Monday, April 25, 2011
College】 【weak RF RF »RF circuit design techniques】.
<br> <BR> successful RF design must pay careful attention to each step throughout the design process and every detail, this means that beginning in the design will be a thorough, careful planning, and each design step .comprehensive and continuous assessment of progress. .Detailed design skills which most of the domestic electronics companies is lacking in culture. .In recent years, due to bluetooth devices, wireless local area network (WLAN) equipment, and mobile phone demand and growth, to promote the industry more and more concerned about the RF circuit design techniques. .From past to present, RF circuit board design as electromagnetic interference (EMI) problems, as has been the most difficult to control the part of engineers, or even a nightmare. .If you want to design a successful, careful prior planning and attention to detail can be effective. .Radio frequency (RF) circuit board design, in theory, because there are many uncertainties, it is often described as a "black art" (black art). .But this is only an incomplete view, RF circuit board design there are many rules to follow. .However, in practical design, the real practical skills is that when these rules can not be implemented due to various constraints, how a compromise deal with them. .Important RF design issues include: impedance and impedance matching, insulation material and stacked plates, the wavelength and harmonic ... so, this article will focus on RF circuit board with various issues related to the design district. .The types of micro-vias <BR> different nature of the circuit board to be separated, but also do not produce electromagnetic interference in the best case connection, which need to use micro-vias (microvia). .Micro-vias are usually 0.05mm diameter to 0.20mm, these vias are generally divided into three categories, namely, blind holes (blind via), buried vias (bury via) and the through-hole (through via). .Blind hole at the top and bottom surface of printed circuit board, with a certain depth for the line and below the inner surface of the connection lines, the depth of the hole is usually no more than a certain ratio (aperture). .Buried hole is located in the inner connection hole printed circuit board, it does not extend to the circuit board surface. .The two holes are located in the inner layer circuit board, laminated front through-hole forming process used to complete the process via the formation of overlap may also make a few lengths. .The third is called through-hole, this hole through the entire circuit board can be used to achieve internal interconnect or as a component of the adhesion location hole. .Partition techniques used in the design of RF <BR> circuit board, as far as possible the high-power RF amplifier (HPA) and low-noise amplifier (LNA) to isolate, simply, is to stay away from high-power low-noise RF transmitter circuit .receiver circuit. .If the PCB board has a lot of space, you can easily do this. .But usually when a lot of components, PCB space will become very small, so it is very difficult to achieve. .Can put them on both sides of PCB board, or let them turn the work, rather than simultaneously. .Sometimes high-power circuit may also include RF buffer (buffer) and voltage-controlled oscillator (VCO). .Design Division can be divided into physical partitions (physical partitioning) and Electrical Division (Electrical partitioning). .Physical partition layout of the main components involved, location and shielding issues; district can continue to be divided into electrical power distribution, RF alignment, sensitive circuits and signals, grounding and other partitions. .Physical partition layout <BR> components designed to achieve an excellent RF key, the most effective technique is to first fixed in the RF path components, and adjust its position, so to minimize the length of the RF path. .And to stay away from RF input RF output, and as far as possible away from the high-power circuits and low noise circuit. .The most effective method is to stack the circuit boards arranged in the main ground of the second subsurface layer, and RF cable to go as far as possible on the surface. .RF path through the pore size can be reduced not only the path to minimize inductance, but also can reduce the cold solder joint on the main ground point, and reduce the RF energy leakage to other regions within layered plates opportunities. .In the physical space, such as multi-stage linear amplifier circuit is usually sufficient between multiple RF isolated areas, but diplexers, mixers and IF amplifiers are always more than one RF / IF signals interfere with each other .must therefore be careful to minimize this effect. .RF and IF alignment cross walk as far as possible, and as far as they are separated by a ground area. .The correct path on the RF performance of entire PCB board is very important, which is why the layout of components in the mobile phone PCB board is usually the design of the reasons accounting for most of the time. .In the mobile phone PCB board, usually on the low noise amplifier circuit side of a PCB board, and the high power amplifier on the other side, and ultimately by the same plane diplexer to connect them to the end of the RF antenna .and the other end the baseband processor. .This requires some skill to ensure that RF energy does not have holes through from the board side to pass to the other side, the commonly used technique is used in both sides of the blind hole. .Blind holes can be arranged by the both sides of PCB board area without RF interference to the vias to minimize the adverse effects. .Metal shield at times, is unlikely to retain a sufficient number of circuit blocks of the segment, in which case we must consider the metal shield to RF energy in the RF shielded area, but the metal shield has side effects, .For example: manufacturing costs and assembly costs are high. .<BR> Irregular shape of the metal shield is difficult to ensure in the manufacture of high precision, rectangular or square metal shield and the layout of the components subject to some restrictions; metal shield is not conducive to replacement components and fault displacement; .the metal shield must be welded to the ground plane, and the components must maintain a proper distance, so need to occupy valuable PCB board space. .As much as possible to ensure the integrity of the metal shield is very important, so enter the metal shield of the digital signal lines should go as far as possible the inner layer, preferably the signal line layer ground plane is set to the next level. .RF signal lines from the metal shield at the bottom of the small gap at the gap and the ground wiring layer alignment out, but as far as possible gap in the surrounding area with the ground surrounded by the vast number of different signals on the ground floor through multiple vias can be .together. .<BR> Notwithstanding the above shortcomings, but the metal shield is still very effective, and often the only key circuit isolation solution. .Power supply decoupling circuit addition, appropriate and effective chip power supply decoupling (decouple) the circuit is also very important. .Many RF integrated circuit chip linear power supply noise is very sensitive to each chip typically need to adopt up to four capacitors and inductors to filter out all of an isolated power supply noise. .(Figure a) the minimum capacitance value usually depends on the capacitance of its own resonant frequency and pin inductance, C4's value is chosen accordingly. .Values of C3 and C2, because of its relationship pin inductance is relatively large, so RF decoupling effect has to be worse, but they are more suited to filter out low frequency noise signal. .RF decoupling is done by the inductor L1, which allows RF signals from the power line can not be coupled to the chip. .Because all the alignment is a potential can also be received RF signal transmitting antenna, so the radio frequency signal and the critical path, component isolation is necessary. .The physical location of the decoupling components is usually critical. .The layout of these principles are important components: C4 to IC pins as close as possible and grounded, C3 must be closest to the C4, C2 must be closest to the C3, C4 and IC pin connector with the alignment should be as short as possible, which .several components of the ground (especially C4) board should normally be by a ground layer of Lower Dir and the chip ground pin connected. .Component connected with the ground plane should be as close to the PCB through-hole board assembly pads, it is best to use to play on the blind hole in the pad will be connected to minimize inductance, the inductor L1 should be close to C1. .An integrated circuit or amplifier often have an open collector output (open collector), and therefore requires a pullup inductor (pullup inductor) to provide a high impedance load and a low RF impedance DC power supply, the same principle also applies to this .inductance decoupling the power supply side. .Some chips require more power to work, so it may take two to three sets of inductors and capacitors are decoupling handle them, if that is not enough space around the chip, then the decoupling effect may be poor. .In particular, require special attention are: the parallel inductance little close together, as it will form an air core transformer, and mutual interference signal is induced, so the distance between them at least equal to the height of one, or at right angles .order to minimize their mutual inductance. .Electrical Division Electrical Division <BR> physical partition with the same principles, but also contains a number of other factors. .Some parts of modern mobile phones with different operating voltage, and using software to control, to extend battery life. .This means that mobile phones need to have a variety of power, which produces more segregation. .Power usually cable (connector) the introduction, and immediately processed to filter out any decoupling circuit board from external noise, and then through a switch or linear regulator after the power distribution. .In the mobile phone, most circuits are very small DC current, so the trace width is usually not a problem, however, must be a separate power supply for high power amplifier designed as a high current line width, so that the instantaneous firing .state voltage drop (voltage drop) can be minimized. .In order to avoid too much current consumption, need to use multiple vias to the current passing from one layer to another layer. .In addition, if not in high-end power amplifier power supply pin decoupling it fully, then the high-power noise will be radiated to the entire board, and bring a variety of problems. .High power amplifier ground is very important, and often need to design a metal shield..