RF circuit board design is often described as a kind of “black art” due to many theoretical uncertainties, but this view is only partially correct, RF circuit board design also has many guidelines to follow and should not be ignored.
In practical design, however, the real trick is to compromise these guidelines and rules when they cannot be implemented precisely because of various design constraints. Of course, there are many important RF design topics worth discussing, including impedance and impedance matching, insulating layer material and laminated plate, as well as wavelength and standing wave, so these have a great impact on the EMC and EMI of mobile phones. The following is a summary of the conditions that must be met in the design of RF layout of mobile phone PCB board:
High power RF amplifier (HPA) and low noise amplifier (LNA) should be separated.
Isolate high-power RF amplifiers (HPA) and low-noise amplifiers (LNA) as much as possible. In short, keep high-power RF transmitting circuits away from low-power RF receiving circuits. Mobile phone features more, a lot of components, but PCB space is small, and considering the wiring design process limit is the highest, all of these on the design skills of the requirements is higher.
It may be necessary to design four or six layers of PCBS to work interchangeably instead of simultaneously. High-power circuits may also sometimes include RF buffers and voltage-controlled oscillators (VCO). Make sure there is at least one piece of land in the high power area on the PCB board, preferably without any holes in it. Of course, the more copper, the better. Sensitive analog signals should be as far away from high-speed digital and RF signals as possible.
Physical partition, electrical partition design partition
It can be decomposed into physical and electrical partitions. Physical partition mainly involves the layout, orientation and shielding of components. Electrical partitions can continue to be decomposed into partitions for power distribution, RF wiring, sensitive circuits and signals, and grounding, etc.
1. We discuss physical partitioning.
Device layout is the key to an excellent RF design. The most effective technique is to first fix the components on the RF path and adjust their orientation to minimize the length of the RF path, keep the input away from the output, and separate the high power circuit from the low power circuit as far as possible.
The most effective circuit board stacking method is to arrange the main ground (the main ground) in the second layer below the surface layer, and as far as possible the RF line on the surface layer. Minimizing the size of the through-hole on the RF path not only reduces path inductance, but also reduces the number of virtual solder spots on the main ground and the chance of RF energy leakage to other areas of the laminate. In physical space, linear circuits such as multistage amplifiers are usually sufficient to isolate multiple RF regions from each other, but duplexers, mixers, and intermediate frequency amplifiers/mixers always have multiple RF/IF signals interfering with each other, so care must be taken to minimize this effect.
2. RF and IF wiring should be crossed as far as possible, and separate a piece of ground between them as far as possible.
The correct RF path is very important for the performance of the whole PCB board, which is why the component layout usually takes most of the time in the design of the PCB board of mobile phone. In the design of PCB board of mobile phone, it is usually possible to place the circuit of low-noise amplifier on one side of PCB board, and the high-power amplifier on the other side, and finally connect them to the antenna of RF end and baseband processor end on the same side through duplexer. Some tricks are required to ensure that a straight through hole does not transfer RF energy from one side of the plate to the other. A common technique is to use blind holes on both sides. The adverse effect of the straight through hole can be minimized by placing the straight through hole in an area free from RF interference on both sides of the PCB board.
It is not always possible to ensure sufficient isolation between multiple circuit blocks. In this case, consideration must be given to the use of a metal shield to shield RF energy within the RF area. The metal shield must be welded to the ground and must be kept at an appropriate distance from the components, thus occupying valuable PCB space. It is very important to ensure the integrity of the shield as much as possible. The digital signal line entering the metal shield should go through the inner layer as much as possible, and it is better to go through the layer of PCB below the layer. RF signal lines can go out from the small gap at the bottom of the metal shield and the wiring layer at the ground break. However, as much ground as possible should be laid around the break. The ground on different layers can be connected through multiple holes.
3. Appropriate and effective decoupling of chip power supply is also very important.
Many RF chips with linear lines are very sensitive to power supply noise, and usually each chip requires up to four capacitors and an isolating 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 supply. The same principle applies to decoupling the supply at the inductor end.
Some chip need more power to work, so you may need two or three sets of capacitance and inductance to decoupling on them respectively, few inductance parallel together, because this will form a tubular transformer and mutual induction interference signal, so the distance between them must be at least equal the height of one of the devices, or a right Angle are lined up in the mutual inductance to a minimum.
4. The principle of electrical partitioning is largely the same as that of physical partitioning, but there are other factors involved.
Some parts of the phone use different operating voltages, which are controlled by software to extend the battery life. This means the phone needs to run on multiple power sources, which creates more problems with isolation.
Power is usually brought in from the connector and immediately decoupled to filter out any noise from outside the circuit board, which is then allocated after a set of switches or voltage regulators. Most of the circuits on a phone’s PCB have fairly small dc currents, so routing width is usually not a problem, but a single large current line as wide as possible must be used for the power supply of the high power amplifier to minimize the transmission voltage drop. In order to avoid too much current loss, it is necessary to use multiple holes to transfer the current from one layer to another. In addition, if it is not sufficiently decoupled at the power pins of the high power amplifier, the high power noise will radiate to the entire plate and cause a variety of problems.
Grounding of high power amplifiers is critical and it is often necessary to design a metal shield for them. In most cases, it is also critical to ensure that RF outputs are kept away from RF inputs. 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 appropriate phases and amplitudes, they are likely to produce self-excited oscillations. At best, they will work stably at any temperature or voltage.
In fact, they can become unstable and add noise and intermodulation signals to RF signals. If the rf signal lines have to be wound from the input to the output of the filter this can seriously impair the bandpass characteristics of the filter. In order to isolate the input and output well, first of all, a field must be arranged around the filter, and then a field should be arranged in the lower area of the filter and connected with the main field around the filter. It is also a good idea to keep the signal line that needs to pass through the filter as far away from the filter pin as possible.
In addition, the grounding of all parts of the whole plate should be very careful, or a coupling channel will be introduced. Sometimes it is possible to choose a single or balanced RF signal line. The principles of cross interference and EMC/EMI apply here as well. Balancing RF signal lines can reduce noise and cross-talk if they are properly routed, but their impedance is usually high and it can be difficult to actually wire them to maintain a reasonable line width to get an impedance that matches the source, routing, and load. Buffers can be used to improve isolation because they divide the same signal into two parts and are used to drive different circuits. In particular, buffers may be needed to drive multiple mixers for local oscillations.
When the mixer reaches common-mode isolation at the RF frequency, it will not work properly. Buffers are good at isolating impedance changes at different frequencies so that circuits do not interfere with each other. Buffers are of great help to the design. They can be closely followed by the circuit to be driven, thus making the high-power output wiring very short. Since the input signal level of buffers is relatively low, they are not easy to cause interference to other circuits on the board. Voltage-controlled oscillators (VCO) convert varying voltages into varying frequencies, a feature used for high-speed channel switching, but they also convert a small amount of noise over the control voltage into a small change in frequency, which adds noise to the RF signal.
5. The following aspects must be considered in order to ensure no noise increase:
First, the desired bandwidth of the control line may range from DC to 2MHz, and filtering out the noise of such a wide band is almost impossible. Second, the VCO control line is usually part of a feedback loop that controls the frequency and can introduce noise in many places, so the VCO control line must be handled with great care. Ensure that the ground beneath the RF line is solid and that all components are firmly connected to the main ground and isolated from other noisy lines.
In addition, to ensure that the VCO power supply is sufficiently decoupled, special attention must be paid to the VCO because the RF output of the VCO is often a relatively high level and the VCO output signal can easily interfere with other circuits. In fact, the VCO is often placed at the end of the RF area and sometimes requires a metal shield. The resonant circuit (one for the transmitter and one for the receiver) is related to the VCO, but has its own characteristics. Simply put, a resonant circuit is a parallel resonant circuit with a capacitive diode that helps set the VCO operating frequency and modulate speech or data into a RF signal. All VCO design principles apply to resonant circuits as well. Resonant circuits are usually very sensitive to noise because they contain a considerable number of components, have a wide distribution on the board, and generally operate at a very high RF frequency.
Signals are usually placed on adjacent pins of the chip, but these signal pins need to work with relatively large inductors and capacitors, which in turn requires that these inductors and capacitors be located close together and connected back to a noise-sensitive control loop. This is not easy to do.
Automatic gain control (AGC) amplifiers are also a problem area, both in transmitting and receiving circuits. AGC amplifiers are usually effective at filtering out noise, but the ability of mobile phones to handle rapid variations in the strength of the transmitted and received signals requires a fairly wide bandwidth for AGC circuits, which makes it easy for AGC amplifiers on certain critical circuits to introduce noise. The design of AGC circuits must follow good analog design techniques, which are associated with very short operational amplifier input pins and very short feedback paths, both of which must be kept away from RF, IF, or high-speed digital signal routing.
Likewise, a good grounding is essential, and the power supply of the chip must be well decoupled. If you have to go a long way from the input or output, it’s best to go to the output, which usually has a much lower impedance and is not susceptible to 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 digital circuits away from analog circuits as much as possible, which is also applicable to RF PCB designs. Public to simulate and used for shielding and separated signal lines are usually equally important, in the early design stage, therefore, careful planning and thoughtful components layout and complete the layout of * assessment is very important, also should make the RF circuit away from analog circuit and some essential digital signal, and all of the RF cables, welding plate and components should be around as much as possible to fill the grounding copper, and connected to the main ground as soon as possible. If RF routing must cross signal lines, try to place a layer of ground between them along the RF routing to connect to the main ground. If this is not possible, make sure that they are cross-linked to minimize capacitive coupling, while placing as much ground as possible around each RF wire and connecting them to the main ground.
In addition, minimizing the distance between parallel RF lines minimizes the inductive coupling. A solid, monolithic floor is best isolated when placed directly below the surface, although a little care in design can be used in other ways. Place as much cloth as possible on each layer of the PCB and connect them to the main floor. Line up as close together as possible to increase the number of plots in the internal signal layer and power distribution layer, and adjust the wiring properly so that you can connect the ground through the hole to the surface of the isolated plot. Dissociation should be avoided on each layer of the PCB as they can pick up or inject noise like a small antenna. In most cases, if you can’t connect them to the main site, you’d better remove them.
Three, in the mobile phone PCB board design, should pay attention to several aspects 1, power supply, ground handling
Even if the wiring in the entire PCB board is completed very well, but due to the power supply, ground wire is not considered and caused by the interference, will make the product performance decline, sometimes even affect the success rate of the product. So to the wiring of electric, ground wire should treat seriously, the noise interference that produces place of electric, ground wire drops minimum limit, in order to assure the quality of the product. For every engineer engaged in the design of electronic products, the cause of the noise generated between ground wire and power line is well understood. The following is the description of noise reduction and suppression:
(1) it is well known to add decoupling capacitors between power supply and ground wire.
(2) try to widen the width of power supply and ground wire, preferably the ground wire is wider than the power line, their relationship is: ground wire > power line > signal line, usually the width of the signal line is: 0.2 ~ 0.3mm, the most through the width of 0.05 ~ 0.07mm, the power line is 1.2 ~ 2.5 mm. A digital PCB can be used with a wide ground wire to form a circuit, that is, a ground network (the ground of an analog circuit cannot be used in this way).
(3) use a large area of copper layer as a ground wire, in the printed circuit board is not used to connect with the ground as a ground wire. Or make it multilayer board, power supply, ground wire each occupy a layer.
2. Common ground processing of digital circuit and analog circuit
Many PCBS today are no longer single-function circuits (digital or analog) but are a mix of digital and analog circuits. Therefore, it is necessary to consider the interference between them when wiring, especially the noise interference on the ground. The sensitivity of the high frequency digital circuits, analog circuits, the signal wire, high-frequency signal lines as far as possible away from the sensitive analog devices, for the ground, moving the PCB to the outside world is only one node, so must be within the PCB processing, mold has problem, and inside the plate to digital and analog is actually are divided between them, just connected with the outside world in PCB interface (e.g., plugs, etc.). There is a short connection between digital and analog, note that there is only one connection point. There are also inconsistencies on the PCB, which is determined by the system design.
3. Signal lines are distributed on the electrical (ground) layer
In multilayer PCB wiring, because in the signal line layer did not finish the line there is not much left, and more layers