Radio frequency (RF) circuit board design is often described as a "black art" because of the many uncertainties in theory, but this view is only partially correct, and RF circuit board designs have many guidelines that can be followed and should not The law of neglect.

However, in practical design, the real practical skills is when these guidelines and rules because of various design constraints can not be accurately implemented when they are compromised. Of course, there are many important RF design issues worthy of discussion, including impedance and impedance matching, insulation materials and laminated boards and wavelengths and standing waves, so these phones on the EMC, EMI effects are great, the following on the mobile phone PCB board In the design of RF layout must meet the conditions to be summarized:

1.1 As far as possible to high-power RF amplifier (HPA) and low noise amplifier (LNA) isolated, simply put, is to make high-power RF transmitter circuit away from low-power RF receiver circuit. Mobile phone features more, many components, but the PCB space is small, taking into account the wiring design process is limited to the highest, all of these requirements on the design skills is relatively high. This time may need to design four to six PCB, and let them work alternately, rather than work at the same time. High power circuits may also include RF buffers and voltage controlled oscillators (VCOs). To ensure that the PCB board on the high-power area at least one block, the best above no holes, of course, copper the more the better. Sensitive analog signals should be as far away as possible from high-speed digital signals and RF signals.

1.2 design partition can be decomposed into physical partitions and electrical partitions. Physical partitioning mainly involves component layout, orientation and shielding and other issues; electrical partition can continue to be broken down into power distribution, RF traces, sensitive circuits and signals, and grounding and other partitions.

1.2.1 We discuss the problem of physical partitioning. Component layout is the key to achieving an excellent RF design. The most efficient technique is to first fix the components on the RF path and adjust the orientation to minimize the length of the RF path so that the input is far from the output and as far as possible To separate high-power circuits and low-power circuits.

The most efficient circuit board stacking method is to arrange the main ground (main) in the second layer under the surface, and as far as possible the RF line on the surface. Minimizing the vias on the RF path not only reduces the path inductance, but also reduces the potential solder joints on the main ground and reduces RF energy leakage to other areas of the board. In physical space, linear circuits such as multistage amplifiers are usually sufficient to isolate multiple RF regions from each other, but duplexers, mixers and IF amplifiers / mixers always have multiple RF / IF The signals interfere with each other, so care must be taken to minimize this effect.

1.2.2 RF and IF traces should be crossed as far as possible and, as far as possible, between them. The correct RF path is very important for the performance of the entire PCB board, which is why the component layout is usually in the handset PCB board design for most of the time reasons. In the design of the mobile phone PCB board, the low noise amplifier circuit can usually be placed on one side of the PCB board, and the high power amplifier is placed on the other side and finally connected to the RF side and baseband processing on the same side by the duplexer On the antenna side. Some techniques are needed to ensure that the through-hole does not transfer RF energy from the side of the board to the other, and the common technique is to use blind holes on both sides. The adverse effects of the through-holes can be minimized by arranging the straight through holes in the area where both sides of the PCB are not subject to RF interference. It is sometimes possible to ensure adequate isolation between multiple circuit blocks. In this case, it is necessary to consider the use of metal shields to shield RF energy into the RF area. The metal shield must be welded to the ground and must be kept An appropriate distance, so need to take up valuable PCB board space. As far as possible to ensure the integrity of the shield is very important to enter the metal shield of the digital signal line should be as far as possible inside the inner layer, and the best layer of the underlying layer of PCB is the formation. RF signal lines can be from the bottom of the metal shield and the gap at the bottom of the wiring layer to go out, but the gap around as much as possible to cloth, the different layers of the ground can be connected through a number of holes The

1.2.3 Proper and efficient chip power supply decoupling is also very important. Many RF chips that integrate linear lines are very sensitive to noise from the power supply. Typically, each chip requires up to four capacitors and an isolated inductor to ensure that all power supply noise is filtered out. An integrated circuit or amplifier often has an open drain output, so a pull-up inductor is required to provide a high impedance RF load and a low impedance DC power supply. The same principle applies to decoupling the power supply at this inductor. Some chips require multiple power supplies to work, so you may need two to three sets of capacitors and inductors to decouple them separately, and the inductors are very close together in parallel because it will form an air-core transformer and interfere with each other Signal, so the distance between them is at least equivalent to the height of one of the devices, or at right angles to minimize their mutual inductance.

1.2.4 The principle of electrical partitioning is generally the same as the physical partition, but also contains some other factors. Some parts of the phone with different operating voltage, and with software to control it to extend the battery life. This means that the phone needs to run a variety of power, and this has brought more problems to the isolation. The power supply is usually introduced from the connector and is immediately decoupled to filter out any noise from the outside of the board and then distributed over a set of switches or regulators. Most of the circuits on the PC PCB have a very small DC current, so the trace width is usually not a problem, however, the power supply for the high power amplifier must be taken as a single wide current line as much as possible to minimize the transmission voltage drop The In order to avoid too much current loss, multiple vias need to be used to transfer current from one layer to another. In addition, if power decoupling can not be adequately decoupled at the power supply pin of the high power amplifier, high power noise will radiate to the entire board and bring a variety of problems. The grounding of the high power amplifier is critical and it is often necessary to design a metal shield for it. In most cases, it is also critical to ensure that the RF output is far from the RF input. This also applies to amplifiers, buffers and filters. In the worst case, if the outputs of the amplifiers and buffers are fed back to their inputs with the appropriate phase and amplitude, they are likely to produce self-oscillating. In the best case, they will be able to work stably at any temperature and voltage conditions. In practice, they may become unstable and add noise and intermodulation signals to the RF signal. If the RF signal line has to be looped back from the input of the filter, this may severely damage the bandpass characteristics of the filter. In order to provide good isolation of the input and output, it must first be around the filter, and then the lower layer of the filter should be laid together and connected to the main ground around the filter. It is also a good idea to keep the signal lines that need to pass through the filter as far as possible from the filter pins.

In addition, the entire board on all parts of the ground must be very careful, otherwise it will introduce a coupling channel. Sometimes you can choose to take a single-ended or balanced RF signal line, the principle of cross-interference and EMC / EMI is equally applicable here. Balanced RF signal lines can reduce noise and cross-interference if the alignment is correct, but their impedance is usually high and keep a reasonable linewidth to get a matching signal source, trace and load impedance, actual wiring possible There will be some difficulties. The buffer can be used to improve the isolation effect because it divides the same signal into two parts and is used to drive different circuits, especially the local oscillator may require a buffer to drive multiple mixers. When the mixer reaches the common-mode isolation at the RF frequency, it will not work properly. The buffers can well isolate the impedance changes at different frequencies so that the circuits do not interfere with each other. The buffers are of great help to the design, and they can be followed by the need to be driven by the circuit so that the high power output traces are very short, and because the input signal levels of the buffers are relatively low, they are not easy Circuit caused by interference. The VCO can convert the varying voltage to a varying frequency, which is used for high-speed channel switching, but they also convert the trace noise on the control voltage to a small frequency change, RF signal increases noise.

1.2.5 to ensure that no noise must be considered from the following aspects: First, the control line of the expected bandwidth range from DC to 2MHz, and by filtering to remove such a wide band noise is almost impossible; Second, VCO The control line is usually part of a feedback loop that controls the frequency, and it is possible to introduce noise in many places, so the VCO control line must be handled with great care. Make sure that the lower ground of the RF trace is solid, and that all the components are firmly connected to the main ground and are isolated from other traces that may cause noise. In addition, to ensure that the VCO power supply has been fully decoupled, because the VCO's RF output is often a relatively high level, VCO output signal is easy to interfere with other circuits, it must pay special attention to the VCO. In fact, VCOs tend to be placed at the end of the RF area, and sometimes it needs a metal shield. The resonant circuit (one for the transmitter and the other for the receiver) is related to the VCO, but also has its own characteristics. Simply put, the resonant circuit is a parallel resonant circuit with a capacitive diode that helps to set the VCO operating frequency and modulate the voice or data onto the RF signal. All VCO design principles apply to resonant circuits as well. The resonant circuit is usually very sensitive to noise because the resonant circuit contains a large number of components, the board is distributed over a wide area and usually runs at a very high RF frequency. The signals are usually arranged on the adjacent feet of the chip, but these signal pins need to work with relatively large inductors and capacitors to work, which in turn requires that the locations of these inductors and capacitors must be very close and back A noise-sensitive control loop. It is not easy to do this.

Automatic gain control (AGC) amplifiers are also prone to problems where both the transmit and receive circuits have AGC amplifiers. AGC amplifiers are often able to effectively filter out noise, but because the phone has the ability to handle rapid changes in transmit and receive signal strength, the AGC circuit is required to have a fairly wide bandwidth, which makes it easy to introduce AGC amplifiers on some critical circuits noise. Designing AGC lines must follow good analog circuit design techniques, which are related to very short op amp input pins and very short feedback paths, both of which must be remote from RF, IF or high speed digital signal traces. Likewise, good grounding is also essential, and the chip's power supply must be well decoupled. If it is necessary to take a long line at the input or output, it is best at the output, usually the output impedance is much lower, and it is not easy to sense noise. Usually the higher the signal level, the easier it is to introduce noise into other circuits. In all PCB designs, it is a general principle to keep a digital circuit away from an analog circuit as much as possible, and it also applies to RFPCB designs. Public analog ground and ground for shielding and separating signal lines are usually equally important, so it is important to carefully plan, consider the layout of the components and the thorough layout at the early stages of the design. The lines are away from analog lines and some very critical digital signals, and all RF traces, pads and components should be filled with as much copper as possible and connected as far as possible to the main ground. If the RF traces must pass through the signal lines, try to connect them along the RF traces between them and the main ground. If it is not possible, make sure that they are crossed, which minimizes capacitive coupling and, as far as possible, spreads around each RF trace and connects them to the main ground. In addition, minimizing the distance between parallel RF traces can minimize inductive coupling. A solid piece of ground directly on the surface layer under the first layer, the isolation effect is the best, although careful design of other practices also work. In each layer of the PCB board, should be laid as much as possible, and connect them to the main ground. Keep the traces as far as possible to increase the number of parcels of the internal signal layer and the power distribution layer and adjust the alignment so that you can arrange the ground connection holes to the isolation plots on the surface. Avoid creating free on the PCB layers, as they pick up or inject noise as a small antenna. In most cases, if you can not connect them to the Lord, then you'd better take them out.