and third layers, minimizing coupling between the two layers. Of course, other layers can be defined in the same way (especially when the board has different layers), but the above structure is a proven success example.    (13) a large area of the power supply layer can make the VCC cabling easy, but this structure is often a trigger for deterioration of the system performance, in a large plane to connect all the power leads will not be able to avoid the noise transmission between the pins. Conversely, if a star topology is used, the coupling between different power pins can be mitigated.    provides a star-connected VCC cabling scheme, which is taken from the evaluation Board of the MAX2826 IEEE 802.11a/g transceiver. The diagram establishes a primary VCC node, from which the power cords of different branches are drawn, powering the power supply pins of the RF ICS. Each power PIN provides space isolation between pins using separate leads, helping to reduce coupling between them. In addition, each lead has a certain parasitic inductance, which is exactly what we want, which helps to filter out high-frequency noise on the power cord.   When using a star topology VCC lead, it is also necessary to take appropriate power decoupling, while the decoupling capacitor has a certain parasitic inductance. In fact, the capacitance is equivalent to a series of RLC circuit, the capacitance in the low frequency band plays a leading role, but after the self-excited oscillation frequency (SRF): , the capacitance impedance will show an inductive. It can be seen that capacitors are decoupled only when the frequency is close to or below their SRF, where they exhibit low resistance.    gives the typical S11 parameters under different capacitance, from which the SRF can be clearly seen, as well as the greater the capacitance, the better the decoupling performance at lower frequencies (the lower the impedance presented).   It is best to place a large capacitance capacitor, such as 2.2μf, at the main node of the VCC star topology. The capacitance has a lower SRF, which is effective for eliminating low-frequency noise and establishing a stable DC voltage. Each supply pin of the IC requires a low-capacity capacitor (such as 10nF) to filter out high-frequency noise that may be coupled to the power cord. For those power supply pins that supply noise-sensitive circuitry, an external two bypass capacitor may be required. For example, a 10pF capacitor in parallel with a 10nF capacitor provides a bypass that provides a wider frequency range decoupling to minimize the noise impact on the supply voltage. Each power supply pin needs to be carefully inspected to determine how much decoupling power is requiredAnd what frequency points the actual circuit is susceptible to noise.   Good power decoupling technology combined with a rigorous PCB layout, VCC lead (star topology) provides a solid foundation for any RF system design. While there are other factors that can reduce system performance in real-world designs, having a "noise-free" power supply is a fundamental element in optimizing system performance .  EMC specifications for RF PCB design 1-Layer distribution1.1 Double panel, the top layer is the signal layer, the bottom surface is the ground plane. 1.2 Four layer board, the top layer is the signal layer, the second layer is the ground plane, the third layer goes the power supply, the control line. Special cases (such as RF signal lines to pass through the shielding wall), on the third floor to go some RF signal lines. A large area of land is required for each floor. 1.2 Four layer board, the top layer is the signal layer, the second layer is the ground plane, the third layer goes the power supply, the control line. Special cases (such as RF signal lines to pass through the shielding wall), on the third floor to go some RF signal lines. A large area of land is required for each floor. 2 Grounding2.1 Large area ground to reduce the impedance of the ground plane, to achieve good grounding effect, it is recommended to comply with the following requirements: a) RF PCB grounding requirements of large area grounding; b) in the Microstrip printed circuit, the bottom surface is ground, must ensure smooth and flat; d) tighten the screw to close the shielding chamber, and the tightening screw spacing is less than λ/20 (depending on the case). 2.2 Packet close to the ground in accordance with the structure of the circuit and the current size of the entire circuit into a relatively independent group, each group of circuits near the ground to form a circuit, to adjust the high-frequency filter capacitance in each group direction, reduce the power circuit. Note that the grounding wire should be short and straight, prohibit cross overlap, reduce the interference caused by the common ground impedance. 2.3 RF Device Grounding surface-mount RF devices and filter capacitors need to be grounded, in order to reduce the device grounding inductance, requires: a) at least 2 wire-paved copper foil, b) with at least 2 metallized vias near the device pin nearby ground. c) increase over over bore diameter and parallel several vias. D) The bottom of some components is grounded metal shell, in the projection area of the component to add some grounding holes, the surface layer shall not be routed. 2.4 Microstrip Circuit of the grounding Microstrip printed circuit terminal single ground hole diameter must be greater than the width of the Microstrip line, or the terminal mass into a large number of small holes in the way of grounding. 2.5 grounding process requirements a) under the condition that the process allows, the distance between the pad and the vias can be shortened; b) on the premise that the process allows, the Earth's large pad can be directly covered by at least 6 grounding vias (the exact number varies depending on the size of the pad); c) when the grounding wire needs to walk a certain distance, the length of the wire should be shortened, and no more than λ /20, in order to prevent the antenna effect caused by the signal radiation; d) except for special purposes, there shall be no isolated copper foil, the copper foil must be added ground over the hole; e) prohibit the grounding wire on the end of the open terminal 3 Shielding3.1 RF signals can be radiated in the air medium. The greater the space distance, the lower the operating frequency, the smaller the parasitic coupling at the input and output ends, the greater the isolation degree. The typical spatial isolation of the PCB is approximately 50dB. 3.2 Sensitive circuit and strong radiation source circuit to be shielded, but if the design process is difficult (such as space or cost limits, etc.), can not add, but to do the final decision. These circuits are: a) the front end of the receiving circuit is a sensitive circuit, the signal is very small, to use shielding. b) the RF unit and the intermediate frequency unit shall be shielded. If signal of the receiving channel will cause large interference to the RF signal, the RF signal of the transmitting channel will also cause radiation interference to the intermediate frequency signal. c) Oscillation circuit: strong radiation source, to the local vibration source to separate shielding, due to the high level of the local oscillator, the other units to form a large radiation interference. d) power amplifier and antenna: strong radiation source, strong signal, to be shielded. e) Digital signal processing circuit: a strong radiation source, high-speed digital signal, the steep upper and lower edge of the analog RF signal interference. f) Cascade Amplification Circuit: The total gain may exceed the spatial isolation of the output to the input, thus satisfying one of the oscillation conditions, the circuit may be self-excited. If the circuit of the cavity in the same frequency gain exceeds 30-50db, the PCB board must be welded or installed metal shielding plate, increase the degree of isolation. The actual design should consider the frequency, the power, the gain situation to decide whether to add shielding plate. g) Cascade filter, switch, attenuation circuit: in the same shielding cavity, the Cascade filter circuit of the out-of-band attenuation, cascade switch circuit isolation, cascade attenuation circuit attenuation must be less than 30-50db. If this value is exceeded, a metal shielding plate must be soldered or installed on the PCB board to increase the isolation level. The actual design should consider the frequency, the power, the gain situation to decide whether to add shielding plate. h) The transceiver unit should be shielded when mixed. i) when the number-mode mixing, the clock line to be covered with copper skin isolation or shielding. 4 Shielding materials and methods4.1 Commonly used shielding materials are high-conductivity materials, such as copper, copper foil, aluminum plate, aluminum foil. Steel plate or metal coating, conductive coating, etc. 4.2 Electrostatic shielding is primarily used to prevent the effects of static and constant magnetic fields. Should pay attention to two basic points, namely the perfect shielding body and good grounding properties. 4.3 Electromagnetic shielding is mainly used to prevent the influence of alternating magnetic field or alternating electromagnetic field, the shielding body has a good conductive continuity, the shield must be connected with the circuit in the common ground reference plane, the PCB should be shielded and shielded circuit to be as close as possible. 4.4 For some sensitive circuit, there is a strong radiation source of the circuit can design a PCB welding shielding cavity, PCB in the design to add "over omits Wall", is on the PCB with the shielding chamber wall close to the site with the grounding of the hole. The requirements are as follows: a) There are two rows above the hole; b) Two rows of holes staggered each other; c) the same row of the hole spacing to less than λ/20;d) grounding PCB copper foil and shielded cavity wall crimp site is forbidden to have solder resistance. 4.5 RF signal line in the top layer through the shielding wall, the shielding cavity in the corresponding position open a slot door, the door is tall in 0.5mm, the door width to ensure that the shield wall after the installation of the signal line and the distance between the shielding body is greater than 1mm. 5 Shielding cover Design5.1 The basic structure of the metal shielding cavity  5.1.1 This kind of shielding cover is widely used, 27. The material is generally thin aluminum alloy, the manufacturing process is generally used stamping bending or pressure casting process, the shielding cover has more screw holes, easy screw fixation. Some aluminum alloy caps and absorbing materials are required to enhance shielding performance. RF PCB needs to be installed in the shielding chamber, to choose the appropriate shielding cavity size, so that its minimum resonant frequency is much higher than the operating frequency, preferably more than 10 times times, see Appendix G "Metal Shielding cavity size design."  5.1.2 Shielding cavity height is generally the first layer of medium thickness of 15-20 times or more, in the shielding cavity area, to improve the minimum resonant frequency of the shielding cavity, the need to increase the aspect ratio, to avoid the square cavity.   5.2 Metal shielding cavity to the PCB layout of the process requirements  5.2.1 shielding cover and PCB contact with the body design should consider the PCB bottom surface of the device height, especially the plug-in device pin out of the height. The  5.2.2 should consider the size of the screw-down area to prevent damage to the surface line or device during assembly. RF Power Amplifier Board due to the size of the structure, the size of the single board is relatively small, it is generally required screw installation space (no cloth area) at least on the outside of the mounting hole pad. Screw mounting space See table 5  .5.2.3 metal shielding cover itself cost and assembly cost is very expensive, and irregular metal shielding cover in manufacturing difficult to ensure high precision and high flatness, but also the layout of components are limited; The metal shielding cover is not conducive to component replacement and fault locating.  5.2.4 as far as possible to ensure the integrity of the shielding cover is very important, enter the metal shielding cover of the digital signal line should be as far as possible the inner layer, the RF signal line from the bottom of the metal shielding cover of the small notch and the ground gap in the wiring layer on the road, but the gap around to as much as possible to cloth some ground, The ground on different layers can be connected together through multiple passes.  5.2.5 to ensure the assembly and rework, the metal shield around the 5mm range can not exceed the height of the device, Chip small device to the shielding cover should be more than 2mm, the other device distance requirements of more than 3mm, and placed towards the best to meet the convenient maintenance direction. The  5.2.6 metal shield can not have more than the height of the device, and the device top to the shielding surface of the distance to meet the safety requirements.  5.2.7 need to consider the SMA microstrip socket with the PCB board contact height matching, otherwise the welding reliability has influence. 29, the design must consider the PCB thickness tolerance (±10%), Metal shielding cavity machining error (±0.05mm). It is recommended that the height gap between SMA microstrip receptacle and PCB is not more than 0.5mm, and the socket and pad are not allowed to have obvious deviationPoor.   5.2.8 due to the special case of the amplifier board design, allow 2 pieces of the signal between the single board through the shielding cover, and connected with the fly line,   

RF route and Ground

For instance.　　We will be the multi-layer circuit board RF line simulation, in order to better make a comparison, the simulation of the PCB is divided into the surface of the pavement and the floor after the two blocks of the simulation comparison; the surface is not paved with the PCB file as shown in 1 (two wire width): Figure 1a: Line width 0.1016 mm RF line (surface paving) Figure 1b: line width 0.35 mm RF line (surface paving front) Figure 1: The surface is not paved on the PCB first the line width of the two plates (before the surface paving) by Allegro into the siwave, the target line with a 50ω port.　　For the different line widths of 0.1016mm and 0.35mm, our simulation results 2 shows that the curve shown in the figure is S21, the simulation frequency range is 800mhz-1ghz. Figure 2a: Surface S21 (line width 0.1016mm) Figure 2b: Surface of the S21 (line width 0.35mm) Figure 2: The surface is not paved S21 by the figure can be seen, in the 800mhz-1ghz range, the simulation of the data is displayed as the decimal point of one to two bits of the order of magnitude, The loss of 0.35mm is one order of magnitude smaller than the 0.1016mm line, because the 0.35mm line width has a characteristic impedance close to 50 Ω under the stack condition of the board.　　Therefore, we can indirectly verify that the impedance calculation (with the line width constraint) has a certain effect. Next we do the same simulation (800mhz-1ghz) after the surface paving, and import the PCB file like. Figure 3a:0.1016 mm RF line (surface paving) Figure 3b:0.35 mm RF line (surface paving) Figure 3: PCB simulation results after surface paving, such as: Figure 4a: The surface of the floor after the S21 (0.1016mm)

"5" Design checklist

Figure 4b: S21 after surface paving (0.35mm) Figure 4: The surface of the ground after the S21 by the figure, the simulation data shows that the line loss of the transmission line is already 1-2 db order of magnitude, of course, 0.35 mm loss is significantly less than 0.1016 mm.　　Another obvious phenomenon is the simulation results with respect to the non-paved ground, with the increase in frequency from 800MHz to 1GHz, the loss tends to be larger.　　We can get such a result from the simulation results: 1. The RF line is best to go 50 ohms, can reduce wire loss; 2. The surface paving is actually a part of the RF signal energy coupled to the ground, resulting in a certain loss. Therefore, the surface of the PCB should be paved with some attention. Try to stay away from the RF cable. Engineering experience is greater than 1.5 times times the line width. first, layout precautions(1) Structural design requirements before the PCB layout needs to understand the structure of the product. The structure needs to be reflected on the PCB board. For example, the outer thickness of the cavity shell, the thickness of the spacer cavity, the size of the chamfer radius and the size of the screw in the compartment, etc. (in other words, the structure is designed according to the finished PCB on the outline (structural part) of the specific design). In general, the outer cavity thickness is 4mm, the lumen width is 3mm, the dispensing process is 2mm; the chamfer radius is 2.5mm. The bottom left corner of the PCB board is the origin, the compartment needs an integer multiple of grid 0.5, the minimum need to do a grid of 0.1 integer times. This is advantageous to the structure processor processing, the error control is more accurate. Of course, this needs to be designed according to the customer's requirements. shown in the PCB design after the completion of the structure outline diagram:(2) Layout requires layout first to layout the RF link, and then layout the other circuits. A RF link layout considerations are fully based on the sequencing of the schematic (input to output, including the position of each component and the spacing between components and components. The distance between components and components should not be too large, such as π net. ) is laid out in a "one" or "L" shape. In the actual RF link layout, due to product space constraints, it is impossible to fully implement, which forces us to layout into a "U" shape.  Layout into U-shape is not not possible, but need to be in the middle of the compartment to isolate the left and right, do a good shielding. There is also a need to add a compartment in the transverse. That is, use the compartment to isolate the shape around a glyph. This is mainly due to the need to isolate the part is very sensitive or easy to interfere with other circuits, in addition, there may be a zigzag input to the output of the circuit's gain is too large, also need to use the compartment to separate it (if the gain is too large, the cavity is too large, may cause self-excitation.)  ）。 B chip peripheral Circuit Layout RF device peripheral circuit layout strictly refer to datasheet above requirements for layout, subject to space constraints can be adjusted; the peripheral circuit layout of the digital chip is not much to say. second, wiring precautionsAccording to the 50 Ohm impedance line width for wiring, as far as possible from the center of the pad, line into line, as far as possible to walk on the surface. To make a 45-degree angle or arc line where a corner is required, it is recommended to turn on either side of the capacitor or resistor. If you encounter the device line matching requirements, please strictly follow the reference value datasheet above the length of the line. For example, the length of the line between the amplifier tube and the capacitor (or the length of the line between the inductors) is required, and so on. in the PCB design, in order to make the high-frequency circuit board design more reasonable, anti-jamming performance is better, should be considered in the following aspects (general practice):(1) Reasonable choice of layer in the PCB design of high-frequency circuit board wiring, the use of intermediate plane as power and ground layer, can play the role of shielding, effectively reduce parasitic inductance, shorten the signal line length, reduce the cross-interference between the signal. (2) The route line must follow the angle of 45° corner or arc turn, this can reduce the high-frequency signal transmission and mutual coupling. (3) The length of the line, the shorter the better, the shorter the two line parallel distance, the better. (4) The number of vias is less than the better. (5) Inter-layer cabling direction between the direction of the vertical direction should be taken, that is, the top layer is horizontal, the bottom is the vertical direction, so that the interference between the signal can be reduced. (6) Apply copper to increase grounding of copper can reduce the interference between the signal. (7) Parcel ground to the important signal line processing, can significantly improve the anti-jamming ability of the signal, of course, the interference source can also be processed, so that it cannot interfere with other signals. (8) Signal line signal lines can not loop, you need to follow the daisy chain wiring.

Third, grounding treatment

(1) RF link grounding Radio frequency part uses the multi-point grounding method to ground treatment. RF link copper gap generally 30mil to 40mil used more. Both sides need to hit the grounding hole, and the spacing as far as possible to keep consistent. On the RF path to ground capacitance resistance grounding pad, as far as possible to hit the ground hole. Grounding pads on the device require grounding vias.

(2) Cavity shell grounding hole in order to allow better contact between the cavity shell and the PCB board. Generally, two rows of grounding holes and staggered mode placement, 06 is shown. The PCB compartment is required to open the window, shown in 07. PCB Bottom ground copper skin contact with the floor of the place need to open window processing, so that it better contact. As shown in Figure 08 (the upper part of the PCB board in contact with the base): PCB compartment grounding hole diagram PCB compartment open window diagram PCB Bottom window diagram (3) screw placement (need to understand structural knowledge) in order to make the PCB and the base and the cavity shell closer contact (better shielding) need to place the screw on the PCB board Pin hole position. Screw placement between PCB and cavity: Place a screw at each intersection of the compartment. In the actual design, it is difficult to realize, can be adjusted according to the function of the module circuit. However, there must be a screw on the four corners of the chamber shell. Cavity case screw Diagram The screw placement between the PCB and the base: there is a need for a screw in each small cavity in the cavity, and the number of the screw depending on the cavity size (the larger the cavity, the more screws are placed). The general principle is to place the screw on the diagonal of the cavity. The screws must be placed next to the SMA head or other connectors. The SMA head or connector does not deform the PCB board during the insertion and extraction process. In-cavity screw diagram

Key points of RF circuit design